Are runtime libraries inherently dynamic libraries?

Question

I am cross compiling for a system with an OpenMP parallelized program, but when I run on the target, I get the error:

can't load library 'libgomp.so.1'

After looking around, I see that it's an OpenMP runtime library. Is there any was to statically link the library it on the compiler host machine, or does it need to be present on the target machine? If it can be statically linked, then what makes a runtime library different from a dynamic library? Could one statically or dynamically link any library, provided the environment was correct?

Answer 1

The term "runtime library" is usually used for the standard library and environment needed to run your program. In the case of a C program, it's the C standard library, maybe some other libraries specific for your compiler, and some object files linked to your program to set up the standard C environment.

A "runtime library" can be a dynamic library, or even a collection of multiple dynamic libraries. But it can also be one or more static libraries as well.

Answer 2

Dynamic libraries are convenient way of providing runtime libraries, as many programs will potentially want to link against such a library. And this is why dynamic linking and dynamic libraries are out there - the first process that requires particular dynamic library will cause it to be loaded to memory. Later, this instance of dynamic library can be reused by many processes.

Imagine if you'd have tens of process running and each of them statically link against say C runtime library. I assume memory consumption would grow rather significantly compared to the case where each of these processes would link against single DLL.

In my opinion, if library will be used by many different processes (for example DirectX might be such library) it might be more efficient to provide it as dynamic library. Otherwise, static linking is preferable.

Answer 3

You can selectively statically link certain libraries by providing certain linker options. For libgomp it would be something like:

gcc -o executable foo.o bar.o -Wl,-static -lgomp -Wl,-Bdynamic -lpthread -lother -llibs

Any library listed between -Wl,-static and -Wl,-Bdynamic will be linked in statically. -fopenmp should not be present in the linking command as it expands to linker flags that get appended after the user supplied options, and therefore libpthread should be listed explicitly. It also means that even simple OpenMP programs have to be compiled and linked in two separate steps for static linking to work.

Example:

// foo.c
#include <stdio.h>
#include <omp.h>

int main(void)
{
   #pragma omp parallel
   printf("Hello world from thread %d\n", omp_get_thread_num());
   return 0;
}

Traditional compilation:

$ gcc -fopenmp -o foo foo.c
$ ldd foo
     linux-vdso.so.1 =>  (0x00007ffff5661000)
     libgomp.so.1 => /usr/lib64/libgomp.so.1 (0x0000003bcfa00000)
     libpthread.so.0 => /lib64/libpthread.so.0 (0x0000003bc2600000)
     libc.so.6 => /lib64/libc.so.6 (0x0000003bc1e00000)
     librt.so.1 => /lib64/librt.so.1 (0x0000003bc3200000)
     /lib64/ld-linux-x86-64.so.2 (0x0000003bc1a00000)

The program is linked against the DSO version of libgomp.

Fully static linking:

$ gcc -fopenmp -static -o foo foo.c
$ ldd foo
     not a dynamic executable

With -static all libraries are linked in statically into the executable.

Linking only libgomp statically:

$ gcc -fopenmp -c foo.c
$ gcc -o foo foo.o -Wl,-static -lgomp -Wl,-Bdynamic -lpthread
$ ldd foo
     linux-vdso.so.1 =>  (0x00007ffdaaf61000)
     libpthread.so.0 => /lib64/libpthread.so.0 (0x0000003bc2600000)
     libc.so.6 => /lib64/libc.so.6 (0x0000003bc1e00000)
     /lib64/ld-linux-x86-64.so.2 (0x0000003bc1a00000)

It is important to maintain the correct order of statically linked objects in that case. If foo.o is placed after -lgomp, a link error results:

$ gcc -o foo -Wl,-static -lgomp -Wl,-Bdynamic foo.o -lpthread
foo.o: In function `main':
foo.c:(.text+0x14): undefined reference to `GOMP_parallel_start'
foo.c:(.text+0x23): undefined reference to `GOMP_parallel_end'
foo.o: In function `main.omp_fn.0':
foo.c:(.text+0x3b): undefined reference to `omp_get_thread_num'
collect2: ld returned 1 exit status

Any object file resulting from source code that contains OpenMP constructs should be placed before -lgomp.