#include <stdio.h>
#include <string.h>
 
int main ( ) {
    char buff[BUFSIZ];      // the input line
    char newbuff[BUFSIZ];   // the results of any editing
    char findme[] = "hello";
    char replacewith[] = "world";
    FILE *in, *out;
 
    in = fopen( "file.txt", "r" );
    out= fopen( "new.txt", "w" );
 
    while ( fgets( buff, BUFSIZ, in ) != NULL ) {
        if ( strstr( buff, findme ) != NULL ) {
            // do 1 or more replacements
            // the result should be placed in newbuff
            // just watch you dont overflow newbuff...
        } else {
            // nothing to do - the input line is the output line
            strcpy( newbuff, buff );
        }
        fputs( newbuff, out );
    }
 
    fclose( in );
    fclose( out );
    return 0;
}