GUN C的标准文档:
也就是glibc
https://www.gnu.org/software/libc/
http://man7.org/linux/man-pages/dir_section_3.html
POSIX C 的标准文档:
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/contents.html
其他:
https://en.wikipedia.org/wiki/C_standard_library
https://en.wikipedia.org/wiki/C_POSIX_library
https://en.wikipedia.org/wiki/GNU_C_Library