Dangerous memory leakage without compiler warnings in C Language.

dont_assign_string.c

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

int main()
{
    char* foo = malloc(4);
    char* old_foo = foo;
    foo[0] = 'f';
    foo[1] = 'o';
    foo[2] = 'o';
    foo[3] = '\0';

    printf("\n%lx", (unsigned long) foo);
    printf("\t%lx", (unsigned long) old_foo);

    printf("\n%3s %3s", foo, old_foo);
    foo = "FOO"; ///< Memory leak

    printf("\n%lx", (unsigned long) foo);
    printf("\t%lx", (unsigned long) old_foo);

    printf("\n%3s %3s", foo, old_foo);
    return 0;
}

output

5c2eb0	5c2eb0
foo foo
de9057	5c2eb0
FOO foo