Using malloc to allocate memory dynamically, I receive an error when trying to compile in cygwin

Question

I am trying to allocate memory for a 3 by 5 2-dimensional array. When compiling this code, I receive this error in the cygwin terminal:

2darraymalloc.c: In function 'main':
2darraymalloc.c:7:9: warning: incompatible implicit declaration of built-in function 'malloc'    a[i] = malloc(sizeof(int)*5);

#include <stdio.h>

void main() {
    int *a[3], i, j;

    for (i = 0 ; i < 3 ; i++) {
        a[i] = malloc(sizeof(int) * 5);
    }

    for (i = 0 ; i < 3 ; i++) {
        for (j = 0 ; j < 5 ; j++) {
            a[i][j] = i + 2 * j;
        }
    }

    printf("%d", *a[2]);
}

[Related reading](http://stackoverflow.com/q/605845/440558) (which also contains the solution to your problem). — Some programmer dude, Oct 03 '15 at 18:37

Iharob Al Asimi · Answer 1 · 2015-10-03T18:39:21.917

0

You need to include stdlib.h, also always check if malloc() did return a valid pointer, on error it returns NULL, and main() should return int, a valid signature for main() in your case could be int main(void).

Always enable compilation warnings, add this to the gcc invocation

gcc -Wall -Wextra -Werror -o my-program-name my-source.c

edited Oct 03 '15 at 18:39

answered Oct 03 '15 at 18:33

Iharob Al Asimi

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Thank you, the #include solved my problem – Michael Oct 03 '15 at 18:37
@Michael I know, that's why I told you to add it. – Iharob Al Asimi Oct 03 '15 at 18:38
I was just commenting to let you know that it was just the stdlib.h, not the main function. It's fine to have void main() and not return anything – Michael Oct 03 '15 at 20:30
@Michale NO IT'S NOT. Please respect what I said by doing research. It's not correct to have `void main()`, not with the compiler you are using. Please enable compiler warnings and you will see. And if you intend to ignore warnings please don't write c code. – Iharob Al Asimi Oct 03 '15 at 20:32
easy.. i appreciate the help regardless – Michael Oct 03 '15 at 20:33
Yes you should, but really try to learn things right. That would make me happier. – Iharob Al Asimi Oct 03 '15 at 20:34

score 0 · Answer 2 · edited Oct 03 '15 at 20:32

0

In C functions that do not have a prototype defined are assumed, by default, to return int. Since you did not include stdlib.h it leaves malloc() undeclared and therefore assumed to return int. The statement a[i] = malloc(...) has on the left side a int pointer and on the right side an int - thus the error message.

edited Oct 03 '15 at 20:32

Iharob Al Asimi

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6
59
97

answered Oct 03 '15 at 19:17

AndersK

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Using malloc to allocate memory dynamically, I receive an error when trying to compile in cygwin

2 Answers2