STDIN_FILENO   0

Descriptor ID of the standard input file.

STDOUT_FILENO   1

Descriptor ID of the standard output file.

STDERR_FILENO   2

Descriptor ID of the standard error file.

SEEK_SET   0

Relocation starts from the file header.

SEEK_CUR   1

Relocation starts from the position of the currently accessed file.

SEEK_END   2

Relocation starts from the end of the file.

NULL   ((void*)0)

NULL.

F_OK   0

Existing file.

R_OK   4

Readable file.

W_OK   2

Writable file.

X_OK   1

Executable file.

pipe (int pipefd[2])

int 

Creates an anonymous pipe.

close (int fd)

int 

Closes a file with a specified file descriptor.

dup (int oldfd)

int 

Copies a specified file descriptor.

dup2 (int oldfd, int newfd)

int 

Copies the descriptor of the target file to a specified descriptor.

lseek (int fd, off_t offset, int whence)

off_t 

Sets the offset of the pointer to the file.

fsync (int fd)

int 

Synchronizes a file associated with a specified file descriptor to the storage device.

read (int fd, void buf, size_t size)

ssize_t 

Reads the file contents and saves them in a specified buffer location.

write (int fd, const void buf, size_t size)

ssize_t 

Writes the specified content to the file.

pread (int fd, void buf, size_t count, off_t offset)

ssize_t 

Reads data whose offset is offset and length is count from fd to the buffer.

pwrite (int fd, const void buf, size_t count, off_t offset)

ssize_t 

Writes data from the buffer to fd whose offset is offset and length is count.

unlink (const char path)

int 

Deletes a specified file.

unlinkat (int fd, const char path, int flag)

int 

Deletes a specified file.

rmdir (const char path)

int 

Deletes a directory.

truncate (const char path, off_t length)

int 

Truncates a file to a specified size based on the file path.

ftruncate (int fd, off_t length)

int 

Truncates a file to a specified length.

access (const char path, int mode)

int 

Checks whether a file has the corresponding permission.

chdir (const char path)

int 

Switches the current working directory to a specified directory.

getcwd (char buf, size_t size)

char  

Obtains the current working directory.

alarm (unsigned int seconds)

unsigned int 

Arranges a signal to be sent to the current process after the number of seconds specified by seconds.

sleep (unsigned seconds)

unsigned 

Sleeps for a period of time.

pause (void)

int 

Waits for signal.

fork (void)

pid_t 

Creates a new process that inherits from the user-mode data of its parent process.

execve (const char path, char const arg[], char const envp[])

int 

Executes a specified user program file in ELF format.

execv (const char path, char const arg[])

int 

Executes a specified user program file in ELF format.

execle (const char path, const char arg,…)

int 

Executes a specified user program file in ELF format.

execl (const char path, const char arg,…)

int 

Executes a specified user program file in ELF format.

execvp (const char path, char const arg[])

int 

Executes a specified user program file in ELF format.

execlp (const char path, const char arg,…)

int 

Executes a specified user program file in ELF format.

_exit (int status)

_Noreturn void 

Exits the process immediately and closes all opened file descriptors in the process.

swab (const void from, void to, ssize_t n)

void 

Swaps bytes.

getpid (void)

pid_t 

Obtains the process ID.

getppid (void)

pid_t 

Obtains the parent process ID.

getpgrp (void)

pid_t 

Obtains the ID of the process group of the calling process.

getpgid (pid_t pid)

pid_t 

Obtains the ID of the process group whose process ID is specified by pid.

setpgid (pid_t pid, pid_t pgid)

int 

Sets the ID of the process group whose process ID is specified by pid.

getopt (int argc, char const argv[], const char optstring)

int 

Parses command-line arguments based on the specified option.

getuid (void)

uid_t 

Obtains the real user ID (UID) of the calling process.

geteuid (void)

uid_t 

Obtains the effective user ID (UID) of the calling process.

getgid (void)

gid_t 

Obtains the real group ID (GID) of the calling process.

getegid (void)

gid_t 

Obtains the effective group ID (GID) of the calling process.

getgroups (int size, gid_t list[])

int 

Obtains a list of supplementary user group IDs specific to the calling process.

setuid (uid_t uid)

int 

Sets the real user ID for the calling process.

seteuid (uid_t euid)

int 

Sets the effective user ID of the calling process.

setgid (gid_t gid)

int 

Sets the user group ID of the calling process.

setegid (gid_t egid)

int 

Sets the effective user group ID of the calling process.

pathconf (const char path, int name)

long 

Obtains the configuration value of a file.

setreuid (uid_t ruid, uid_t euid)

int 

Sets the real and effective user IDs of the calling process.

setregid (gid_t rgid, gid_t egid)

int 

Sets the real and effective user group IDs of the calling process.

setpgrp (void)

pid_t 

Sets the process group ID of the calling process.

usleep (unsigned useconds)

int 

Sleeps for several microseconds.

ualarm (unsigned value, unsigned interval)

unsigned 

Sets a microsecond-level timer.

setgroups (size_t size, const gid_t list)

int 

Sets the supplementary user group list of the calling process.

setresuid (uid_t ruid, uid_t euid, uid_t suid)

int 

Sets the real, effective, and saved user IDs of the calling process.

setresgid (gid_t rgid, gid_t egid, gid_t sgid)

int 

Sets the real, effective, and saved group IDs of the calling process.

getresuid (uid_t ruid, uid_t euid, uid_t suid)

int 

Obtains the real, effective, and saved user IDs of the calling process.

getresgid (gid_t rgid, gid_t egid, gid_t sgid)

int 

Obtains the real, effective, and saved user group IDs of the calling process.

get_current_dir_name (void)

char  

Obtains the name of the current working directory.

sbrk (intptr_t increment)

void  

getpagesize (void)

int 

Obtains the memory page size.

chown (const char pathname, uid_t owner, gid_t group)

int 

Changes the user and group ownership of a file.