Replacement of a word within a string - c

Question

I am trying to replace a word within a sentence with another word.
For ex. if str1="strong" and str2="weak", then the 1st string in str should get changed to "If the core is weak, you can't go wrong".
I am getting segmentation fault(core dumped) error.
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
void func(char *str,char *str1,char *str2,int l1,int l2)
{
int i,m,j,c;
for(i=0;*(str+i)!='\0';i++)
{
if(*(str+i)==*str1)
{
for(m=i,j=0;j<l1;m++,j++) /*checking the equality of the two words*/
{
if(*(str+m)!=*(str1+j))
break;
else
c=1;
}
if(c==1)
{
for(j=i;*(str+j)!='\0';j++)
*(str+j+l2-l1)=*(str+j);
for(m=0,j=i;(*(str2+m)!='\0');j++,m++)
*(str+j)=*(str2+m);
}
}
}
}
int main()
{
char *str[]={
"If the core is strong, you can't go wrong",
"Canada's wonder of the world",
"Processing a sorted array",
"Was that a joke?",
"I wanna be a millionaire",
"Your post is not properly formatted"
};
int i,l1,l2,k,j;
char str1[10],str2[10];
l1=strlen(str1);
l2=strlen(str2);
printf("Enter string 1 : ");
scanf("%s",str1);
printf("\nEnter string 2 : ");
scanf("%s",str2);
for(k=0;k<=5;k++)
func(str[k],str1,str2,l1,l2);
for(i=0;i<6;i++)
{
printf("\n");
for(j=0;*(str[i]+j)!='0';j++)
printf("%c",*(str[i]+j));
}
return 0;
}

Answer 1

char *str[]={
"If the core is strong, you can't go wrong",
"Canada's wonder of the world",
"Processing a sorted array",
"Was that a joke?",
"I wanna be a millionaire",
"Your post is not properly formatted"
};
This creates a array of character pointers and each pointer points to a string literal and if you modify a string literal it will result in segmentation fault since it is in read-only memory. this is the reason that you are getting segmentation fault.
Do check your logical errors too in your code.

Answer 2

Your code is unfortunately wrong on many levels. Others have already pointed out the string literal issue, and the issue with strlen() before scanf() in main(), but there is more wrong:
1. You try to edit the string in-place. What happens if you replace a sub-string with a longer string? You try to move the string forward. But that means you write past the end of the memory allocated to the string. This is undefined behaviour and may crash.
2. In func():
[snip]
if(c==1)
{
for(j=i;*(str+j)!='\0';j++)
*(str+j+l2-l1)=*(str+j);
for(m=0,j=i;(*(str2+m)!='\0');j++,m++)
*(str+j)=*(str2+m);
}
You iterate until the source string is null-terminated, but you don't copy the null-terminator! Later on, you try to printf() the string (character by character, for some unfathomable reason, also not checking for a null-terminator but for an ASCII '0'...), which will then crash due to the lack of null-termination.
To solve your issues, you can do the following:
1. Walk through the string using strstr() to find the sub-string to replace. Count the number of occurrences of the sub-string.
2. Use strlen() in func() to determine the length of the sub-string and the replacement. Multiply the difference by the number of occurrences of the sub-string to find how much longer or shorter your new string will be. Add the difference to strlen() of the original string to find out how much memory to allocate (Remember to allocate an extra byte for null-termination).
3. Allocate the needed memory with malloc().
4. Use a combination of strcpy()/strncpy()/memmove()/strstr()/whatever standard library functions you need to assemble your new string.