test.c

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

char *foo() {
	char *home = getenv("HOME");

	char *result = malloc(strlen(home) + strlen("/foo") + 1);
	strcpy(result, home);
	result = strcat(result, "/foo");
	return result;
}
int main() {
	printf("%s\n", foo());
	printf("%s\n", foo());
	printf("%s\n", foo());
}