GNU gcc/ld – wrapping a call to symbol with caller and callee defined in the same object file

by

You have to weaken and globalize the symbol using objcopy.

-W symbolname
--weaken-symbol=symbolname
    Make symbol symbolname weak. This option may be given more than once.
--globalize-symbol=symbolname
    Give symbol symbolname global scoping so that it is visible outside of the file in which it is defined. This option may be given more than once.

This worked for me

bar.c:

#include <stdio.h>
int foo(){
  printf("Wrap-FU\n");
}

foo.c:

#include <stdio.h>

void foo(){
printf("foo\n");
}

int main(){
printf("main\n");
foo();
}

Compile it

$ gcc -c foo.c bar.c

Weaken the foo symbol and make it global, so it’s available for linker again.

$ objcopy foo.o --globalize-symbol=foo --weaken-symbol=foo foo2.o

Now you can link your new obj with the wrap from bar.c

$ gcc -o nowrap foo.o #for reference
$ gcc -o wrapme foo2.o bar.o

Test

$ ./nowrap 
main
foo

And the wrapped one:

$ ./wrapme 
main
Wrap-FU

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