How to find the whole path to a library using the C preprocessor?

Question

I'm looking for a simple bash script which, when given the name of a system header, will return its full path from which it would be read in a #include <header> statement. I already have an analogous thing for looking up the library archive used by linker.

ld -verbose -lz -L/some/other/dir | grep succeeded | sed -e 's/^\s*attempt to open //' -e 's/ succeeded\s*$//'

For example, this will return the path of libz archive ( /lib/x86_64-linux-gnu/libz.so on my system).

For the requested script I know that I could take a list of include directories used by gcc and search them for the file myself, but I'm looking for a more accurate simulation of what's happening inside the preprocessor (unless it's that simple).

Answer 1

You can use the preprocessor to do the work:

user@host:~$ echo "#include <stdio.h>" > testx.c && gcc -M testx.c | grep 'stdio.h'
testx.o: testx.c /usr/include/stdc-predef.h /usr/include/stdio.h \

You can add a bit bash-fu to cut the part you are interested in

Answer 2

Pipe the input to preprocessor and then process the output. Gcc preprocessor output inserts # lines with information and flags that you can parse.

$ f=stdlib.h
$ echo "#include <$f>" | gcc -xc -E - | sed '\~# [0-9]* "\([^"]*/'"$f"'\)" 1 .*~!d; s//\1/'
/usr/include/stdlib.h

It can output multiple files, because gcc has #include_next and can improperly detect in some complicated cases where multiple filenames are included with the same name, like in f=limits.h . So you could also filter exactly second line, knowing that the first line is always going to be stdc-predef.h :

$ f=limits.h; echo "#include <$f>" | gcc -xc -E - | sed '/# [0-9]* "\([^"]*\)" 1 .*/!d;s//\1/' | sed '2!d'
/usr/lib/gcc/x86_64-pc-linux-gnu/10.1.0/include-fixed/limits.h

But really search the include paths yourself, it's not that hard:

$ f=limits.h; echo | gcc -E -Wp,-v - 2>&1 | sed '\~^ /~!d; s/ //' | while IFS= read -r path; do if [[ -e "$path/$f" ]]; then echo "$path/$f"; break; fi; done
/usr/lib/gcc/x86_64-pc-linux-gnu/10.1.0/include-fixed/limits.h