CommandLine 2.0 Library Manual

Introduction

This document describes the CommandLine argument processing library. It willshow you how to use it, and what it can do. The CommandLine library uses adeclarative approach to specifying the command line options that your programtakes. By default, these options declarations implicitly hold the value parsedfor the option declared (of course this can be changed).

Although there are a lot of command line argument parsing libraries outthere in many different languages, none of them fit well with what I needed. Bylooking at the features and problems of other libraries, I designed theCommandLine library to have the following features:

• Speed: The CommandLine library is very quick and uses little resources. Theparsing time of the library is directly proportional to the number ofarguments parsed, not the number of options recognized. Additionally,command line argument values are captured transparently into user definedglobal variables, which can be accessed like any other variable (and with thesame performance).
• Type Safe: As a user of CommandLine, you don’t have to worry aboutremembering the type of arguments that you want (is it an int? a string? abool? an enum?) and keep casting it around. Not only does this help preventerror prone constructs, it also leads to dramatically cleaner source code.
• No subclasses required: To use CommandLine, you instantiate variables thatcorrespond to the arguments that you would like to capture, you don’tsubclass a parser. This means that you don’t have to write anyboilerplate code.
• Globally accessible: Libraries can specify command line arguments that areautomatically enabled in any tool that links to the library. This ispossible because the application doesn’t have to keep a list of arguments topass to the parser. This also makes supporting dynamically loaded optionstrivial.
• Cleaner: CommandLine supports enum and other types directly, meaning thatthere is less error and more security built into the library. You don’t haveto worry about whether your integral command line argument accidentally gotassigned a value that is not valid for your enum type.
• Powerful: The CommandLine library supports many different types of arguments,from simple boolean flags to scalars arguments (strings,integers, enums, doubles), to lists of arguments. This ispossible because CommandLine is…
• Extensible: It is very simple to add a new argument type to CommandLine.Simply specify the parser that you want to use with the command line optionwhen you declare it. Custom parsers are no problem.
• Labor Saving: The CommandLine library cuts down on the amount of grunt workthat you, the user, have to do. For example, it automatically provides a-help option that shows the available command line options for your tool.Additionally, it does most of the basic correctness checking for you.
• Capable: The CommandLine library can handle lots of different forms ofoptions often found in real programs. For example, positional arguments,ls style grouping options (to allow processing ‘ls -lad’naturally), ld style prefix options (to parse ‘-lmalloc-L/usr/lib’), and interpreter style options.This document will hopefully let you jump in and start using CommandLine in yourutility quickly and painlessly. Additionally it should be a simple referencemanual to figure out how stuff works.

Quick Start Guide

This section of the manual runs through a simple CommandLine’ification of abasic compiler tool. This is intended to show you how to jump into using theCommandLine library in your own program, and show you some of the cool things itcan do.

To start out, you need to include the CommandLine header file into your program:

#include "llvm/Support/CommandLine.h"

Additionally, you need to add this as the first line of your main program:

int main(int argc, char **argv) {
  cl::ParseCommandLineOptions(argc, argv);
  ...
}

… which actually parses the arguments and fills in the variable declarations.

Now that you are ready to support command line arguments, we need to tell thesystem which ones we want, and what type of arguments they are. The CommandLinelibrary uses a declarative syntax to model command line arguments with theglobal variable declarations that capture the parsed values. This means thatfor every command line option that you would like to support, there should be aglobal variable declaration to capture the result. For example, in a compiler,we would like to support the Unix-standard ‘-o <filename>’ option to specifywhere to put the output. With the CommandLine library, this is represented likethis:

cl::opt<string> OutputFilename("o", cl::desc("Specify output filename"), cl::value_desc("filename"));

This declares a global variable “OutputFilename” that is used to capture theresult of the “o” argument (first parameter). We specify that this is asimple scalar option by using the “cl::opt” template (as opposed to the“cl::list” template), and tell the CommandLine library that the datatype that we are parsing is a string.

The second and third parameters (which are optional) are used to specify what tooutput for the “-help” option. In this case, we get a line that looks likethis:

USAGE: compiler [options]
 
OPTIONS:
  -h                - Alias for -help
  -help             - display available options (-help-hidden for more)
  -o <filename>     - Specify output filename

Because we specified that the command line option should parse using thestring data type, the variable declared is automatically usable as a realstring in all contexts that a normal C++ string object may be used. Forexample:

...
std::ofstream Output(OutputFilename.c_str());
if (Output.good()) ...
...

There are many different options that you can use to customize the command lineoption handling library, but the above example shows the general interface tothese options. The options can be specified in any order, and are specifiedwith helper functions like cl::desc(…), so there are no positionaldependencies to remember. The available options are discussed in detail in theReference Guide.

Continuing the example, we would like to have our compiler take an inputfilename as well as an output filename, but we do not want the input filename tobe specified with a hyphen (ie, not -filename.c). To support this style ofargument, the CommandLine library allows for positional arguments to bespecified for the program. These positional arguments are filled with commandline parameters that are not in option form. We use this feature like this:

cl::opt<string> InputFilename(cl::Positional, cl::desc("<input file>"), cl::init("-"));

This declaration indicates that the first positional argument should be treatedas the input filename. Here we use the cl::init option to specify an initialvalue for the command line option, which is used if the option is not specified(if you do not specify a cl::init modifier for an option, then the defaultconstructor for the data type is used to initialize the value). Command lineoptions default to being optional, so if we would like to require that the useralways specify an input filename, we would add the cl::Required flag, and wecould eliminate the cl::init modifier, like this:

cl::opt<string> InputFilename(cl::Positional, cl::desc("<input file>"), cl::Required);

Again, the CommandLine library does not require the options to be specified inany particular order, so the above declaration is equivalent to:

cl::opt<string> InputFilename(cl::Positional, cl::Required, cl::desc("<input file>"));

By simply adding the cl::Required flag, the CommandLine library willautomatically issue an error if the argument is not specified, which shifts allof the command line option verification code out of your application into thelibrary. This is just one example of how using flags can alter the defaultbehaviour of the library, on a per-option basis. By adding one of thedeclarations above, the -help option synopsis is now extended to:

USAGE: compiler [options] <input file>
 
OPTIONS:
  -h                - Alias for -help
  -help             - display available options (-help-hidden for more)
  -o <filename>     - Specify output filename

… indicating that an input filename is expected.

