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Where are syscalls located in glibc source

Asked 2011-06-29T03:54:36.967
26

So I was looking through the linux glibc source and I don't see where it actually does anything. The following is from io/chdir.c but it is indicative of many of the source files. What's going on here? Obviously I am missing something. What's the secret, where does it make a system call or actually do something?

stub_warning is some legacy craziness. __set_errno seems to be a simple macro that sets errno. And while I find a million usages of weak_alias I don't see it defined anywhere.

Is there a helpful guide to understanding how glibc works somewhere? 

#include <errno.h>
#include <stddef.h>
#include <unistd.h>

/* Change the current directory to PATH.  */
int
__chdir (path)
     const char *path;
{
  if (path == NULL)
    {
      __set_errno (EINVAL);
      return -1;
    }

  __set_errno (ENOSYS);
  return -1;
}
stub_warning (chdir)

weak_alias (__chdir, chdir)
#include <stub-tag.h>
c linux glibc
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2 Answers

7

That's a generic stub that is used if another definition doesn't exist; weak_alias is a cpp macro which tells the linker that __chdir should be used when chdir is requested, but only if no other definition is found. (See weak symbols for more details.)

chdir is actually a system call; there will be per-OS system call bindings in the gibc source tree, which will override the stub definition with a real one that calls into the kernel. This allows glibc to present a stable interface across systems which may not have all of the system calls that glibc knows about.

answered 2011-06-29T04:01:05.253
6

Note that the actual system calls aren't defined anywhere in the source tree - they're generated at build time from syscalls.list (linked is the one in sysdeps/unix, there are additional ones further down), a series of macros in sysdep.h (linked linux/i386), and a script that actually generates the source files.

answered 2011-06-29T04:11:29.420

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