How do I know how many slots are in an array after mallocing it?

Question

#include <stdlib.h>
#include <stdio.h>

#define MAX_NUM 65536
struct Test{
int t;
char args[16][65];

};


int main(int argc, char ** argv) {
//
struct Test *array = malloc(MAX_NUM * sizeof(struct Test*));
printf("%d", sizeof(*array));
for(int i = 0; i < sizeof(*array); i++) {
    printf("%d", i);
    printf(" ");
    array[i].t = 0;

}
array[421].t = 5;
printf("%d", array[421].t);
free(array);
return 0;

}

Hi, how do I determine how many slots of an array I have after mallocing it? When I print sizeof(*array) I get the number 1044, but when I try to run through the loop and set a test value in each slot I only get to 282 before segfaulting. What is the difference between sizeof and malloc? Why does my array only accept values up to 282 when the size of says it is 1044? Why can I edit a slot at 421 but it segfaults up to 282 in my loop? Thank you

Answer 1

I think you mean

struct Test *array = malloc(MAX_NUM * sizeof(struct Test));

Or am I missing something ?

Answer 2

For a statically-allocated array, you can use sizeof(array)/sizeof(*array):

• The total amount of memory that array occupies is sizeof(array).

• The amount of memory that a single entry occupies is sizeof(*array).

• Hence, the number of entries in array is sizeof(array)/sizeof(*array).

For a dynamically-allocated array, however, you will have to use the value of MAX_NUM, since array is a pointer, hence sizeof(array) = size of a pointer, which is 4 or 8 bytes (depending on your system).

Answer 3

The malloc is wrong: you must use sizeof(struct Test) without *. You should also use MAX_NUM as number of iterations.

sizeof(*array) returns the size of the first element of the array so sizeof(struct Test), not the length of the array.

#include <stdlib.h>
#include <stdio.h>

#define MAX_NUM 65536
struct Test{
int t;
char args[16][65];

};


int main(int argc, char ** argv) {
//Changed Key to Test type
struct Test *array = (Struct Test*)malloc(MAX_NUM * sizeof(struct Test));
printf("%d", sizeof(*array));
for(int i = 0; i < MAX_NUM; i++) {
    printf("%d", i);
    printf(" ");
    array[i].t = 0;

}
array[421].t = 5;
printf("%d", array[421].t);
free(array);
return 0;

}