C: string replace in loop (c beginner) - c

Question

I need to replace a strings in some text. I found this function here at 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);
for (i = 0; s[i] != '\0'; i++) {
if (strstr(&s[i], old) == &s[i]) {
count++;
i += oldlen - 1;
}
}
ret = malloc(i + count * (newlen - oldlen));
if (ret == NULL)
exit(EXIT_FAILURE);
i = 0;
while (*s) {
if (strstr(s, old) == s) {
strcpy(&ret[i], new);
i += newlen;
s += oldlen;
} else
ret[i++] = *s++;
}
ret[i] = '\0';
return ret;
}
This function works for me fine for single replacement. But i need to replace a whole array "str2rep" to "replacement". So what i'm trying to do(im just a 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 even works locally, If I build it from single file... But this is asterisk module, so when this is being executed, asterisk stops with:
* glibc detected * /usr/sbin/asterisk: double free or corruption (!prev): 0x00007fa720006310 *

Answer 1

Issues:
ret = malloc(i + count * (newlen - oldlen)); is too small. Need + 1.
Consider what happens with replace("", "", ""). If your SO ref is this, it is wrong too.
Questionable results mixing signed/unsigned. count is signed. newlen, oldlen are unsigned.
I think the original code works OK, but I do not like using the wrap-around nature of unsigned math when it can be avoided which is what happens when newlen < oldlen.
// i + count * (newlen - oldlen)
size_t newsize = i + 1; // + 1 for above reason
if (newlen > oldlen) newsize += count * (newlen - oldlen);
if (newlen < oldlen) newsize -= count * (oldlen - newlen);
ret = malloc(newsize);
Insure enough space. #hyde Various approaches available here.
// strcpy(text, newsms);
if (strlen(newsms) >= sizeof text) Handle_Error();
strcpy(text, newsms);
Minor
No need for casts
// newsms = replace(text, (char *) str2rep[j], (char *) replacement[j]);
newsms = replace(text, str2rep[j], replacement[j]);
Better to use size_t for i. A pedantic solution would also use size_t count.
// int i;
size_t i;

Answer 2

I will suggest something that to me looks a bit more clear as an alternative, in place of a proper dynamic string implementation. Exception handling is left as an exercise for the reader to add. :)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char *appendn(char *to, char *from, int length)
{
return strncat(realloc(to, strlen(to) + length + 1), from, length);
}
char *replace(char *string, char *find, char *sub)
{
char *result = calloc(1, 1);
while (1)
{
char *found = strstr(string, find);
if (!found)
break;
result = appendn(result, string, found - string);
result = appendn(result, sub, strlen(sub));
string = found + strlen(find);
}
return appendn(result, string, strlen(string));
}
int main()
{
const char text[] = "some [1] with [2] to [3] with other [2]";
char *find[] = {"[1]", "[2]", "[3]", NULL};
char *sub[] = {"text", "words", "replace"};
char *result, *s;
int i;
result = malloc(sizeof(text));
(void) strcpy(result, text);
for (i = 0; find[i]; i ++)
{
s = replace(result, find[i], sub[i]);
free(result);
result = s;
}
(void) printf("%s\n", result);
free(result);
}