Boolean Arguments

In addition to input and output filenames, we would like the compiler example tosupport three boolean flags: “-f” to force writing binary output to aterminal, “—quiet” to enable quiet mode, and “-q” for backwardscompatibility with some of our users. We can support these by declaring optionsof boolean type like this:

cl::opt<bool> Force ("f", cl::desc("Enable binary output on terminals"));
cl::opt<bool> Quiet ("quiet", cl::desc("Don't print informational messages"));
cl::opt<bool> Quiet2("q", cl::desc("Don't print informational messages"), cl::Hidden);

This does what you would expect: it declares three boolean variables(“Force”, “Quiet”, and “Quiet2”) to recognize these options. Notethat the “-q” option is specified with the “cl::Hidden” flag. Thismodifier prevents it from being shown by the standard “-help” output (notethat it is still shown in the “-help-hidden” output).

The CommandLine library uses a different parser for different data types.For example, in the string case, the argument passed to the option is copiedliterally into the content of the string variable… we obviously cannot do thatin the boolean case, however, so we must use a smarter parser. In the case ofthe boolean parser, it allows no options (in which case it assigns the value oftrue to the variable), or it allows the values “true” or “false” to bespecified, allowing any of the following inputs:

compiler -f          # No value, 'Force' == true
compiler -f=true     # Value specified, 'Force' == true
compiler -f=TRUE     # Value specified, 'Force' == true
compiler -f=FALSE    # Value specified, 'Force' == false

… you get the idea. The bool parser just turns the string values intoboolean values, and rejects things like ‘compiler -f=foo’. Similarly, thefloat, double, and int parsers work like you would expect, using the‘strtol’ and ‘strtod’ C library calls to parse the string value into thespecified data type.

With the declarations above, “compiler -help” emits this:

USAGE: compiler [options] <input file>
 
OPTIONS:
  -f     - Enable binary output on terminals
  -o     - Override output filename
  -quiet - Don't print informational messages
  -help  - display available options (-help-hidden for more)

and “compiler -help-hidden” prints this:

USAGE: compiler [options] <input file>
 
OPTIONS:
  -f     - Enable binary output on terminals
  -o     - Override output filename
  -q     - Don't print informational messages
  -quiet - Don't print informational messages
  -help  - display available options (-help-hidden for more)

This brief example has shown you how to use the ‘cl::opt’ class to parsesimple scalar command line arguments. In addition to simple scalar arguments,the CommandLine library also provides primitives to support CommandLine optionaliases, and lists of options.

Argument Aliases

So far, the example works well, except for the fact that we need to check thequiet condition like this now:

...
  if (!Quiet && !Quiet2) printInformationalMessage(...);
...

… which is a real pain! Instead of defining two values for the samecondition, we can use the “cl::alias” class to make the “-q” option analias for the “-quiet” option, instead of providing a value itself:

cl::opt<bool> Force ("f", cl::desc("Overwrite output files"));
cl::opt<bool> Quiet ("quiet", cl::desc("Don't print informational messages"));
cl::alias     QuietA("q", cl::desc("Alias for -quiet"), cl::aliasopt(Quiet));

The third line (which is the only one we modified from above) defines a “-q”alias that updates the “Quiet” variable (as specified by the cl::aliasoptmodifier) whenever it is specified. Because aliases do not hold state, the onlything the program has to query is the Quiet variable now. Another nicefeature of aliases is that they automatically hide themselves from the -helpoutput (although, again, they are still visible in the -help-hidden output).

Now the application code can simply use:

...
  if (!Quiet) printInformationalMessage(...);
...

… which is much nicer! The “cl::alias” can be used to specify analternative name for any variable type, and has many uses.

Selecting an alternative from a set of possibilities

So far we have seen how the CommandLine library handles builtin types likestd::string, bool and int, but how does it handle things it doesn’tknow about, like enums or ‘int*’s?

The answer is that it uses a table-driven generic parser (unless you specifyyour own parser, as described in the Extension Guide). This parser mapsliteral strings to whatever type is required, and requires you to tell it whatthis mapping should be.

Let’s say that we would like to add four optimization levels to our optimizer,using the standard flags “-g”, “-O0”, “-O1”, and “-O2”. Wecould easily implement this with boolean options like above, but there areseveral problems with this strategy:

• A user could specify more than one of the options at a time, for example,“compiler -O3 -O2”. The CommandLine library would not be able to catchthis erroneous input for us.
• We would have to test 4 different variables to see which ones are set.
• This doesn’t map to the numeric levels that we want… so we cannot easilysee if some level >= “-O1” is enabled.To cope with these problems, we can use an enum value, and have the CommandLinelibrary fill it in with the appropriate level directly, which is used like this:

enum OptLevel {
  g, O1, O2, O3
};
 
cl::opt<OptLevel> OptimizationLevel(cl::desc("Choose optimization level:"),
  cl::values(
    clEnumVal(g , "No optimizations, enable debugging"),
    clEnumVal(O1, "Enable trivial optimizations"),
    clEnumVal(O2, "Enable default optimizations"),
    clEnumVal(O3, "Enable expensive optimizations")));
 
...
  if (OptimizationLevel >= O2) doPartialRedundancyElimination(...);
...

This declaration defines a variable “OptimizationLevel” of the“OptLevel” enum type. This variable can be assigned any of the values thatare listed in the declaration. The CommandLine library enforces thatthe user can only specify one of the options, and it ensure that only valid enumvalues can be specified. The “clEnumVal” macros ensure that the commandline arguments matched the enum values. With this option added, our help outputnow is:

USAGE: compiler [options] <input file>
 
OPTIONS:
  Choose optimization level:
    -g          - No optimizations, enable debugging
    -O1         - Enable trivial optimizations
    -O2         - Enable default optimizations
    -O3         - Enable expensive optimizations
  -f            - Enable binary output on terminals
  -help         - display available options (-help-hidden for more)
  -o <filename> - Specify output filename
  -quiet        - Don't print informational messages

In this case, it is sort of awkward that flag names correspond directly to enumnames, because we probably don’t want a enum definition named “g” in ourprogram. Because of this, we can alternatively write this example like this:

enum OptLevel {
  Debug, O1, O2, O3
};
 
cl::opt<OptLevel> OptimizationLevel(cl::desc("Choose optimization level:"),
  cl::values(
   clEnumValN(Debug, "g", "No optimizations, enable debugging"),
    clEnumVal(O1        , "Enable trivial optimizations"),
    clEnumVal(O2        , "Enable default optimizations"),
    clEnumVal(O3        , "Enable expensive optimizations")));
 
...
  if (OptimizationLevel == Debug) outputDebugInfo(...);
...

By using the “clEnumValN” macro instead of “clEnumVal”, we can directlyspecify the name that the flag should get. In general a direct mapping is nice,but sometimes you can’t or don’t want to preserve the mapping, which is when youwould use it.

