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: =========

For one thing, this: sizeof(char) in your realloc should be sizeof(char*), and the types aren't comparable (assigning a const char* to a char*), but thats a different issue. (i.e. matches is the wrong type). You also never initialize matches to NULL before starting this thing up, which is critical for the first iteration to work. — WhozCraig
– WhozCraig, Commented Feb 8, 2014 at 20:08
char **matches = NULL; and char **more_matches = NULL; now the code works, but I get the same warnings when finishing it segfaults. And by the way, printf outputs data twice. — bsteo
– bsteo, Commented Feb 8, 2014 at 20:13

WhozCraig · Accepted Answer · 2014-02-08 20:18:32Z

A number of things wrong:

Using the wrong sizeof() when resizing your matches pointer array
Failing to initialize matches to NULL before the parsing begins.
Using the wrong type for the matches and more_matches pointers.
Using a volatile pointer to read-only memory for subject
Casting malloc and realloc is never good in C
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");
    }
}

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