Re: Best way to find and replace substrings in a char buffer?

This is how dynamic allocation can be done. I haven't testet this
carefully, so there might be errors in calculating needed size for new
buffer, but you'll get the idea
#include <stdio.h>
#include <malloc.h>
#include <string.h>
#include <assert.h>

#define CHECK_BUFFER(buffer,capacity,length,append_length) \
    if ( (length) + (append_length) > (capacity)) { \
        printf("Buffer Capacity %d is not enought for %d\n", (capacity),
(length) + (append_length));\
        capacity = (capacity)*2 > (length) + (append_length) ?
(capacity*2) : (length) + 2*(append_length);\
        printf("New Capacity is %d\n", (capacity));\
        buffer = (char*)realloc(buffer, (capacity)); \
                  assert(buffer!=NULL);\
    }

char *replace_str(char *str, char *orig, char *rep)
{
  int buffer_capacity = 10;
  int buffer_length = 0;
  char* buffer = (char*)malloc(sizeof(char)*buffer_capacity);
  buffer[0] = '\0';

  char *p;
  char *begin = str; //points on the beginning of string
  int origLen = strlen(orig);
  int rep_length = strlen(rep);

  while ( p = strstr(begin, orig))
  {
    CHECK_BUFFER(buffer, buffer_capacity, buffer_length, p-begin);
    strncat(buffer, begin, p-begin);
    buffer_length+=(p-begin);

    CHECK_BUFFER(buffer, buffer_capacity, buffer_length, rep_length);
    strcat(buffer, rep);
    buffer_length+=rep_length;
    begin = p+origLen;
  }
  CHECK_BUFFER(buffer, buffer_capacity, buffer_length, strlen(str) -
(begin - str));
  strncat(buffer, begin, strlen(str) - (begin - str));
  //ATTENTION : buffer is not freed!!!
  return buffer;
}

int main()
{
  char * str = replace_str("Hello, world!", "world", "Miami");
  puts(str);
  free(str);// not that wierd as it might look like,
        // strdup for example also returns buffer that must be freed
  return 0;
}

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