C undefined behaviour with fscanf

Question

I have a code snippet below. On macOS, I have run it in Xcode and CLion with the same odd result. On the other hand, on Linux when compiled with gcc it runs flawlessly. I would like to know whether the code at any point produces undefined behaviour. The input file it tries to parse is the vigenére table, you know, 26-character rows with the latin alphabet, and the letters are shifted left by 1 on the 1 lower row. Every line is terminated by CRLF. The expected output is the table printed out on the console. The unexpected part is that az least 1 line is displayed incorrectyl on macOS. Here’s the input btw: https://pastebin.com/QnucTAFs (I dont know however, whether the corresponding line endings got preserved)

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

char ** parse(char *path) {
    FILE *f = fopen(path, "r");
    char **table = (char**)malloc(sizeof(char*) * 26);
    int i = -1;

    do table[++i] = (char*)malloc(sizeof(char) * 27);
    while (fscanf(f, "%s", table[i]) > 0);

    return table;
}

int main() {
    char **table = parse("Vtabla.dat");

    for (int i = 0; i < 26; i++) {
        for (int x = 0; x < 26; x++)
            printf("%c", table[i][x]);
        printf("\n");
    }

    return 0;
}

Answer 1

There is a bug in this code, in this loop:

do table[++i] = (char*)malloc(sizeof(char) * 27);
while (fscanf(f, "%s", table[i]) > 0);

The table holds 26 pointers, but on the iteration that fscanf() fails, the table variable's 27th pointer gets initialized via malloc in the previous step. This corrupts the data in table on my system. You can convince yourself of this by bumping the 26 in this line to 27 and see if your problems disappear:

char **table = (char**)malloc(sizeof(char*) * 26);

My rework of the code:

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

#define LETTERS 26

char **parse(char *path) {
    char **table = calloc(LETTERS, sizeof(char *));

    FILE *f = fopen(path, "r");

    for (int i = 0; i < LETTERS; i++) {
        table[i] = (char *) calloc(LETTERS+1, sizeof(char));

        if (fscanf(f, "%s", table[i]) <= 0) {
            break;
        }
    }

    fclose(f);

    return table;
}

int main() {
    char **table = parse("Vtabla.dat");

    for (int i = 0; i < LETTERS; i++) {
        for (int j = 0; j < LETTERS; j++)
            printf("%c", table[i][j]);

        printf("\n");
        free(table[i]);
    }

    free(table);
    return 0;
}

Answer 2

The discussion in the comments have been lively, but the OP seems interested in a narrower concern that many experienced developers are, so I'll post an answer that's not strictly on point to the question, but demonstrates the wider concerns.

I believe many of us have skipped error checking for conditions that we consider highly unlikely, but "file not found" or "file is malformed" are not even close to that category. This attempts to address that, plus it closes the file after reading, plus it replaces a magic number ("26") with a constant.

While reading each input line, this will overflow the buffer if there happen to be too many characters, but I will leave this limit-check as an exercise to the reader.

Malformed user input is so common that it's imperative that one check it.

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

#define ALPHABET_SIZE  26

char ** parse(const char *path) {

    FILE *f = fopen(path, "r");

    if (f == 0)
        errx(EXIT_FAILURE, "Cannot open input file %s (err=%s)", path, strerror(errno));

    char **table = malloc(sizeof(char*) * ALPHABET_SIZE);
    int i = -1;

    do
    {
      // BUG: overflows the table - see cdlane's answer
      table[++i] = malloc(ALPHABET_SIZE + 1);

                         // TODO: what if line is too long? Or too short?
    } while (i < ALPHABET_SIZE  &&  fscanf(f, "%s", table[i]) > 0);

    if (i != ALPHABET_SIZE)
        errx(EXIT_FAILURE, "Not enough input lines");

    fclose(f);

    return table;
}

int main() {
    char **table = parse("Vtabla.dat");

    for (int i = 0; i < ALPHABET_SIZE; i++) {
        for (int x = 0; x < ALPHABET_SIZE; x++)
            printf("%c", table[i][x]);
        printf("\n");
    }

    return 0;
}