Named Alternatives

Another useful argument form is a named alternative style. We shall use thisstyle in our compiler to specify different debug levels that can be used.Instead of each debug level being its own switch, we want to support thefollowing options, of which only one can be specified at a time:“—debug-level=none”, “—debug-level=quick”,“—debug-level=detailed”. To do this, we use the exact same format as ouroptimization level flags, but we also specify an option name. For this case,the code looks like this:

enum DebugLev {
  nodebuginfo, quick, detailed
};
 
// Enable Debug Options to be specified on the command line
cl::opt<DebugLev> DebugLevel("debug_level", cl::desc("Set the debugging level:"),
  cl::values(
    clEnumValN(nodebuginfo, "none", "disable debug information"),
     clEnumVal(quick,               "enable quick debug information"),
     clEnumVal(detailed,            "enable detailed debug information")));

This definition defines an enumerated command line variable of type “enumDebugLev”, which works exactly the same way as before. The difference here isjust the interface exposed to the user of your program and the help output bythe “-help” option:

USAGE: compiler [options] <input file>
 
OPTIONS:
  Choose optimization level:
    -g          - No optimizations, enable debugging
    -O1         - Enable trivial optimizations
    -O2         - Enable default optimizations
    -O3         - Enable expensive optimizations
  -debug_level  - Set the debugging level:
    =none       - disable debug information
    =quick      - enable quick debug information
    =detailed   - enable detailed debug information
  -f            - Enable binary output on terminals
  -help         - display available options (-help-hidden for more)
  -o <filename> - Specify output filename
  -quiet        - Don't print informational messages

Again, the only structural difference between the debug level declaration andthe optimization level declaration is that the debug level declaration includesan option name ("debug_level"), which automatically changes how the libraryprocesses the argument. The CommandLine library supports both forms so that youcan choose the form most appropriate for your application.

Parsing a list of options

Now that we have the standard run-of-the-mill argument types out of the way,lets get a little wild and crazy. Lets say that we want our optimizer to accepta list of optimizations to perform, allowing duplicates. For example, wemight want to run: “compiler -dce -constprop -inline -dce -strip”. In thiscase, the order of the arguments and the number of appearances is veryimportant. This is what the “cl::list” template is for. First, start bydefining an enum of the optimizations that you would like to perform:

enum Opts {
  // 'inline' is a C++ keyword, so name it 'inlining'
  dce, constprop, inlining, strip
};

Then define your “cl::list” variable:

cl::list<Opts> OptimizationList(cl::desc("Available Optimizations:"),
  cl::values(
    clEnumVal(dce               , "Dead Code Elimination"),
    clEnumVal(constprop         , "Constant Propagation"),
   clEnumValN(inlining, "inline", "Procedure Integration"),
    clEnumVal(strip             , "Strip Symbols")));

This defines a variable that is conceptually of the type“std::vector<enum Opts>”. Thus, you can access it with standard vectormethods:

for (unsigned i = 0; i != OptimizationList.size(); ++i)
  switch (OptimizationList[i])
     ...

… to iterate through the list of options specified.

Note that the “cl::list” template is completely general and may be used withany data types or other arguments that you can use with the “cl::opt”template. One especially useful way to use a list is to capture all of thepositional arguments together if there may be more than one specified. In thecase of a linker, for example, the linker takes several ‘.o’ files, andneeds to capture them into a list. This is naturally specified as:

...
cl::list<std::string> InputFilenames(cl::Positional, cl::desc("<Input files>"), cl::OneOrMore);
...

This variable works just like a “vector<string>” object. As such, accessingthe list is simple, just like above. In this example, we used thecl::OneOrMore modifier to inform the CommandLine library that it is an errorif the user does not specify any .o files on our command line. Again, thisjust reduces the amount of checking we have to do.

Collecting options as a set of flags

Instead of collecting sets of options in a list, it is also possible to gatherinformation for enum values in a bit vector. The representation used by thecl::bits class is an unsigned integer. An enum value is represented by a0/1 in the enum’s ordinal value bit position. 1 indicating that the enum wasspecified, 0 otherwise. As each specified value is parsed, the resulting enum’sbit is set in the option’s bit vector:

bits |= 1 << (unsigned)enum;

Options that are specified multiple times are redundant. Any instances afterthe first are discarded.

Reworking the above list example, we could replace cl::list with cl::bits:

cl::bits<Opts> OptimizationBits(cl::desc("Available Optimizations:"),
  cl::values(
    clEnumVal(dce               , "Dead Code Elimination"),
    clEnumVal(constprop         , "Constant Propagation"),
   clEnumValN(inlining, "inline", "Procedure Integration"),
    clEnumVal(strip             , "Strip Symbols")));

To test to see if constprop was specified, we can use the cl:bits::isSetfunction:

if (OptimizationBits.isSet(constprop)) {
  ...
}

It’s also possible to get the raw bit vector using the cl::bits::getBitsfunction:

unsigned bits = OptimizationBits.getBits();

Finally, if external storage is used, then the location specified must be oftype unsigned. In all other ways a cl::bits option is equivalent to acl::list option.

Adding freeform text to help output

As our program grows and becomes more mature, we may decide to put summaryinformation about what it does into the help output. The help output is styledto look similar to a Unix man page, providing concise information about aprogram. Unix man pages, however often have a description about what theprogram does. To add this to your CommandLine program, simply pass a thirdargument to the cl::ParseCommandLineOptions call in main. This additionalargument is then printed as the overview information for your program, allowingyou to include any additional information that you want. For example:

int main(int argc, char **argv) {
  cl::ParseCommandLineOptions(argc, argv, " CommandLine compiler example\n\n"
                              "  This program blah blah blah...\n");
  ...
}

would yield the help output:

**OVERVIEW: CommandLine compiler example
 
  This program blah blah blah...**
 
USAGE: compiler [options] <input file>
 
OPTIONS:
  ...
  -help             - display available options (-help-hidden for more)
  -o <filename>     - Specify output filename

Grouping options into categories

If our program has a large number of options it may become difficult for usersof our tool to navigate the output of -help. To alleviate this problem wecan put our options into categories. This can be done by declaring optioncategories (cl::OptionCategory objects) and then placing our options intothese categories using the cl::cat option attribute. For example:

cl::OptionCategory StageSelectionCat("Stage Selection Options",
                                     "These control which stages are run.");
 
cl::opt<bool> Preprocessor("E",cl::desc("Run preprocessor stage."),
                           cl::cat(StageSelectionCat));
 
cl::opt<bool> NoLink("c",cl::desc("Run all stages except linking."),
                     cl::cat(StageSelectionCat));

The output of -help will become categorized if an option category isdeclared. The output looks something like

OVERVIEW: This is a small program to demo the LLVM CommandLine API
USAGE: Sample [options]
 
OPTIONS:
 
  General options:
 
    -help              - Display available options (-help-hidden for more)
    -help-list         - Display list of available options (-help-list-hidden for more)
 
 
  Stage Selection Options:
  These control which stages are run.
 
