Thread: sizeof in a function

does not seem to matter how I pass it, I only get two elements in a function. BUT, I think the real question here is how to pass an array, then do sizeof on it, to elevate the ( an ) unknown size of the array?

Code:

#include <stdio.h>

void fun_array(int  a[]) 
//void fun_array(int* a) 
{
    int size = 0, j;
    size = sizeof(&a)/sizeof(int);
    //size = sizeof(a)/sizeof(a[0]);
    printf("size %d\n", size);
    for (j = 0; j < size; j++)
        printf("%d\n", a[j]);
     
}
 
void fun_array2(int d[])
{
 for (int s = 0; s < 6; s++)
        printf("d = %d\n", d[s]);
    
}
int main()
{
    int a[10] = {1, 2, 3, 4, 5, 6};
    fun_array(a);
    
    for (int s = 0; s < sizeof(a)/sizeof(a[0]); s++)
        printf("\n\nin Main: %d\n", a[s]);
        
    fun_array2(a);
 
return 0;
}

Code:

userx@slackwhere:~/bin
$ ./arrayPassFunction
size 2
1
2


in Main: 1
in Main: 2
in Main: 3
in Main: 4
in Main: 5
in Main: 6
in Main: 0
in Main: 0
in Main: 0
in Main: 0

d = 1
d = 2
d = 3
d = 4
d = 5
d = 6

When you pass in an array, it is converted to a pointer to the first element, so when you do the sizeof trick inside a called function, it ends up being wrong.
sizeof(int*)/sizeof(int) is different from say sizeof(int[10])/sizeof(int).

The solution is to use a size parameter, and to make sure that you only use the sizeof trick where your array is defined.
fun_array(a, sizeof(a)/sizeof(a[0]));

Originally Posted by whiteflags

When you pass in an array, it is converted to a pointer to the first element, so when you do the sizeof trick inside a called function, it ends up being wrong.
sizeof(int*)/sizeof(int) is different from say sizeof(int[10])/sizeof(int).

The solution is to use a size parameter, and to make sure that you only use the sizeof trick where your array is defined.
fun_array(a, sizeof(a)/sizeof(a[0]));

that makes sense, as I seen something that is how it was set up, but to just put the length in the second pram, but I was trying to get around that. that trick still allows the unknown to be had. because even when I was casting to int it was messing up. thanks