如何使用指针函数？

Question

for exp : this program isn't working to me.. can anyone explian to me how to use correctly a way to use in this program pointers to functions the program gives me runtime error

the program should be generic and returns the size of any given array

#include<stdio.h>
#include<string.h>

int ReturnSize_INT(void *a)
{
    a = (int*)a;
    int count  = 0 ;
    int *p = (int*)a;
    while(p != NULL)
    {
        count++;
        p++;
    }
    return count;
}
int ReturnSize_char(void *a)
{
    a = (char*)a;
    int count  = 0 ;
    char *p = (char*)a;
    while(p != NULL)
    {
        count++;
        p++;
    }
    return count;
}
int ReturnSize_float(void *a)
{
    a = (int*)a;
    int count  = 0 ;
    float *p = (float*)a;
    while(p != NULL)
    {
        count++;
        p++;
    }
    return count;
}

int main()
{

    int a [] = {1,2,5,7,8};
    char b[]={'a','b','c','\0'};
    float c [] = {2.75,5.25,7.27,4.25};
    int (*ReturnSize)(void *e);
    ReturnSize = ReturnSize_INT;
    printf("%d",ReturnSize(&a));

    return 0;
}

Answer 1

Pass a to the function - not &a . The name of the array is a pointer to the start of the array.

printf("%d",ReturnSize(a));

Secondly, there are no terminators in the data arrays. You could use a nominal value such as -1 for example. Note that the pointer p is derefenced ( *p ) to get the value p points to:

int a [] = {1,2,5,7,8,-1};  // in main

int ReturnSize_INT(void *a)
{
    a = (int*)a;
    int count  = 0 ;
    int *p = (int*)a;
    while(*p != -1)
    {
        count++;
        p++;
    }
    return count;
}