    -E                 - Run preprocessor stage.
    -c                 - Run all stages except linking.

In addition to the behaviour of -help changing when an option category isdeclared, the command line option -help-list becomes visible which willprint the command line options as uncategorized list.

Note that Options that are not explicitly categorized will be placed in thecl::GeneralCategory category.

Reference Guide

Now that you know the basics of how to use the CommandLine library, this sectionwill give you the detailed information you need to tune how command line optionswork, as well as information on more “advanced” command line option processingcapabilities.

Positional Arguments

Positional arguments are those arguments that are not named, and are notspecified with a hyphen. Positional arguments should be used when an option isspecified by its position alone. For example, the standard Unix grep tooltakes a regular expression argument, and an optional filename to search through(which defaults to standard input if a filename is not specified). Using theCommandLine library, this would be specified as:

cl::opt<string> Regex   (cl::Positional, cl::desc("<regular expression>"), cl::Required);
cl::opt<string> Filename(cl::Positional, cl::desc("<input file>"), cl::init("-"));

Given these two option declarations, the -help output for our grepreplacement would look like this:

USAGE: spiffygrep [options] <regular expression> <input file>
 
OPTIONS:
  -help - display available options (-help-hidden for more)

… and the resultant program could be used just like the standard greptool.

Positional arguments are sorted by their order of construction. This means thatcommand line options will be ordered according to how they are listed in a .cppfile, but will not have an ordering defined if the positional arguments aredefined in multiple .cpp files. The fix for this problem is simply to defineall of your positional arguments in one .cpp file.

Specifying positional options with hyphens

Sometimes you may want to specify a value to your positional argument thatstarts with a hyphen (for example, searching for ‘-foo’ in a file). Atfirst, you will have trouble doing this, because it will try to find an argumentnamed ‘-foo’, and will fail (and single quotes will not save you). Notethat the system grep has the same problem:

$ spiffygrep '-foo' test.txt
Unknown command line argument '-foo'.  Try: spiffygrep -help'
 
$ grep '-foo' test.txt
grep: illegal option -- f
grep: illegal option -- o
grep: illegal option -- o
Usage: grep -hblcnsviw pattern file . . .

The solution for this problem is the same for both your tool and the systemversion: use the ‘—‘ marker. When the user specifies ‘—‘ on thecommand line, it is telling the program that all options after the ‘—‘should be treated as positional arguments, not options. Thus, we can use itlike this:

$ spiffygrep -- -foo test.txt
  ...output...

Determining absolute position with getPosition()

Sometimes an option can affect or modify the meaning of another option. Forexample, consider gcc’s -x LANG option. This tells gcc to ignore thesuffix of subsequent positional arguments and force the file to be interpretedas if it contained source code in language LANG. In order to handle thisproperly, you need to know the absolute position of each argument, especiallythose in lists, so their interaction(s) can be applied correctly. This is alsouseful for options like -llibname which is actually a positional argumentthat starts with a dash.

So, generally, the problem is that you have two cl::list variables thatinteract in some way. To ensure the correct interaction, you can use thecl::list::getPosition(optnum) method. This method returns the absoluteposition (as found on the command line) of the optnum item in thecl::list.

The idiom for usage is like this:

static cl::list<std::string> Files(cl::Positional, cl::OneOrMore);
static cl::list<std::string> Libraries("l", cl::ZeroOrMore);
 
int main(int argc, char**argv) {
  // ...
  std::vector<std::string>::iterator fileIt = Files.begin();
  std::vector<std::string>::iterator libIt  = Libraries.begin();
  unsigned libPos = 0, filePos = 0;
  while ( 1 ) {
    if ( libIt != Libraries.end() )
      libPos = Libraries.getPosition( libIt - Libraries.begin() );
    else
      libPos = 0;
    if ( fileIt != Files.end() )
      filePos = Files.getPosition( fileIt - Files.begin() );
    else
      filePos = 0;
 
    if ( filePos != 0 && (libPos == 0 || filePos < libPos) ) {
      // Source File Is next
      ++fileIt;
    }
    else if ( libPos != 0 && (filePos == 0 || libPos < filePos) ) {
      // Library is next
      ++libIt;
    }
    else
      break; // we're done with the list
  }
}

Note that, for compatibility reasons, the cl::opt also supports anunsigned getPosition() option that will provide the absolute position ofthat option. You can apply the same approach as above with a cl::opt and acl::list option as you can with two lists.

The cl::ConsumeAfter modifier

The cl::ConsumeAfter formatting option is used to construct programs thatuse “interpreter style” option processing. With this style of optionprocessing, all arguments specified after the last positional argument aretreated as special interpreter arguments that are not interpreted by the commandline argument.

As a concrete example, lets say we are developing a replacement for the standardUnix Bourne shell (/bin/sh). To run /bin/sh, first you specify optionsto the shell itself (like -x which turns on trace output), then you specifythe name of the script to run, then you specify arguments to the script. Thesearguments to the script are parsed by the Bourne shell command line optionprocessor, but are not interpreted as options to the shell itself. Using theCommandLine library, we would specify this as:

cl::opt<string> Script(cl::Positional, cl::desc("<input script>"), cl::init("-"));
cl::list<string>  Argv(cl::ConsumeAfter, cl::desc("<program arguments>..."));
cl::opt<bool>    Trace("x", cl::desc("Enable trace output"));

which automatically provides the help output:

USAGE: spiffysh [options] <input script> <program arguments>...
 
OPTIONS:
  -help - display available options (-help-hidden for more)
  -x    - Enable trace output

At runtime, if we run our new shell replacement as `spiffysh -x test.sh -a -x-y bar’, the Trace variable will be set to true, the Script variablewill be set to “test.sh”, and the Argv list will contain ["-a", "-x","-y", "bar"], because they were specified after the last positional argument(which is the script name).

There are several limitations to when cl::ConsumeAfter options can bespecified. For example, only one cl::ConsumeAfter can be specified perprogram, there must be at least one positional argument specified, there mustnot be any cl::list positional arguments, and the cl::ConsumeAfter optionshould be a cl::list option.

