Why do we write -D_REENTRANT while compiling C code using threads

Question

We write this statement when we are compiling a C program that has threads implemented in them. I could not understand why we use -D_REENTRANT here. e.g gcc t1.c -lpthread -D_REENTRANT

Answer 1

Actually, the recommended way to compile with threads in GCC is using the -pthread option. It is equivalent to -lpthread -D_REENTRANT so you have actually no problem.

The flags do the following:

• -lpthread instructs the linker to use the appropriate library versions for thread compatibility.

• -D_REENTRANT tells the compiler to use the declarations (functions, types, ...) necessary for thread usage.

Answer 2

Compilers like gcc use -D name to predefine name as a macro with definition 1.

In the program source code and header files, you will see compiler directives that check for _REENTRANT and does something when this macro is true or 1.

If this macro is not passed to the compiler, then the compiler directive _REENTRANT would be false or 0.

Take this example from /usr/include/features.h.

#if defined _REENTRANT || defined _THREAD_SAFE
# define __USE_REENTRANT    1
#endif

You will see that it tells the compiler what to do if _REENTRANT is defined.

Finally, you have to link your code agains pthread library so you can use pthread_*() family like pthread_create(), pthread_join().

When -lpthread is passed to the linker, the code gets linked with libpthread.so.

Answer 3

You don't have to write it. But it is recommended.

Defining _REENTRANT causes the compiler to use thread safe (i.e. re-entrant) versions of several functions in the C library.