Using memset to initialize an integer array

Question

I have a malloc'd array as follows:

int* buf;
buf = (int*)malloc(sizeof(int) * N)

where N is the number of elements in the integer array. I am trying to set all N elements in the array to a specific value, say 4 (so buf = [4, 4, 4, ..., 4]).

For intents and purposes, this is mostly an experiment, and so I am trying to only use memset, without for loops. Is this possible?

With memset, I do:

memset(buf, 4, sizeof(int)*N);

which I assumed places a 4 everywhere in the array in memory, but this doesn't seem to work as expected. Suppose N = 20, then the output is:

[1,6,8,4,3,0,0,9,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,,0,0,0,0]

which is not what I expected, but what I do notice is it has set the 4th element (which does correspond to sizeof(int)*N) to 4. I thought memset would set it all to 4, similar to the string case?

memset sets bytes, not ints, even though the prototype shows the second parameter as type int. Read the man pages for memset again. — cdarke
– cdarke, Commented Jan 17, 2019 at 22:47
Can you give a minimum working example? As indicated above, memset fills all the byte in memory with the same char and cannot be used to initiliaze ints. But your results are somehow weird. — Alain Merigot
– Alain Merigot, Commented Jan 17, 2019 at 22:50
Output of 1,6,8,4,3,0,0,9,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,,0,0,0,0 makes little sense. Post an minimal reproducible example — chux
– chux, Commented Jan 18, 2019 at 3:21

0___________ · Accepted Answer · 2019-01-17 23:18:42Z

-1

Here you have two examples

void memsetint(int *table, const int value, size_t size)
{
    while(size--)
    {
        *table++ = value;
    }
}

#define gmemset(table, value, size) _Generic((&table[0]),   \
    int * : memsetint,                                        \
    double * : memsetdouble) (table, value, size)

void memsetdouble(double *table, const double value, size_t size)
{
    while(size--)
    {
        *table++ = value;
    }
}

int main()
{
    double d[1000];

    gmemset(d, 45.34, 1000);
    printf("%f\n", d[300]);

    return 0;
}

#include <string.h>

void mymemset(void *buff, const void *value, const size_t size, size_t count)
{
    unsigned char *b = buff;
    while(count--)
    {
        memcpy(b, value, size);
        b += size;
    }
}

int main()
{
    int x[1000];
    int value = 5;

    mymemset(x, (int []){10}, sizeof(int), 1000);
    mymemset(x, &value, sizeof(int), 1000);

    return 0;
}

edited Jan 17, 2019 at 23:18

answered Jan 17, 2019 at 23:06

0___________

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3 Comments

0___________ Over a year ago

DV Why? Revenge?

David C. Rankin Over a year ago

No telling, sometimes is probably just has to do with the phase of the moon, or users with 3-char names. Never can tell.

0___________ Over a year ago

@DavidC.Rankin Another one interesting. Probably same person 4 topics - some of them very old

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