Avoid segfault from `struct S as[] = {{NULL}};` with `struct S { const char *array[ARG_MAX]; };`?

Question

On Apple clang version 12.0.5 (clang-1205.0.22.11) with gcc -ansi the following produces a segfault:

#include <stdlib.h>

#define ARG_MAX 1024 * 1024

struct S { const char *array[ARG_MAX]; };

int main(void) {
    struct S as[] = {{NULL}};
    return EXIT_SUCCESS;
}

ARG_MAX is defined in sys/syslimits.h as 1024 * 1024, defined above explicitly.

How do I avoid the segfault?

Answer 1

Large arrays with automatic storage duration (aka local variables) is to be avoid on most implementations as they typically use a stack with a fixed and rather limited size, e.g. in the rang 1-8 MB. If the object size exceeds the available stack, all kind of things may happen. Including seg fault.

How do I avoid the segfault?

Use dynamic allocation like:

#define ARG_MAX (1024 * 1024)

struct S { const char *array[ARG_MAX]; };
int main(void) {
    struct S * as = calloc(1, sizeof *as);
    if (as == NULL) exit(EXIT_FAILURE);

    ... use as[0] ...

    free(as);
    return EXIT_SUCCESS;
}

Most implementations have much more memory available for dynamic allocated objects. It's typically called heap memory.

BTW: It's legal but a bit strange that your code makes an array with a single element.