Specify OpenMP to GCC

Question

For OpenMP, when my code is using the functions in its API (for example, omp_get_thread_num()) without using its directives (such as those #pragma omp ...),

why directly specifying libgomp.a to gcc instead of using -fopenmp doesn't work, such as
```
gcc hello.c /usr/lib/gcc/i686-linux-gnu/4.4/libgomp.a  -o hello
```
Update: I just found that linking to libgomp.a does not work, but linking to libgomp.so works. Does it mean OpenMP can not be static linked?
Why -fopenmp only works without specifying the library files
```
gcc hello.c -fopenmp -o hello
```
Update: In other words, when using -fopenmp, why explicit linking to libgomp.so is not required?
Why does this also compile:
```
gcc hello.c -L/usr/lib/gcc/i686-linux-gnu/4.4/ -lgomp -o hello
```
Will this ignore OpenMP directives in the code if there is any?

Thanks and regards!

From GCC documentation: -fopenmp flag also arranges for automatic linking of the OpenMP runtime library. See here: gcc.gnu.org/onlinedocs/libgomp/… — user1436916
– user1436916, Commented May 26, 2013 at 15:17

Chris Frederick · Accepted Answer · 2011-06-15 20:03:40Z

34

In general, keep in mind that the directives and the functions are different things; the former are controlled by -fopenmp and the latter are controlled by linking to the OpenMP library.

(Updated to incorporate comments) Try using the -fopenmp and -static options to statically link OpenMP. Because this implies -lgomp -lrt, the following command won't compile correctly unless you also specify the location of librt.a.
```
gcc hello.c /usr/lib/gcc/i686-linux-gnu/4.4/libgomp.a  -o hello
```
(Updated to incorporate comments) I imagine that the following command is compiling correctly because the OpenMP library is already in your library path and your system's dynamic linker is automatically linking with libgomp.so.
```
gcc hello.c -fopenmp -o hello
```
The following command is probably compiling properly because it is linking to the shared object for OpenMP (libgomp.so). Note that the -static option is not used.
```
gcc hello.c -L/usr/lib/gcc/i686-linux-gnu/4.4/ -lgomp -o hello
```
If you don't specify the -fopenmp option, OpenMP directives should be ignored.

edited Jun 15, 2011 at 20:03

answered Jun 15, 2011 at 5:25

Chris Frederick

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4 Comments

Tim Over a year ago

Thanks! I am considering the code with API and without directives. (1) I just found that linking to libgomp.a does not work, but linking to libgomp.so works. Does it mean OpenMP can not be static linked? (2) when using -fopenmp, why explicit linking to libgomp.so is not required?

ejd Over a year ago

(1) You can statically link OpenMP by using "-fopenmp -static". What you will find is that when this is specified it uses "-lgomp -lrt". (2) If the system supports the dynamic linker then it is used and when you specify -fopenmp it will link with libgomp.so.

Royi Over a year ago

It seems both the compiler and the linker requires the -fopenmp flag.

ChrisZZ Over a year ago

@ejd Hi, I would like to know how can people know "-fopenmp -static" links to openmp and rt libraries. I do man gcc and search -fopenmp -static but not found result, and search -fopenmp not get result of rt library.

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