Use of #pragma in C

Question

What are some uses of #pragma in C, with examples?

Answer 1

I would generally try to avoid the use of #pragmas if possible, since they're extremely compiler-dependent and non-portable. If you want to use them in a portable fashion, you'll have to surround every pragma with a #if/#endif pair. GCC discourages the use of pragmas, and really only supports some of them for compatibility with other compilers; GCC has other ways of doing the same things that other compilers use pragmas for.

For example, here's how you'd ensure that a structure is packed tightly (i.e. no padding between members) in MSVC:

#pragma pack(push, 1)
struct PackedStructure
{
  char a;
  int b;
  short c;
};
#pragma pack(pop)
// sizeof(PackedStructure) == 7

Here's how you'd do the same thing in GCC:

struct PackedStructure __attribute__((__packed__))
{
  char a;
  int b;
  short c;
};
// sizeof(PackedStructure == 7)

The GCC code is more portable, because if you want to compile that with a non-GCC compiler, all you have to do is

#define __attribute__(x)

Whereas if you want to port the MSVC code, you have to surround each pragma with a #if/#endif pair. Not pretty.