C - Last element of char* array

For below sample code;

char* g_commands[]={
    "abcd",
    NULL
};

int main()
{
    
    g_commands[1] = "efg";
    
    char ** tmp = &g_commands[0];
    
    for(; *tmp != NULL; tmp++){
        printf("%s", *tmp);
    }

    return 0;
}

since tmp is pointing to the pointers in g_commands array in a loop, after I assign "efg" to g_commands[1], I expect the loop create a segmentation fault since the last element of g_commands is not null anymore. But the program finishes without exception and prints abcdefg successfully.

Why is it so? Does the compiler add NULL to the end of char* array as well?

The program has undefined behavior. In particular it means that a program can produce as an expected or as unexpected result.

I expect the loop create a segmentation fault since the last element of g_commands is not null anymore

The program works without a segmentation fault because the array g_commands

char* g_commands[]={
    "abcd",
    NULL
};

is defined in the global namespace and there is no other definition of an object after the array. Such a declaration has static storage duration and compilers usually set this memory to zeroes.

If you will move the definition in main like

#include <stdio.h>
/*
char* g_commands[]={
    "abcd",
    NULL
};
*/
int main()
{
    char* g_commands[]={
        "abcd",
        NULL
    };
    
    g_commands[1] = "efg";
    
    char ** tmp = &g_commands[0];
    
    for(; *tmp != NULL; tmp++){
        printf("%s", *tmp);
    }

    return 0;
}

then the probability that a segmentation fault will occur is very high.