Internal vs External Storage

By default, all command line options automatically hold the value that theyparse from the command line. This is very convenient in the common case,especially when combined with the ability to define command line options in thefiles that use them. This is called the internal storage model.

Sometimes, however, it is nice to separate the command line option processingcode from the storage of the value parsed. For example, lets say that we have a‘-debug’ option that we would like to use to enable debug information acrossthe entire body of our program. In this case, the boolean value controlling thedebug code should be globally accessible (in a header file, for example) yet thecommand line option processing code should not be exposed to all of theseclients (requiring lots of .cpp files to #include CommandLine.h).

To do this, set up your .h file with your option, like this for example:

// DebugFlag.h - Get access to the '-debug' command line option
//
 
// DebugFlag - This boolean is set to true if the '-debug' command line option
// is specified.  This should probably not be referenced directly, instead, use
// the DEBUG macro below.
//
extern bool DebugFlag;
 
// DEBUG macro - This macro should be used by code to emit debug information.
// In the '-debug' option is specified on the command line, and if this is a
// debug build, then the code specified as the option to the macro will be
// executed.  Otherwise it will not be.
#ifdef NDEBUG
#define LLVM_DEBUG(X)
#else
#define LLVM_DEBUG(X) do { if (DebugFlag) { X; } } while (0)
#endif

This allows clients to blissfully use the LLVM_DEBUG() macro, or theDebugFlag explicitly if they want to. Now we just need to be able to setthe DebugFlag boolean when the option is set. To do this, we pass anadditional argument to our command line argument processor, and we specify whereto fill in with the cl::location attribute:

bool DebugFlag;                  // the actual value
static cl::opt<bool, true>       // The parser
Debug("debug", cl::desc("Enable debug output"), cl::Hidden, cl::location(DebugFlag));

In the above example, we specify “true” as the second argument to thecl::opt template, indicating that the template should not maintain a copy ofthe value itself. In addition to this, we specify the cl::locationattribute, so that DebugFlag is automatically set.

Option Attributes

This section describes the basic attributes that you can specify on options.

• The option name attribute (which is required for all options, exceptpositional options) specifies what the option name is. This option isspecified in simple double quotes:

cl::opt<bool> Quiet("quiet");

• The cl::desc attribute specifies a description for the option to beshown in the -help output for the program. This attribute supportsmulti-line descriptions with lines separated by ‘n’.

• The cl::value_desc attribute specifies a string that can be used tofine tune the -help output for a command line option. Look here for anexample.

• The cl::init attribute specifies an initial value for a scalaroption. If this attribute is not specified then the command line option valuedefaults to the value created by the default constructor for thetype.

Warning

If you specify both cl::init and cl::location for an option, youmust specify cl::location first, so that when the command-line parsersees cl::init, it knows where to put the initial value. (You will get anerror at runtime if you don’t put them in the right order.)

• The cl::location attribute where to store the value for a parsed commandline option if using external storage. See the section on Internal vsExternal Storage for more information.

• The cl::aliasopt attribute specifies which option a cl::alias option isan alias for.

• The cl::values attribute specifies the string-to-value mapping to be usedby the generic parser. It takes a list of (option, value, description)triplets that specify the option name, the value mapped to, and thedescription shown in the -help for the tool. Because the generic parseris used most frequently with enum values, two macros are often useful:

  • The clEnumVal macro is used as a nice simple way to specify a tripletfor an enum. This macro automatically makes the option name be the same asthe enum name. The first option to the macro is the enum, the second isthe description for the command line option.
  • The clEnumValN macro is used to specify macro options where the optionname doesn’t equal the enum name. For this macro, the first argument isthe enum value, the second is the flag name, and the second is thedescription.You will get a compile time error if you try to use cl::values with a parserthat does not support it.

• The cl::multi_val attribute specifies that this option takes has multiplevalues (example: -sectalign segname sectname sectvalue). This attributetakes one unsigned argument - the number of values for the option. Thisattribute is valid only on cl::list options (and will fail with compileerror if you try to use it with other option types). It is allowed to use allof the usual modifiers on multi-valued options (besidescl::ValueDisallowed, obviously).

• The cl::cat attribute specifies the option category that the optionbelongs to. The category should be a cl::OptionCategory object.

• The cl::callback attribute specifies a callback function that iscalled when an option is seen, and can be used to set other options,as in option B implies option A. If the option is a cl::list,and cl::CommaSeparated is also specified, the callback will fireonce for each value. This could be used to validate combinations orselectively set other options.

cl::opt<bool> OptA("a", cl::desc("option a"));
cl::opt<bool> OptB(
    "b", cl::desc("option b -- This option turns on option a"),
    cl::callback([&](const bool &) { OptA = true; }));
cl::list<std::string, cl::list<std::string>> List(
  "list",
  cl::desc("option list -- This option turns on options a when "
           "'foo' is included in list"),
  cl::CommaSeparated,
  cl::callback([&](const std::string &Str) {
    if (Str == "foo")
      OptA = true;
  }));

Option Modifiers

Option modifiers are the flags and expressions that you pass into theconstructors for cl::opt and cl::list. These modifiers give you theability to tweak how options are parsed and how -help output is generated tofit your application well.

These options fall into five main categories:

• Hiding an option from -help output
• Controlling the number of occurrences required and allowed
• Controlling whether or not a value must be specified
• Controlling other formatting options
• Miscellaneous option modifiersIt is not possible to specify two options from the same category (you’ll get aruntime error) to a single option, except for options in the miscellaneouscategory. The CommandLine library specifies defaults for all of these settingsthat are the most useful in practice and the most common, which mean that youusually shouldn’t have to worry about these.

Hiding an option from -help output

The cl::NotHidden, cl::Hidden, and cl::ReallyHidden modifiers areused to control whether or not an option appears in the -help and-help-hidden output for the compiled program:

• The cl::NotHidden modifier (which is the default for cl::opt andcl::list options) indicates the option is to appear in both helplistings.

• The cl::Hidden modifier (which is the default for cl::alias options)indicates that the option should not appear in the -help output, butshould appear in the -help-hidden output.

• The cl::ReallyHidden modifier indicates that the option should not appearin any help output.

Controlling the number of occurrences required and allowed

This group of options is used to control how many time an option is allowed (orrequired) to be specified on the command line of your program. Specifying avalue for this setting allows the CommandLine library to do error checking foryou.

The allowed values for this option group are:

• The cl::Optional modifier (which is the default for the cl::opt andcl::alias classes) indicates that your program will allow either zero orone occurrence of the option to be specified.

