12.27

FILE *fpin, *fpout;
fpin = fdopen(sockfd, "r");
fpout = fdopen(sockfd, "w");
// read and write
fclose(fpin);
fclose(fpout);

ref: 10.11

fdopen open 2 stream on same sockfd, fdclose will close sockfd under stream.
if you call fclose 2 stream on the same sockfd, the second fclose will fail.

image one thread execute code and open 2 stream on fd N. after execution of line
fclose(fpin);, program create another thread and execute the same code.

but

after fclose(fpin); in thread 1, fd N is reusable again. assume thread 2 use
the fd N again: thread 1 execute line fclose(fpout); close the fd that thread
2 is using. it’ll cause something unpredicted.