Extracting C / C++ function prototypes

I use ctags

# p = function declaration, f = function definition
ctags -x --c-kinds=fp /usr/include/hal/libhal.h

Also works with C++

ctags -x --c++-kinds=pf --language-force=c++ /usr/include/c++/4.4.1/bits/deque.tcc

Note, you may need to add include paths, do this using the -I /path/to/includes.

The tool cproto does what you want and allows to tune the output to your requirements.

Note: This tool also only works for C files.

I use ctags and jq

ctags --output-format=json --totals=no --extras=-F --fields=nP file1.c |
jq -sr 'sort_by(.line) | .[].pattern | ltrimstr("/^") | rtrimstr("$/") | . + ";"'

http://cfunctions.sourceforge.net

(This only does C and a limited subset of C++. Disclaimer: this is my program.)

If you have universal-ctags (https://ctags.io), --_xformat option may be useful though you need sed and tr commands to get what you want.

$ cat input.c 
struct object *new_object (struct
                           /* COMMENT */
                           param
                           /* IGNORE ME */
                           *p)
{
    return NULL;
}
int main (void)
{
    return 0;
}
$ ./ctags -o - --kinds-C=f --kinds-C++=f -x --_xformat='%{typeref} %{name} %{signature};' input.c | tr ':' ' ' | sed -e 's/^typename //'
struct object * new_object (struct param * p);
int main (void);
$

This is similar to the answer posted by Steve Ward but this one requires sed, and tr instead of jq.