strcpy() Delete everything else - c

Question

I need to read a file and look for a word and replace with a new word but it's not working as it should:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main() {
FILE * f = fopen("text" , "rb" );
if(f == NULL ){
perror("error ");
return 1;
}
char chain[100];
while(!feof(f)){
fgets(chain, 100 , f);
}
printf("%s", chain);
fclose(f);
printf("\n \n ;D \n");
return 0;
}
And this is how I replace the old word:
char str[] ="This is a hiall samplesss friends string";
char * pch;
pch = strstr (str,"hiall");
strncpy (pch,"sam",5);
puts (str);
thanks

Answer 1

That strncpy is copying 5 chars from "sam" into pch. Note however that "sam" has only 3 characters, so it is copying the three characters plus the \0 terminator. That's why it is "deleting" the rest of the string: it is adding a terminator right after "sam", so you are getting This is a sam in the output.
If you strncpy only 3 characters, you'll get this:
This is a samll samplesss friends string
If what you want is something like find-and-replace (i.e. replacing "hiall" with "sam" and getting This is a sam samplesss friends string), you need to move the rest of the string backwards. A simple strncpy won't work because the destination memory overlaps with the target. For this, you can either use memmove, or use an auxiliary buffer:
char str[] = "This is a hiall samplesss friends string";
char* pch;
char* old_word = "hiall";
char* new_word = "sam";
size_t len_old = strlen(old_word);
size_t len_new = strlen(new_word);
pch = strstr(str, old_word);
assert(len_new <= len_old);
if (pch) {
char* rest = pch + len_old;
size_t len_rest = strlen(rest);
char* aux = malloc(len_rest + 1);
strncpy(aux, rest, len_rest + 1);
strncpy(pch, new_word, len_new);
strncpy(pch + len_new, aux, len_rest + 1);
free(aux);
puts(str);
}
Note that this will only work if new_word is the same size or shorter than old_word. If new_word is longer, you won't be able to edit in-place (in the string str itself), unless the original string has extra memory for it (e.g. if you declared it with str[1000] to guarantee you can increase its size enough — then the code above would work). The safest approach if you can't plan ahead what size new_word is going to be would be to allocate a new string:
char str[] ="This is a hiall samplesss friends string";
char* pch;
char* old_word = "hiall";
char* new_word = "sam";
pch = strstr(str, old_word);
size_t len_str = strlen(str);
size_t len_new = strlen(new_word);
size_t len_old = strlen(old_word);
if (pch) {
char* new_str = malloc(len_str - len_old + len_new + 1);
ptrdiff_t pos_word = pch - str;
strncpy(new_str, str, pos_word);
strncpy(new_str + pos_word, new_word, len_new);
strncpy(new_str + pos_word + len_new, pch + len_old,
len_str - pos_word - len_old + 1);
puts(new_str);
}
(Edit: addressed issues pointed out by David Bowling in the comments.)