9.31 __attribute__((const)) function attribute

The const function attribute specifies that a function examines only its arguments, and has no effect except for the return value. That is, the function does not read or modify any global memory.

If a function is known to operate only on its arguments then it can be subject to common sub-expression elimination and loop optimizations.
This is a much stricter class than __attribute__((pure)) because functions are not permitted to read global memory.


This function attribute is a GNU compiler extension that the ARM compiler supports. It has the keyword equivalent __pure.


#include <stdio.h>

// __attribute__((const)) functions do not read or modify any global memory
int my_double(int b) __attribute__((const));
int my_double(int b) {
  return b*2;

int main(void) {
    int i;
    int result;
    for (i = 0; i < 10; i++)
       result = my_double(i);
       printf (" i = %d ; result = %d \n", i, result);
Related concepts
4.17 Functions that return the same result when called with the same arguments
Related reference
9.45 __attribute__((pure)) function attribute
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