Passing a pointer to array of string pointers

Question

In the example below if I uncomment the printing of matches[1] and matches[2] in the print_and_modify function it fails(Illegal instruction: 4 on my Mac OS X mavericks).

What confuses me is why matches[0] works fine ? Any help is greatly appreciated.

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

void print_and_modify(char ***matches) {
  printf("%s\n", *matches[0]);
  /* printf("%s\n", *matches[1]); */
  /* printf("%s", *matches[2]); */
}

int main(int argc, char **argv)
{

  char **matches;

  matches = malloc(3 * sizeof(char*));

  matches[0] = malloc(10 * sizeof(char));
  matches[1] = malloc(10 * sizeof(char));
  matches[2] = malloc(10 * sizeof(char));

  char *test1 = "test1";
  char *test2 = "test2";
  char *test3 = "test3";

  strncpy(matches[0],test1,5);
  strncpy(matches[1],test2,5);
  strncpy(matches[2],test3,5);
  print_and_modify(&matches);

  printf("======\n");
  printf("%s\n", matches[0]);
  printf("%s\n", matches[1]);
  printf("%s\n", matches[2]);
}

Please excuse the contrived example. I am trying to learn some C.

How else would you pass a pointer to a *char[] to a function ? — Mohan
– Mohan, Commented Sep 2, 2015 at 1:44
use (*matches)[0],(*matches)[1]and (*matches)[2] instead of. — BLUEPIXY
– BLUEPIXY, Commented Sep 2, 2015 at 1:50

Sergey Kalinichenko · Accepted Answer · 2015-09-02 01:52:15Z

1

It's operator precedence: [] has higher precedence than *, so you need to force the desired precedence:

void print_and_modify(char ***matches) {
    printf("%s\n", (*matches)[0]);
    printf("%s\n", (*matches)[1]);
    printf("%s", (*matches)[2]);
}

Demo.

Note that you do not need to pass a triple pointer, unless you wish to modify the allocated array itself. If you pass a double pointer, you would be able to modify the content of the individual strings, and also replace strings with other strings by de-allocating or re-allocating the char arrays.

answered Sep 2, 2015 at 1:52

Sergey Kalinichenko

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2 Comments

SwiftMango Over a year ago

[] is called subscript, * is called dereference.

Mohan Over a year ago

@dasblinkenlight - I do plan to modify the allocated array. I would like for the function to be able to increase the size of the array. Additionally I would also like for print_and_modify to be able to initialize it if its not been initialized yet. Thanks for the help!

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