2. Pointers and Function Arguments
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 Since C passes arguments to functions by value, there is no direct way for the called function to alter a variable in the calling function. For instance, a sorting routine might exchange two out-of-order arguments with a function called swap. |
Call by Value
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int a = 5, b = 7;
swap(a, b);
void swap(int x, int y) /* WRONG */
{
int temp;
temp = x;
x = y;
y = temp;
}
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| – Because of call by value, swap can't affect the arguments a and b in the routine that called it. The function above swaps copies of a and b. |
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Call by Reference
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int a = 5, b = 7;
swap(&a, &b);
/* interchange *px and *py */
void swap(int *px, int *py)
{
int temp;
temp = *px;
*px = *py;
*py = temp;
}
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| Variables passing out of scope |
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| Pointer invalid after variable passes out of scope. |
| What is wrong with this code? |
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#include <stdio.h>
char *get_message ( )
{
char msg[] = “Aren't pointers fun?”;
return msg;
}
int main ( void )
{
char *string = get_message();
puts(string );
return 0;
}
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| – warning: returning address of local variable or temporary |
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