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 statusI have already added -lrt lib, why does it still not compile?
54 Answers
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.6
Change the compile line from
gcc -lrt test.c -o testto
gcc test.c -o test -lrtIn 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?:)
For those of you using super-auto-magical CMAKE like me, try to add:
target_link_libraries(your_binary_name PRIVATE librt.so)to your CMakeLists.txt
Alternatively, replace PRIVATE for PUBLIC as needed..
If you have well established paths to libraries (e.g. all needed libs in /usr/lib), you may be fine with just stating in CMakeLists.txt:
set(CMAKE_CXX_FLAGS "-lrt")
