How can I find the header files of the C programming language in Linux?

locate stdio.h

or

mlocate stdio.h

but locate relies on a database, if you have never updated it

sudo updatedb

you can also enquire gcc to know what are the default directories that are scanned by gcc itself:

gcc -print-search-dirs

gcc -H ... will print the full path of every include file as a side-effect of regular compilation. Use -fsyntax-only in addition to get it not to create any output (it will still tell you if your program has errors). Example (Linux, gcc-4.7):

$ cat > test.c
#include <stdbool.h>
#include <stdio.h>
^D
$ gcc -H -fsyntax-only test.c
. /usr/lib/gcc/x86_64-linux-gnu/4.7/include/stdbool.h
. /usr/include/stdio.h
.. /usr/include/features.h
... /usr/include/x86_64-linux-gnu/bits/predefs.h
... /usr/include/x86_64-linux-gnu/sys/cdefs.h
.... /usr/include/x86_64-linux-gnu/bits/wordsize.h
... /usr/include/x86_64-linux-gnu/gnu/stubs.h
.... /usr/include/x86_64-linux-gnu/bits/wordsize.h
.... /usr/include/x86_64-linux-gnu/gnu/stubs-64.h
.. /usr/lib/gcc/x86_64-linux-gnu/4.7/include/stddef.h
.. /usr/include/x86_64-linux-gnu/bits/types.h
... /usr/include/x86_64-linux-gnu/bits/wordsize.h
... /usr/include/x86_64-linux-gnu/bits/typesizes.h
.. /usr/include/libio.h
... /usr/include/_G_config.h
.... /usr/lib/gcc/x86_64-linux-gnu/4.7/include/stddef.h
.... /usr/include/wchar.h
... /usr/lib/gcc/x86_64-linux-gnu/4.7/include/stdarg.h
.. /usr/include/x86_64-linux-gnu/bits/stdio_lim.h
.. /usr/include/x86_64-linux-gnu/bits/sys_errlist.h

The dots at the beginning of each line count how deeply nested the #include is.

If you use gcc, you can check a specific file with something like:

echo '#include <stdbool.h>' | cpp -H -o /dev/null 2>&1 | head -n1

-H asks the preprocessor to print all included files recursively. head -n1 takes just the first line of output from that, to ignore any files included by the named header (though stdbool.h in particular probably doesn't).

On my computer, for example, the above outputs:

. /usr/lib/gcc/x86_64-linux-gnu/4.6/include/stdbool.h

During the preprocessing all preprocessor directives will be replaced with the actuals. Like macro expansion, code comment removal, including the header file source code etc...

we can check it by using the cpp - C PreProcessor command.

For example in the command line:

cpp Filename.c

displays the preprocessed output.