CODEWITHSUNDEEP
clinuxgccgnulibc
pushkarnk
pushkarnk

Reputation: 443

System call stat() converted to stat64() without any cpp option

I am running 32-bit SUSE Linux with kernel level 3.0.76.

I can see a stat() call in my code translate to stat64() in the strace output, without me specifying any CPP options like _LARGEFILE64_SOURCE or _FILE_OFFSET_BITS=64. 

#include<stdio.h>
#include<sys/stat.h>

int main ( int argc, char * argv[] )
{
    char * path = "nofile";
    struct stat b;
    if (stat(path, &b) != 0) {
    }
}

I compiled this file with gcc with no compiler options/flags.

On running the program, relevant strace output is:

    munmap(0xb770a000, 200704)              = 0
    stat64("nofile", 0xbfb17834)            = -1 ENOENT (No such file or directory)
    exit_group(-1)

Can anyone please tell me how the stat() was converted to stat64() ?

Thanks in advance!

Upvotes: 3

Views: 2759

Answers (2)

user184968
user184968

Reputation:

I can see a stat() call in my code translate to stat64() in the strace output

You need to analyze the way syscall stat is wrapped on your system http://sourceware.org/glibc/wiki/SyscallWrappers (one way that you can see this code is to use gdb: execute this command under gdb: catch syscall stat and when you program will stop on this breakpoint get backtrace and analyze the function that called stat).

From http://linux.die.net/man/2/stat64

Over time, increases in the size of the stat structure have led to three successive versions of stat(): sys_stat() (slot __NR_oldstat), sys_newstat() (slot __NR_stat), and sys_stat64() (new in kernel 2.4; slot __NR_stat64). The glibc stat() wrapper function hides these details from applications, invoking the most recent version of the system call provided by the kernel, and repacking the returned information if required for old binaries.

So it is glibc_wrapper that decides based on sizeof(struct stat) that it should call why stat64.

Upvotes: 3

nos
nos

Reputation: 229244

The answer seems to be found in the stat man page

Over time, increases in the size of the stat structure have led to three successive versions of stat(): sys_stat() (slot __NR_oldstat), sys_newstat() (slot __NR_stat), and sys_stat64() (new in kernel 2.4; slot __NR_stat64). The glibc stat() wrapper function hides these details from applications, invoking the most recent version of the system call provided by the kernel, and repacking the returned information if required for old binaries. Similar remarks apply for fstat() and lstat().

Basically, glibc always call stat64.

If you add a printf("%zu\n", sizeof b); , the struct stat sizes are likely different depending on whether you use _FILE_OFFSET_BITS=64 or not, and glibc converts the struct stat between the kernel and your code.

Upvotes: 5

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