C: implementing str_replace

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Submitted by chantra on Wed, 10/07/2009 - 15:38

And example on how to implement str_replace in C.

This implementation will only replace the value once. If the substring to replace is not found, a copy of the original string is returned.

If you are looking for a version that replace all occurrences of substring, please have a look at str_replace all matching pattern.

/**
 * vim: tabstop=2:shiftwidth=2:softtabstop=2:expandtab
 *
 * str_replace.c implements a str_replace PHP like function
 * Copyright (C) 2009  chantra <chantra__A__debuntu__D__org>
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 *
 * gcc -o str_replace str_replace.c
 */
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
 
 
void usage(char *p){
  fprintf(stderr, "USAGE: %s string tok replacement\n", p );
}
 
/*
 * Create a new string with [substr] being replaced by [replacement] in [string]
 * Returns the new string, or NULL if out of memory.
 * The caller is responsible for freeing this new string.
 */
char *
str_replace( const char *string, const char *substr, const char *replacement ){
  char *tok = NULL;
  char *newstr = NULL;
 
  tok = strstr( string, substr );
  if( tok == NULL ) return strdup( string );
  newstr = malloc( strlen( string ) - strlen( substr ) + strlen( replacement ) + 1 );
  if( newstr == NULL ) return NULL;
  memcpy( newstr, string, tok - string );
  memcpy( newstr + (tok - string), replacement, strlen( replacement ) );
  memcpy( newstr + (tok - string) + strlen( replacement ), tok + strlen( substr ), strlen( string ) - strlen( substr ) - ( tok - string ) );
  memset( newstr + strlen( string ) - strlen( substr ) + strlen( replacement ), 0, 1 );
  return newstr;
}
 
int main( int argc, char **argv ){
 
  char *ns = NULL;
  if( argc != 4 ) {
    usage(argv[0]);
    return 1;
  }
  ns = str_replace( argv[1], argv[2], argv[3] );
  fprintf( stdout, "Old string: %s\nTok: %s\nReplacement: %s\nNew string: %s\n", argv[1], argv[2], argv[3], ns );
  free(ns);
  return 0;
}

Will output:

$ gcc -o str_replace str_replace.c
$ ./str_replace "(uid=%u)" "%u" chantra
Old string: (uid=%u)
Tok: %u
Replacement: chantra
New string: (uid=chantra)
$ ./str_replace "(uid=%u)" "%p" chantra
Old string: (uid=%u)
Tok: %p
Replacement: chantra
New string: (uid=%u)

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