Segfault when copying into malloced memory

Question

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

const int debug = 1;                                                                                                                                                                                           


void copy_array(char* input, char** output, int len) {
  *output = (char *)malloc(len * sizeof(*output));                                                                                                                                                                               
  if (debug) printf("copy_array: len: %x\n", len);                                                                                                                                                             
  for (int i=0; i<len; i++) {                                                                                                                                                                                  
    printf("Pre assignment: %x\n", input[i]);                                                                                                                                                                  
    *output[i] = input[i];                                                                                                                                                                                     
    printf("Post assignment\n");                                                                                                                                                                               
  }

}

int main(void) {

  char input[] = { 49, 27, 0x6d, 20, 0 };                                                                                                                                                                         
  char c;                                                                                                                                                                                                      
  char* output;                                                                                                                                                                                                
  int len = strlen(input);                                                                                                                                                                                     
  copy_array(input, &output, len);                                                                                                                                                                             
  return 0;                                                                                                                                                                                                    
}

is example code where I am getting a segfault for reasons I don't understand at the "*output[i] =..." line.

Does anyone understand what I am doing wrong?

Edit: addressed existing comments re:

missing stdlib.h include
non-null-terminated array
not using sizeof for malloc

Edit 2: the problem is resolved by Antti Haapala: [] binds more tightly than *, so "(*output)[i]" is needed.

Implicit declaration of `malloc`. You've forgotten to include ``. Doesn't your compiler issue any warnings?! — Antti Haapala -- Слава Україні, Dec 29 '18 at 12:00
Strange, you've got a really old compiler if it doesn't issue warnings for that by default — Antti Haapala -- Слава Україні, Dec 29 '18 at 12:07
Ah, 3rd thing: `(*output)[i]`. Easier to use a temporary variable for `char *` instead of always dereferencing the returned pointer. — Antti Haapala -- Слава Україні, Dec 29 '18 at 12:10
The first two "duplicate answers" should be removed - they don't address the segfault question. — Realraptor, Dec 30 '18 at 13:45
they do address the problems of the post as they were originally — Antti Haapala -- Слава Україні, Dec 30 '18 at 13:53

zch · Answer 1 · 2018-12-29T12:04:55.347

2

input is not 0-terminated. Calling strlen on it is undefined behavior.

You are also not allocating enough memory. You need len * sizeof output[0].

edited Dec 29 '18 at 12:04

answered Dec 29 '18 at 11:59

zch

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Thanks. However, it isn't relative to the actual problem. – Realraptor Dec 29 '18 at 12:02
@Realraptor the first one is. the 2nd one is not actually correct, it is just the 3rd mistake of yours that you did. – Antti Haapala -- Слава Україні Dec 29 '18 at 12:13

Segfault when copying into malloced memory

1 Answers1