Using memset for integer array in C

Question

char str[] = \"beautiful earth\";
memset(str, \'*\', 6);
printf(\"%s\", str);

Output:
******ful earth

Like the above use of memset, can we initial

Answer 1

The third argument of memset is byte size. So you should set total byte size of arr[15]

memset(arr, 1, sizeof(arr));

However probably, you should want to set value 1 to whole elements in arr. Then you've better to set in the loop.

for (i = 0; i < sizeof(arr)/sizeof(arr[0]); i++) {
    arr[i] = 1;
}

Because memset() set 1 in each bytes. So it's not your expected.

Answer 2

Short answer, NO.

Long answer, memset sets bytes and works for characters because they are single bytes, but integers are not.

Answer 3

No, you cannot use memset() like this. The manpage says (emphasis mine):

The memset() function fills the first n bytes of the memory area pointed to by s with the constant byte c.

Since an int is usually 4 bytes, this won't cut it.

---

If you (incorrectly!!) try to do this:

int arr[15];
memset(arr, 1, 6*sizeof(int));    //wrong!

then the first 6 ints in the array will actually be set to 0x01010101 = 16843009.

The only time it's ever really acceptable to write over a "blob" of data with non-byte datatype(s), is memset(thing, 0, sizeof(thing)); to "zero-out" the whole struture/array. This works because NULL, 0x00000000, 0.0, are all completely zeros.

---

The solution is to use a for loop and set it yourself:

int arr[15];
int i;

for (i=0; i<6; ++i)    // Set the first 6 elements in the array
    arr[i] = 1;        // to the value 1.

Answer 4

Ideally you can not use memset to set your arrary to all 1.
Because memset works on byte and set every byte to 1.

memset(hash, 1, cnt);

So once read, the value it will show 16843009 = 0x01010101 = 1000000010000000100000001
Not 0x00000001
But if your requiremnt is only for bool or binary value then we can set using C99 standard for C library

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>        //Use C99 standard for C language which supports bool variables

int main()
{
    int i, cnt = 5;
    bool *hash = NULL;
    hash = malloc(cnt);

    memset(hash, 1, cnt);
    printf("Hello, World!\n");

    for(i=0; i<cnt; i++)
        printf("%d ", hash[i]);

    return 0;
}

Output:

Hello, World!
1 1 1 1 1