Use of #pragma in C

Question

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

Answer 1

All answers above make nice explanations for #pragma but I wanted to a add small example

I just want to explain a simple OpenMP example that demonstrate some uses of #pragma to doing its work

OpenMp briefly is an implementation for multi-platform shared-memory parallel programming (then we can say it's machine-specific or operating-system-specific)

let's go to the example

#include 
#include // compile with: /openmp

int main() {
   #pragma omp parallel num_threads(4)
   {
      int i = omp_get_thread_num();
      printf_s("Hello from thread %d\n", i);
   }
}

the output is

Hello from thread 0
Hello from thread 1
Hello from thread 2
Hello from thread 3

Note that the order of output can vary on different machines.

now let me tell you what #pragma did...

it tells the OS to run the some block of code on 4 threads

this is just one of many many applications you can do with the little #pragma

sorry for the outside sample OpenMP