Question

Why does this strncpy() implementation crashes on second run, while the first run works ok?

strncpy

Copy characters from string Copies the first n characters of source
to destination. If the end of the source C string (which is signaled
by a null-character) is found before n characters have been copied,
destination is padded with zeros until a total of n characters have
been written to it.

No null-character is implicitly appended at the end of destination if
source is longer than n (thus, in this case, destination may not be
a null terminated C string).

char *strncpy(char *src, char *destStr, int n)
{
    char *save = destStr; //backing up the pointer to the first destStr char
    char *strToCopy = src; //keeps [src] unmodified

    while (n > 0)
    {
        //if [n] > [strToCopy] length (reaches [strToCopy] end),
        //adds n null-teminations to [destStr]
        if (strToCopy = '\0') 
            for (; n > 0 ; ++destStr)
                *destStr = '\0';

        *destStr = *strToCopy;
        strToCopy++;
        destStr++;
        n--;

        //stops copying when reaches [dest] end (overflow protection)
        if (*destStr == '\0')
            n = 0; //exits loop
    }

    return save;
}

/////////////////////////////////////////////

int main()
{
    char st1[] = "ABC";
    char *st2;
    char *st3 = "ZZZZZ";
    st2 = (char *)malloc(5 * sizeof(char));


    printf("Should be: ZZZZZ\n");
    st3 = strncpy(st1, st3, 0);
    printf("%s\n", st3);

    printf("Should be: ABZZZZZ\n");
    st3 = strncpy(st1, st3, 2);
    printf("%s\n", st3);

    printf("Should be: ABCZZZZZ\n");
    st3 = strncpy(st1, st3, 3);
    printf("%s\n", st3);

    printf("Should be: ABC\n");
    st3 = strncpy(st1, st3, 4);
    printf("%s\n", st3);

    printf("Should be: AB\n");
    st2 = strncpy(st1, st2, 2);
    printf("%s\n", st2);

    printf("Should be: AB\n");
    st2 = strncpy(st1, st2, 4);
    printf("%s\n", st2);
}

Answer 1

You get a segmentation fault because

char *st3 = "ZZZZZ";

the destination is a string literal. String literals must not be modified, and often they are stored in write-protected memory. So when you call

strncpy(st1, st3, n);

with an n > 0, you are trying to modify the string literal and that results in a crash (not necessarily, but usually).

In the copy loop, you have forgotten to dereference strToCopy

if (strToCopy = '\0')

and wrote = instead of ==, so strToCopy is set to NULL, causing further dereferences of strToCopy to invoke undefined behaviour.