CODEWITHSUNDEEP
clinuxlinux-kernelglibclibc
William Breathitt Gray
William Breathitt Gray

Reputation: 11996

Where is the definition of the sync(2) Linux function call for x86_64 processors

I'm trying to find the Linux 3.2.21 x86_64 implementation of the sync(2) Unix function declared in the unistd.h header. Looking at the Linux unistd.h yields this prototype:

/* Make all changes done to all files actually appear on disk.  */
extern void sync (void) __THROW;

So I take that to mean that sync is defined outside of the Linux kernel and looking inside glibc 2.7, which gives me me this definition in glibc-2.17/misc/sync.c:

/* Make all changes done to all files actually appear on disk. */
void
sync ()
{
  __set_errno (ENOSYS);
}

So that means sync does nothing other than set the value of errno.

However, when I disassemble /usr/lib/x86_64-linux-gnu/libc.a on system, I find that the sync section makes a system call, passing the value 162 (so it is doing something).

Looking at the Linux source again at arch/x86/include/asm/unistd_64.h, I see:

#define __NR_sync               162
__SYSCALL(__NR_sync, sys_sync)

Now I'm really confused.

If sync(2) is defined outside of Linux, why is there a system call for it? Where is the definition of sync for the x86_64 architecture?

P.S.: I did find the preprocessor definition of __SYSCALL at arch/x86/kernel/syscall_64.c, but this seems to imply that the sync system call just calls an external function declared as void sys_sync(void). Where is the definition for this function?

Upvotes: 3

Views: 2082

Answers (1)

cnicutar
cnicutar

Reputation: 182744

You are looking for fs/sync.c. See:

SYSCALL_DEFINE0(sync)
{
...

The glibc version you posted is likely what's compiled in the very awkward case where the kernel doesn't expose a sync syscall. In other words it's a stub that's virtually never used.

Upvotes: 7

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