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Common library functions

Checking character types. Handling strings. Doing maths.

The libraries in GCC contain a repertoire of standard functions and macros. There are many different kinds of function and macro in the libraries. Here are a few of the different kinds available, with the header files you can use to access them:

You may find it useful to read the header files yourself. They are usually found in the directories /usr/include and its subdirectories on GNU/Linux systems. The three header files listed above can be found in /usr/include; there is a second version of ctype.h in /usr/include/linux.1


  1. The version of ctype.h in the /usr/include directory proper is the one that comes with glibc; the one in /usr/include/linux is a special version associated with the Linux kernel. You can specify the one you want with a full pathname inside double quotes (for example, #include "/usr/include/linux/ctype.h"), or you can use the -I option of gcc to force GCC to search a set of directories in a specific order. See Building a library, for more information.)