[Erev0s]

Hi guys i need your help for this replace_pattern program.

This is a program that takes a string
i.e.:"The boy was sitting in the room"
then it takes a string to compare
i.e.:"the"
Finally it takes a string to replace the second string
i.e:"a"

The final result is :"a boy was sitting in a room"

I must use this function:
char *replace_pattern(char *s, char *p ,char *np,int firstOnly);

Here is my program ...(i cant find what is wrong)
The bug is in replace_pattern function

Code:

#include<stdio.h>
#include<string.h>

char *replace_pattern(char *s, char *p ,char *np,int firstOnly);

//this funct allocates dynamicly memory for the three strings
//s,p and np
char *mem_alloc(char *buffer);

char buffer_str[81];
char *cur_str,*same_str,*repl_str,*new_str;

int firstOnly,c;

int main(void){
   
   char *str_ptr;
  
   printf("Enter a string and then press enter...\n");
 
   while( *(str_ptr=gets(buffer_str)) == NULL ) 
   printf("try again...\n");

   cur_str=mem_alloc(buffer_str);
   cur_str=buffer_str;

 printf("Enter a string to compare,then press enter...\n"); 
 while( *(str_ptr=gets(buffer_str)) == NULL )
   
   same_str=mem_alloc(buffer_str);
   same_str=buffer_str;
   
  printf("Enter a string to replace,then press enter...\n");  
 while( *(str_ptr=gets(buffer_str)) == NULL )
 printf("try again...\n");
 
   repl_str=mem_alloc(buffer_str);
   repl_str=buffer_str;
   
   printf("\nEnter 0 if you want to check the whole string...");
   printf("\nor enter a different number if you want to find");
   printf("\nand replace the 1st similarity:");
   c=scanf("%d",&firstOnly);
   
   while(c!=1){
	while((c=getchar())!='\n');
	c=scanf("%d",&firstOnly);
	}
   
   new_str=replace_pattern(cur_str, same_str ,repl_str, firstOnly);
 return 0;
}

/*memory allocation for the strings*/

Code:

char *mem_alloc(char *buffer)
{
 char *str;
 
	str=(char *)malloc((strlen(buffer)+1)); 
	if(str==NULL){
	printf("\n memory allocation error! ");
  printf("\n exiting now... ");
  exit(1);
  }
  else return(str);  
}

Replace Pattern function *buggy one*
/* *s is the given string, *p is a string to search in *s,
if we find *p string in *s string we replace with *np string

if firstOnly !=0 i must replace only the first substring with np
else i must replace substrings with np string in the whole string*/

Code:

char *replace_pattern(char *s, char *p ,char *np)
{
char *start_ptr;
int counter=0,i=0,j=0;
 
 counter=strlen(p);
 while((start_ptr=strstr(s,p))!=NULL){
   
   while(s<start_ptr)
   
   buffer_str[i++]=*s++;
	while(j<counter)
   buffer_str[i++]=*np++;
   s+=counter;
   }
   buffer_str[i++]='\0'; 
 return buffer_str;
}