• The cl::ZeroOrMore modifier (which is the default for the cl::listclass) indicates that your program will allow the option to be specified zeroor more times.

• The cl::Required modifier indicates that the specified option must bespecified exactly one time.

• The cl::OneOrMore modifier indicates that the option must be specified atleast one time.
• The cl::ConsumeAfter modifier is described in the Positional argumentssection.

If an option is not specified, then the value of the option is equal to thevalue specified by the cl::init attribute. If the cl::init attribute isnot specified, the option value is initialized with the default constructor forthe data type.

If an option is specified multiple times for an option of the cl::opt class,only the last value will be retained.

Controlling whether or not a value must be specified

This group of options is used to control whether or not the option allows avalue to be present. In the case of the CommandLine library, a value is eitherspecified with an equal sign (e.g. ‘-index-depth=17’) or as a trailingstring (e.g. ‘-o a.out’).

The allowed values for this option group are:

• The cl::ValueOptional modifier (which is the default for bool typedoptions) specifies that it is acceptable to have a value, or not. A booleanargument can be enabled just by appearing on the command line, or it can havean explicit ‘-foo=true’. If an option is specified with this mode, it isillegal for the value to be provided without the equal sign. Therefore‘-foo true’ is illegal. To get this behavior, you must usethe cl::ValueRequired modifier.

• The cl::ValueRequired modifier (which is the default for all other typesexcept for unnamed alternatives using the generic parser) specifies that avalue must be provided. This mode informs the command line library that if anoption is not provides with an equal sign, that the next argument providedmust be the value. This allows things like ‘-o a.out’ to work.

• The cl::ValueDisallowed modifier (which is the default for unnamedalternatives using the generic parser) indicates that it is a runtime errorfor the user to specify a value. This can be provided to disallow users fromproviding options to boolean options (like ‘-foo=true’).

In general, the default values for this option group work just like you wouldwant them to. As mentioned above, you can specify the cl::ValueDisallowedmodifier to a boolean argument to restrict your command line parser. Theseoptions are mostly useful when extending the library.

Controlling other formatting options

The formatting option group is used to specify that the command line option hasspecial abilities and is otherwise different from other command line arguments.As usual, you can only specify one of these arguments at most.

• The cl::NormalFormatting modifier (which is the default all options)specifies that this option is “normal”.

• The cl::Positional modifier specifies that this is a positional argumentthat does not have a command line option associated with it. See thePositional Arguments section for more information.
• The cl::ConsumeAfter modifier specifies that this option is used tocapture “interpreter style” arguments. See this section for moreinformation.

• The cl::Prefix modifier specifies that this option prefixes its value.With ‘Prefix’ options, the equal sign does not separate the value from theoption name specified. Instead, the value is everything after the prefix,including any equal sign if present. This is useful for processing oddarguments like -lmalloc and -L/usr/lib in a linker tool or-DNAME=value in a compiler tool. Here, the ‘l’, ‘D’ and ‘L’options are normal string (or list) options, that have the cl::Prefixmodifier added to allow the CommandLine library to recognize them. Note thatcl::Prefix options must not have the cl::ValueDisallowed modifierspecified.

Controlling options grouping

The cl::Grouping modifier can be combined with any formatting types exceptfor cl::Positional. It is used to implement Unix-style tools (like ls)that have lots of single letter arguments, but only require a single dash.For example, the ‘ls -labF’ command actually enables four different options,all of which are single letters.

Note that cl::Grouping options can have values only if they are usedseparately or at the end of the groups. For cl::ValueRequired, it isa runtime error if such an option is used elsewhere in the group.

The CommandLine library does not restrict how you use the cl::Prefix orcl::Grouping modifiers, but it is possible to specify ambiguous argumentsettings. Thus, it is possible to have multiple letter options that are prefixor grouping options, and they will still work as designed.

To do this, the CommandLine library uses a greedy algorithm to parse the inputoption into (potentially multiple) prefix and grouping options. The strategybasically looks like this:

parse(string OrigInput) {
 
1. string Input = OrigInput;
2. if (isOption(Input)) return getOption(Input).parse();  // Normal option
3. while (!Input.empty() && !isOption(Input)) Input.pop_back();  // Remove the last letter
4. while (!Input.empty()) {
     string MaybeValue = OrigInput.substr(Input.length())
     if (getOption(Input).isPrefix())
       return getOption(Input).parse(MaybeValue)
     if (!MaybeValue.empty() && MaybeValue[0] == '=')
       return getOption(Input).parse(MaybeValue.substr(1))
     if (!getOption(Input).isGrouping())
       return error()
     getOption(Input).parse()
     Input = OrigInput = MaybeValue
     while (!Input.empty() && !isOption(Input)) Input.pop_back();
     if (!Input.empty() && !getOption(Input).isGrouping())
       return error()
   }
5. if (!OrigInput.empty()) error();
 
}

Miscellaneous option modifiers

The miscellaneous option modifiers are the only flags where you can specify morethan one flag from the set: they are not mutually exclusive. These flagsspecify boolean properties that modify the option.

• The cl::CommaSeparated modifier indicates that any commas specified for anoption’s value should be used to split the value up into multiple values forthe option. For example, these two options are equivalent whencl::CommaSeparated is specified: “-foo=a -foo=b -foo=c” and“-foo=a,b,c”. This option only makes sense to be used in a case where theoption is allowed to accept one or more values (i.e. it is a cl::listoption).

• The cl::DefaultOption modifier is used to specify that the option is adefault that can be overridden by application specific parsers. For example,the -help alias, -h, is registered this way, so it can be overriddenby applications that need to use the -h option for another purpose,either as a regular option or an alias for another option.

• The cl::PositionalEatsArgs modifier (which only applies to positionalarguments, and only makes sense for lists) indicates that positional argumentshould consume any strings after it (including strings that start with a “-“)up until another recognized positional argument. For example, if you have two“eating” positional arguments, “pos1” and “pos2”, the string “-pos1-foo -bar baz -pos2 -bork” would cause the “-foo -bar -baz” strings tobe applied to the “-pos1” option and the “-bork” string to be appliedto the “-pos2” option.

• The cl::Sink modifier is used to handle unknown options. If there is atleast one option with cl::Sink modifier specified, the parser passesunrecognized option strings to it as values instead of signaling an error. Aswith cl::CommaSeparated, this modifier only makes sense with a cl::listoption.

Response files

Some systems, such as certain variants of Microsoft Windows and some olderUnices have a relatively low limit on command-line length. It is thereforecustomary to use the so-called ‘response files’ to circumvent thisrestriction. These files are mentioned on the command-line (using the “@file”)syntax. The program reads these files and inserts the contents into argv,thereby working around the command-line length limits.

