C: string replace in loop (c beginner) -

i need replace strings in text. found function here @ stackoverflow:

char *replace(const char *s, const char *old, const char *new) {     char *ret;     int i, count = 0;     size_t newlen = strlen(new);     size_t oldlen = strlen(old);      (i = 0; s[i] != '\0'; i++) {         if (strstr(&s[i], old) == &s[i]) {         count++;         += oldlen - 1;         }     }      ret = malloc(i + count * (newlen - oldlen));     if (ret == null)         exit(exit_failure);      = 0;     while (*s) {         if (strstr(s, old) == s) {             strcpy(&ret[i], new);             += newlen;             s += oldlen;         } else             ret[i++] = *s++;     }     ret[i] = '\0';      return ret; } 

this function works me fine single replacement. need replace whole array "str2rep" "replacement". i'm trying do(im beginner)

****     #define maxtext 39016     int l;     int j;     char *newsms = null;     char text[maxtext];     char *str2rep[] = {":q:",":n:"};     char *replacement[] = {"?","\n"};      strcpy((char *)text,(char *)argv[5]);      l = sizeof(str2rep) / sizeof(*str2rep);      for(j = 0; j < l; j++)     {         newsms = replace(text,(char *)str2rep[j],(char *)replacement[j]);         strcpy(text,newsms);         free(newsms);            }      textlen = strlen(text); 

this code works locally, if build single file... asterisk module, when being executed, asterisk stops with:

* glibc detected * /usr/sbin/asterisk: double free or corruption (!prev): 0x00007fa720006310 *

issues:

1. ret = malloc(i + count * (newlen - oldlen)); small. need + 1.
   consider happens replace("", "", ""). if ref this, wrong too.

2. questionable results mixing signed/unsigned. count signed. newlen, oldlen unsigned.
   think original code works ok, not using wrap-around nature of unsigned math when can avoided happens when newlen < oldlen.

   // + count * (newlen - oldlen) size_t newsize = + 1;  // + 1 above reason if (newlen > oldlen) newsize += count * (newlen - oldlen); if (newlen < oldlen) newsize -= count * (oldlen - newlen); ret = malloc(newsize); 

3. insure enough space. @hyde various approaches available here.

   // strcpy(text, newsms); if (strlen(newsms) >= sizeof text) handle_error(); strcpy(text, newsms); 

minor

2. no need casts

   // newsms = replace(text, (char *) str2rep[j], (char *) replacement[j]); newsms = replace(text, str2rep[j], replacement[j]); 

3. better use size_t i. pedantic solution use size_t count.

   // int i; size_t i;