Moving to different Linux build system, getting error: undefined symbol: stat

Question

This may just be an issue with the build system I am migrating to, but I\'ll include differences in the two systems and how I encountered the problem.

My old build s

Answer 1

The problem you are facing is most likely the result of building your shared library with ld. User-level code on UNIX systems should never use ld directly. You should use the compiler driver (g++ in your case) to perform the link instead.

Example:

// t.c
#include <sys/stat.h>

void fn(const char *p)
{
  struct stat st;
  stat(p, &st);
}

gcc -fPIC -c t.c
ld -shared -o t.so t.o
nm t.so | grep stat
             U stat       ## problem: this library is not linked correctly

Compare to correctly linked library:

gcc -shared -o t.so t.o
nm t.so | grep stat

0000000000000700 t stat
0000000000000700 t __stat
                 U __xstat@@GLIBC_2.2.5

To find where the above local stat symbol came from, you could do this:

gcc  -shared -o t.so t.o -Wl,-y,stat
t.o: reference to stat
/usr/lib/x86_64-linux-gnu/libc_nonshared.a(stat.oS): definition of stat

Finally, the reason U stat disappears with optimization:

gcc -E t.c | grep -A2 ' stat '

extern int stat (const char *__restrict __file,
  struct stat *__restrict __buf) __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__nonnull__ (1, 2)));

gcc -E t.c -O | grep -A2 ' stat '

__attribute__ ((__nothrow__ , __leaf__)) stat (const char *__path, struct stat *__statbuf)
{
   return __xstat (1, __path, __statbuf);

That's right: you get different preprocessed source depending on the optimization level.