Top-Level Classes and Functions

Despite all of the built-in flexibility, the CommandLine option library reallyonly consists of one function cl::ParseCommandLineOptions and three mainclasses: cl::opt, cl::list, and cl::alias. This section describesthese three classes in detail.

The cl::getRegisteredOptions function

The cl::getRegisteredOptions function is designed to give a programmeraccess to declared non-positional command line options so that how they appearin -help can be modified prior to calling cl::ParseCommandLineOptions.Note this method should not be called during any static initialisation becauseit cannot be guaranteed that all options will have been initialised. Hence itshould be called from main.

This function can be used to gain access to options declared in libraries thatthe tool writer may not have direct access to.

The function retrieves a StringMap that maps the optionstring (e.g. -help) to an Option*.

Here is an example of how the function could be used:

using namespace llvm;
int main(int argc, char **argv) {
  cl::OptionCategory AnotherCategory("Some options");
 
  StringMap<cl::Option*> &Map = cl::getRegisteredOptions();
 
  //Unhide useful option and put it in a different category
  assert(Map.count("print-all-options") > 0);
  Map["print-all-options"]->setHiddenFlag(cl::NotHidden);
  Map["print-all-options"]->setCategory(AnotherCategory);
 
  //Hide an option we don't want to see
  assert(Map.count("enable-no-infs-fp-math") > 0);
  Map["enable-no-infs-fp-math"]->setHiddenFlag(cl::Hidden);
 
  //Change --version to --show-version
  assert(Map.count("version") > 0);
  Map["version"]->setArgStr("show-version");
 
  //Change --help description
  assert(Map.count("help") > 0);
  Map["help"]->setDescription("Shows help");
 
  cl::ParseCommandLineOptions(argc, argv, "This is a small program to demo the LLVM CommandLine API");
  ...
}

The cl::ParseCommandLineOptions function

The cl::ParseCommandLineOptions function is designed to be called directlyfrom main, and is used to fill in the values of all of the command lineoption variables once argc and argv are available.

The cl::ParseCommandLineOptions function requires two parameters (argcand argv), but may also take an optional third parameter which holdsadditional extra text to emit when the -help option is invoked.

The cl::ParseEnvironmentOptions function

The cl::ParseEnvironmentOptions function has mostly the same effects ascl::ParseCommandLineOptions, except that it is designed to take values foroptions from an environment variable, for those cases in which reading thecommand line is not convenient or desired. It fills in the values of all thecommand line option variables just like cl::ParseCommandLineOptions does.

It takes four parameters: the name of the program (since argv may not beavailable, it can’t just look in argv[0]), the name of the environmentvariable to examine, and the optional additional extra text to emit when the-help option is invoked.

cl::ParseEnvironmentOptions will break the environment variable’s value upinto words and then process them using cl::ParseCommandLineOptions.Note: Currently cl::ParseEnvironmentOptions does not support quoting, soan environment variable containing -option "foo bar" will be parsed as threewords, -option, "foo, and bar", which is different from what youwould get from the shell with the same input.

The cl::SetVersionPrinter function

The cl::SetVersionPrinter function is designed to be called directly frommain and before cl::ParseCommandLineOptions. Its use is optional. Itsimply arranges for a function to be called in response to the —versionoption instead of having the CommandLine library print out the usual versionstring for LLVM. This is useful for programs that are not part of LLVM but wishto use the CommandLine facilities. Such programs should just define a smallfunction that takes no arguments and returns void and that prints outwhatever version information is appropriate for the program. Pass the address ofthat function to cl::SetVersionPrinter to arrange for it to be called whenthe —version option is given by the user.

The cl::opt class

The cl::opt class is the class used to represent scalar command lineoptions, and is the one used most of the time. It is a templated class whichcan take up to three arguments (all except for the first have default valuesthough):

namespace cl {
  template <class DataType, bool ExternalStorage = false,
            class ParserClass = parser<DataType> >
  class opt;
}

The first template argument specifies what underlying data type the command lineargument is, and is used to select a default parser implementation. The secondtemplate argument is used to specify whether the option should contain thestorage for the option (the default) or whether external storage should be usedto contain the value parsed for the option (see Internal vs External Storagefor more information).

The third template argument specifies which parser to use. The default valueselects an instantiation of the parser class based on the underlying datatype of the option. In general, this default works well for most applications,so this option is only used when using a custom parser.

The cl::list class

The cl::list class is the class used to represent a list of command lineoptions. It too is a templated class which can take up to three arguments:

namespace cl {
  template <class DataType, class Storage = bool,
            class ParserClass = parser<DataType> >
  class list;
}

This class works the exact same as the cl::opt class, except that the secondargument is the type of the external storage, not a boolean value. For thisclass, the marker type ‘bool’ is used to indicate that internal storageshould be used.

The cl::bits class

The cl::bits class is the class used to represent a list of command lineoptions in the form of a bit vector. It is also a templated class which cantake up to three arguments:

namespace cl {
  template <class DataType, class Storage = bool,
            class ParserClass = parser<DataType> >
  class bits;
}

This class works the exact same as the cl::list class, except that the secondargument must be of type unsigned if external storage is used.

The cl::alias class

The cl::alias class is a nontemplated class that is used to form aliases forother arguments.

namespace cl {
  class alias;
}

The cl::aliasopt attribute should be used to specify which option this is analias for. Alias arguments default to being cl::Hidden, and use the aliasedoptions parser to do the conversion from string to data.

The cl::extrahelp class

The cl::extrahelp class is a nontemplated class that allows extra help textto be printed out for the -help option.

namespace cl {
  struct extrahelp;
}

To use the extrahelp, simply construct one with a const char parameter tothe constructor. The text passed to the constructor will be printed at thebottom of the help message, verbatim. Note that multiple cl::extrahelp*can be used, but this practice is discouraged. If your tool needs to printadditional help information, put all that help into a single cl::extrahelpinstance.

For example:

cl::extrahelp("\nADDITIONAL HELP:\n\n  This is the extra help\n");

The cl::OptionCategory class

The cl::OptionCategory class is a simple class for declaringoption categories.

namespace cl {
  class OptionCategory;
}

An option category must have a name and optionally a description which arepassed to the constructor as const char*.

Note that declaring an option category and associating it with an option beforeparsing options (e.g. statically) will change the output of -help fromuncategorized to categorized. If an option category is declared but notassociated with an option then it will be hidden from the output of -helpbut will be shown in the output of -help-hidden.

