tac

The tac Command

tac is cat spelled backward — it prints lines in reverse order:

$ printf "a\nb\nc\n" | tac
c
b
a

The last line becomes the first. The characters within each line are unchanged.

Your Implementation

Write void my_tac(const char *s) that prints lines in reverse order.

The challenge: you do not know the last line until you have scanned through the entire string. You need to store the line positions first, then print them in reverse.

void my_tac(const char *s) {
    const char *starts[64];   // pointer to start of each line
    int count = 0;

    const char *p = s;
    const char *line_start = s;

    while (*p) {
        if (*p == '\n') {
            starts[count++] = line_start;
            line_start = p + 1;
        }
        p++;
    }

    for (int i = count - 1; i >= 0; i--) {
        const char *q = starts[i];
        while (*q && *q != '\n') { putchar(*q); q++; }
        putchar('\n');
    }
}

Pointers as Array Entries

starts[count] = line_start stores a pointer (a memory address) in an array. Later, we use const char *q = starts[i] and walk forward with q++ until we hit '\n'.

This is a key C pattern: an array of pointers pointing into a larger buffer. It is how argv works — argv[0], argv[1], etc. are pointers into the program's argument memory.

Your Task

Implement my_tac that prints lines in reverse order. You can assume the input has at most 64 lines.