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?

4 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. 

Change the compile line from

gcc -lrt test.c -o test 

to

gcc test.c -o test -lrt 

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?:)

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")

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