What is the output of the following C program on a 64-bit system? #include #include int main(void) { int *p = malloc(sizeof *p); free(p); p = NULL; free(p); printf("%s\n", p == NULL ? "safe" : "dangling"); return 0; }

The program crashes with a double free

Memory Allocation · Predict the Output · Question 7

free(), set to NULL, free() again – output Answer, explanation & discussion

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Question 7 Back to Exercise

What is the output of the following C program on a 64-bit system?

main.c

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

int main(void) {
    int *p = malloc(sizeof *p);
    free(p);
    p = NULL;
    free(p);
    printf("%s\n", p == NULL ? "safe" : "dangling");
    return 0;
}

A safe Correct
B dangling
C The program crashes with a double free
D Compilation error

Correct Answer: Option A

The second free() receives NULL, which is a no-op. Without p = NULL; it would be a double free. (Even if malloc() had failed, freeing NULL is fine.)

Output Shrinking a block with realloc() – output
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Output calloc() a struct – are pointer members NULL?
Output C23 auto and nullptr with malloc() – output
Output sizeof p vs sizeof *p after malloc – output
Output calloc() then realloc() – which values survive?

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free(), set to NULL, free() again – output Answer, explanation & discussion

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