Segmentation fault the second time printf is called

Question

I'm writing a simple C program that opens a file, and reads each line into an array, where LISTS is the maximum number of lines in the file that I wish to read.

However, when MASTER_LIST is less than LISTS lines long, I get a segmentation fault on printf() the second time round, but not the first (I've commented to show where).

I'm a little confused by this behaviour, and I'm wondering what causes it and what I can do to circumvent it. Ideally I would stop reading when fgets finds the end of the file.

#define MASTER_LIST "master_list.txt"
#define LINE 64
#define LISTS 32

char **lists = malloc(LISTS * sizeof(char *));
int i; 
for (i = 0; i < LISTS; i++) {
    lists[i] = malloc(LINE * sizeof(char));
}

/*Open the file for reading.*/
FILE *fp = fopen(MASTER_LIST, "r");
if (fp != NULL) {
    /*Each line of the file, up to LISTS is read into lists.*/
    for (i = 0; i < LISTS; i++){

        lists[i] = fgets(lists[i], LINE, fp);
        /*NO SEGFAULT HERE*/ printf("Line Read: %s\n", lists[i]);                   
    }
}

/*print out each line*/
for(i = 0; i < LISTS; i++){
    printf("Are we segfaulting yet? %d\n", i);
    /*HERE I GET A SEGFAULT*/ printf("%s\n", lists[i]);
    printf("How about now? %d\n", i);       
}

Answer 1

fgets returns NULL if it fails to read any characters, which would happen if your file contains less than LISTS lines. Attempting to printf a NULL pointer is undefined behaviour.

---

This is a good time to remind ourselves that undefined behaviour really is undefined. It looks like the printf should crash the first time around. But, the C spec says nothing about what should happen, so your printf just prints (null) (many Linux libcs do this, for example).

Why does the second printf crash, then? It's because you used the pattern

printf("%s\n", lists[i]);

Many compilers will optimize this to

puts(lists[i]);

and on your system, puts does not check for a NULL pointer and so it segfaults.

Moral of the story? Don't ever rely on undefined behaviour, or even expect consistent results.