str_copy.c

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

char *replace_str(char *str, char *orig, char *rep)
{
	static char buffer[64];
	char *p;

	// Is 'orig' even in 'str'?
	if(!(p = strstr(str, orig)))
		return str;

	// Copy characters from 'str' start to 'orig' st$
	strncpy(buffer, str, p-str);
	buffer[p-str] = '\0';

	sprintf(buffer+(p-str), "%s%s", rep, p+strlen(orig));

	return buffer;
}


int main() {

	puts(replace_str("Hello, worlddddd!", "worlddddd", "Miamii"));

	return 0;
}