Builtin parsers

Parsers control how the string value taken from the command line is translatedinto a typed value, suitable for use in a C++ program. By default, theCommandLine library uses an instance of parser<type> if the command lineoption specifies that it uses values of type ‘type’. Because of this,custom option processing is specified with specializations of the ‘parser’class.

The CommandLine library provides the following builtin parser specializations,which are sufficient for most applications. It can, however, also be extended towork with new data types and new ways of interpreting the same data. See theWriting a Custom Parser for more details on this type of library extension.

• The generic parser<t> parser can be used to map strings values to any datatype, through the use of the cl::values property, which specifies themapping information. The most common use of this parser is for parsing enumvalues, which allows you to use the CommandLine library for all of the errorchecking to make sure that only valid enum values are specified (as opposed toaccepting arbitrary strings). Despite this, however, the generic parser classcan be used for any data type.

• The parser specialization is used to convert boolean strings to aboolean value. Currently accepted strings are “true”, “TRUE”,“True”, “1”, “false”, “FALSE”, “False”, and “0”.

• The parser specialization is used for cases where the valueis boolean, but we also need to know whether the option was specified at all.boolOrDefault is an enum with 3 values, BOU_UNSET, BOU_TRUE and BOU_FALSE.This parser accepts the same strings as parser<bool>.

• The parser specialization simply stores the parsed string into thestring value specified. No conversion or modification of the data isperformed.

• The parser specialization uses the C strtol function to parse thestring input. As such, it will accept a decimal number (with an optional ‘+’or ‘-‘ prefix) which must start with a non-zero digit. It accepts octalnumbers, which are identified with a ‘0’ prefix digit, and hexadecimalnumbers with a prefix of ‘0x’ or ‘0X’.

• The parser and parser specializations use the standardC strtod function to convert floating point strings into floating pointvalues. As such, a broad range of string formats is supported, includingexponential notation (ex: 1.7e15) and properly supports locales.

Extension Guide

Although the CommandLine library has a lot of functionality built into italready (as discussed previously), one of its true strengths lie in itsextensibility. This section discusses how the CommandLine library works underthe covers and illustrates how to do some simple, common, extensions.

Writing a custom parser

One of the simplest and most common extensions is the use of a custom parser.As discussed previously, parsers are the portion of the CommandLine librarythat turns string input from the user into a particular parsed data type,validating the input in the process.

There are two ways to use a new parser:

• Specialize the cl::parser template for your custom data type.

This approach has the advantage that users of your custom data type willautomatically use your custom parser whenever they define an option with avalue type of your data type. The disadvantage of this approach is that itdoesn’t work if your fundamental data type is something that is alreadysupported.

• Write an independent class, using it explicitly from options that need it.

This approach works well in situations where you would line to parse anoption using special syntax for a not-very-special data-type. The drawbackof this approach is that users of your parser have to be aware that they areusing your parser instead of the builtin ones.

To guide the discussion, we will discuss a custom parser that accepts filesizes, specified with an optional unit after the numeric size. For example, wewould like to parse “102kb”, “41M”, “1G” into the appropriate integer value. Inthis case, the underlying data type we want to parse into is ‘unsigned’. Wechoose approach #2 above because we don’t want to make this the default for allunsigned options.

To start out, we declare our new FileSizeParser class:

struct FileSizeParser : public cl::parser<unsigned> {
  // parse - Return true on error.
  bool parse(cl::Option &O, StringRef ArgName, const std::string &ArgValue,
             unsigned &Val);
};

Our new class inherits from the cl::parser template class to fill inthe default, boiler plate code for us. We give it the data type that we parseinto, the last argument to the parse method, so that clients of our customparser know what object type to pass in to the parse method. (Here we declarethat we parse into ‘unsigned’ variables.)

For most purposes, the only method that must be implemented in a custom parseris the parse method. The parse method is called whenever the option isinvoked, passing in the option itself, the option name, the string to parse, anda reference to a return value. If the string to parse is not well-formed, theparser should output an error message and return true. Otherwise it shouldreturn false and set ‘Val’ to the parsed value. In our example, weimplement parse as:

bool FileSizeParser::parse(cl::Option &O, StringRef ArgName,
                           const std::string &Arg, unsigned &Val) {
  const char *ArgStart = Arg.c_str();
  char *End;
 
  // Parse integer part, leaving 'End' pointing to the first non-integer char
  Val = (unsigned)strtol(ArgStart, &End, 0);
 
  while (1) {
    switch (*End++) {
    case 0: return false;   // No error
    case 'i':               // Ignore the 'i' in KiB if people use that
    case 'b': case 'B':     // Ignore B suffix
      break;
 
    case 'g': case 'G': Val *= 1024*1024*1024; break;
    case 'm': case 'M': Val *= 1024*1024;      break;
    case 'k': case 'K': Val *= 1024;           break;
 
    default:
      // Print an error message if unrecognized character!
      return O.error("'" + Arg + "' value invalid for file size argument!");
    }
  }
}

This function implements a very simple parser for the kinds of strings we areinterested in. Although it has some holes (it allows “123KKK” for example),it is good enough for this example. Note that we use the option itself to printout the error message (the error method always returns true) in order to geta nice error message (shown below). Now that we have our parser class, we canuse it like this:

static cl::opt<unsigned, false, FileSizeParser>
MFS("max-file-size", cl::desc("Maximum file size to accept"),
    cl::value_desc("size"));

Which adds this to the output of our program:

OPTIONS:
  -help                 - display available options (-help-hidden for more)
  ...
  -max-file-size=<size> - Maximum file size to accept

And we can test that our parse works correctly now (the test program just printsout the max-file-size argument value):

$ ./test
MFS: 0
$ ./test -max-file-size=123MB
MFS: 128974848
$ ./test -max-file-size=3G
MFS: 3221225472
$ ./test -max-file-size=dog
-max-file-size option: 'dog' value invalid for file size argument!

It looks like it works. The error message that we get is nice and helpful, andwe seem to accept reasonable file sizes. This wraps up the “custom parser”tutorial.

Exploiting external storage

Several of the LLVM libraries define static cl::opt instances that willautomatically be included in any program that links with that library. This isa feature. However, sometimes it is necessary to know the value of the commandline option outside of the library. In these cases the library does or shouldprovide an external storage location that is accessible to users of thelibrary. Examples of this include the llvm::DebugFlag exported by thelib/Support/Debug.cpp file and the llvm::TimePassesIsEnabled flagexported by the lib/IR/PassManager.cpp file.

Dynamically adding command line options