Undefined reference to `shm_open' even while compiling with -pthread -lrt

Question

I see a similar question has been asked with no coherent solution. Using the gcc compiler on linux, I get the message:

/tmp/ccPNsJFZ.o: In function `main':
testChTh.c:(.text+0xbfb): undefined reference to `shm_open'
collect2: error: ld returned 1 exit status

after typing gcc -pthread -lrt -o testChTh testChTh.c into the command line. I only use shm_open once in my code, which is:

int shm_fd;

  /* create the shared memory segment */
  shm_fd = shm_open(name, O_RDWR, 0666);

I have the following relevant libraries included:

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

Any insight is much appreciated!

Answer 1

In your case

  gcc -pthread -lrt -o testChTh testChTh.c

is not going to work, as you are providing testChTh.c at last which expects to use librt. You need to write like

 gcc -o testChTh testChTh.c -pthread -lrt

Quoting the online manual (emphasis mine)

-l library

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.