undefined reference 'shm_open', already ad

问题:

I just have a system crash and reinstall Ubuntu 11.10, and my code produces this strange error.

I wrote a simple code sample to test where the problem is:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <sys/stat.h>

int main (void) {

    int i;

    i = shm_open ("/tmp/shared", O_CREAT | O_EXCL, S_IRUSR | S_IWUSR);   printf ("shm_open rc = %d\n", i);

    shm_unlink ("/tmp/shared");

    return (0);
}

and the compile command is

gcc -lrt test.c -o test

The error is:

/tmp/ccxVIUiP.o: In function `main':
test.c:(.text+0x21): undefined reference to `shm_open'
test.c:(.text+0x46): undefined reference to `shm_unlink'
collect2: ld returned 1 exit status

I have already added -lrt lib, why does it still not compile?

回答1:

Libraries at the end:

gcc test.c -o test -lrt

From GCC Link Options:

-llibrary
-l library
    Search the library named library when linking. 
    (The second alternative with the library as a separate argument
    is only for POSIX compliance and is not recommended.)

    It makes a difference where in the command you write this option;
    the linker searches and processes libraries and object files in the
    order they are specified.
    Thus, `foo.o -lz bar.o' searches library `z' after file foo.o but
    before bar.o. If bar.o refers to functions in `z', those functions
    may not be loaded.

回答2:

Change the compile line from

gcc -lrt test.c -o test

to

gcc test.c -o test -lrt

回答3:

In Expert C programming Page 108: <Handy Heuristic> Where to Put Library Options:Always put the -l library options at the rightmost end of your compilation command line. But it doesn't tell why, so i guess this is somewhat a rule?:)