22.4 Function Call Semantics

The meaning of a function call is to compute the specified argument expressions, convert their values according to the function’s declaration, then run the function giving it copies of the converted values. (This method of argument passing is known as call-by-value.) When the function finishes, the value it returns becomes the value of the function-call expression.

Call-by-value implies that an assignment to the function argument variable has no direct effect on the caller. For instance,

#include <stdlib.h>  /* Defines EXIT_SUCCESS. */
#include <stdio.h>   /* Declares printf. */

void
subroutine (int x)
{
  x = 5;
}

void
main (void)
{
  int y = 20;
  subroutine (y);
  printf ("y is %d\n", y);
  return EXIT_SUCCESS;
}

prints ‘y is 20’. Calling subroutine initializes x from the value of y, but this does not establish any other relationship between the two variables. Thus, the assignment to x, inside subroutine, changes only that x.

If an argument’s type is specified by the function’s declaration, the function call converts the argument expression to that type if possible. If the conversion is impossible, that is an error.

If the function’s declaration doesn’t specify the type of that argument, then the default argument promotions apply. See Argument Promotions.