How to copy from static array to dynamically allocated memory

Question

Why does b not hold 1., 2.?

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

#define LEN 2

void main() {
    double a[LEN] = {1, 2};
    double* b = malloc(LEN * sizeof(*b));

    memcpy(b, a, LEN);
    for (size_t i = 0; i < LEN; i++)
    {
        printf("%.2f ", b[i]);
    }
    printf("\n");
}

Instead, I get

> gcc code.c                                                                                                                                                         
> ./a.out                                                                                                                                                                                   
0.00 0.00

Does this answer your question? Copy array to dynamically allocated memory — tstanisl
– tstanisl, Commented Sep 22, 2021 at 22:40
It does show that memcpy is the right tool, but here obviously getting the size right was the problem. — fhchl
– fhchl, Commented Sep 24, 2021 at 8:55

Ted Lyngmo · Accepted Answer · 2021-09-22 21:27:07Z

4

You forgot the sizeof in the memcpy

memcpy(b, a, LEN * sizeof *b);

As noted by @tstanisl in the comments, LEN * sizeof *b is the same as sizeof a, so you could make it:

double* b = malloc(sizeof a);
memcpy(b, a, sizeof a);

Also note that void main() isn't a valid signature for main. It should be int main().

edited Sep 22, 2021 at 21:27

answered Sep 22, 2021 at 20:53

Ted Lyngmo

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9 Comments

fhchl Over a year ago

I JUST realized. Where is the automatic pair programmer tool that hints to these idiotic mistakes?

Ted Lyngmo Over a year ago

@fhchl LOL :-D Yeah, I've needed one of those myself.

tstanisl Over a year ago

it would be simpler and less error prone to use memcpy(b, &a, sizeof a);

Ted Lyngmo Over a year ago

@tstanisl &a makes a double(*)[LEN] instead of a double*. I find the latter clearer/simpler. I can agree to some extent about sizeof a but I went with *b for consistency with the malloc.

n. m. could be an AI Over a year ago

@fhchl gcc -Wall -Werror can help with some of those.

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