Linux POSIX C dynamic string array issue

Question

I have the following code, it searches for some matches with regex (pcre) then adds the matches to a dynamically growing array (so I can make the matches unique)... the problem I get two warnings when compiling and when running the program crashes.

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <pcre.h>

int main() {
  pcre *myregexp;
  const char *error;
  int erroroffset;
  int offsetcount;
  int offsets[(0+1)*3]; // (max_capturing_groups+1)*3
  const char *result;
  int n;
  int count = 1;
  char **matches;
  char **more_matches;
  char *subject = "9,5,3,2,5,6,3,2,5,6,3,2,2,2,5,0,5,5,6,6,1,";
  myregexp = pcre_compile("\\d,", PCRE_MULTILINE|PCRE_DOTALL, &error, &erroroffset, NULL);

  if (myregexp != NULL) {
    offsetcount = pcre_exec(myregexp, NULL, subject, strlen(subject), 0, 0, offsets, (0+1)*3);

    while (offsetcount > 0) {

      if (pcre_get_substring(subject, offsets, offsetcount, 0, &result) >= 0) {
        printf("%s\n", result);
        more_matches = (char *) realloc(matches, count * sizeof(char));
        if (more_matches!=NULL) {
          matches=more_matches;
          matches[count-1]=result;
          count++;
        }
        else {
          free(matches);
          puts("Error (re)allocating memory");
          exit(1);
       }
      }

      offsetcount = pcre_exec(myregexp, NULL, subject, strlen(subject), offsets[1], 0, offsets, (0+1)*3);
    }
    for (n=0; n<count; n++) printf("%s\n", matches[n]);
    free(matches);
  } else {
      printf("Syntax error in REGEX at erroroffset\n");
  }

}

casting something wrong?

$ gcc -o pcre_ex_arr pcre_ex_arr.c -lpcre
pcre_ex_arr.c: In function 'main':
pcre_ex_arr.c:29: warning: assignment from incompatible pointer type
pcre_ex_arr.c:32: warning: assignment discards qualifiers from pointer target type
$ ./pcre_ex_arr
2,
*** glibc detected *** ./pcre_ex_arr: realloc(): invalid pointer: 0xb7fcfb80 ***
======= Backtrace: =========

Answer 1

A number of things wrong:

1. Using the wrong sizeof() when resizing your matches pointer array
2. Failing to initialize matches to NULL before the parsing begins.
3. Using the wrong type for the matches and more_matches pointers.
4. Using a volatile pointer to read-only memory for subject
5. Casting malloc and realloc is never good in C
6. The math on your count is flaky-at-best. It should start at 0, not 1

See below:

int main()
{
    pcre *myregexp;
    const char *error;
    int erroroffset;
    int offsetcount;
    int offsets[(0+1)*3]; // (max_capturing_groups+1)*3
    const char *result;
    int n;
    int count = 0;
    const char **matches = NULL;
    const char **more_matches;

    char subject[] = "9,5,3,2,5,6,3,2,5,6,3,2,2,2,5,0,5,5,6,6,1,";
    myregexp = pcre_compile("\\d,", PCRE_MULTILINE|PCRE_DOTALL, &error, &erroroffset, NULL);

    if (myregexp != NULL) {
        offsetcount = pcre_exec(myregexp, NULL, subject, strlen(subject), 0, 0, offsets, (0+1)*3);

        while (offsetcount > 0) {

            if (pcre_get_substring(subject, offsets, offsetcount, 0, &result) >= 0)
            {
                printf("%s\n", result);
                more_matches = realloc(matches, (count+1)* sizeof(*more_matches));
                if (more_matches!=NULL)
                {
                    matches=more_matches;
                    matches[count++]=result;
                }
                else
                {
                    free(matches);
                    puts("Error (re)allocating memory");
                    exit(1);
                }
            }

            offsetcount = pcre_exec(myregexp, NULL, subject, strlen(subject), offsets[1], 0, offsets, (0+1)*3);
        }

        for (n=0; n<count; n++) printf("%s\n", matches[n]);
        free(matches);

    } else {
        printf("Syntax error in REGEX at erroroffset\n");
    }
}