Does fscanf move the file pointer backwards?

Question

These are the contents of my file, 'unsorted.txt' :

3 robert justin trump

This is my code:

#include <stdio.h>

int main(void) {
    FILE *f = fopen("unsorted.txt", "r");
    char n;
    printf("%d\n", ftell(f));
    fscanf(f, "%s", &n);
    int l = n - '0';
    printf("%d %d\n", l, ftell(f));
    return 0;
}

on execution it gives the following output:

0
3 -1

why did it return -1 in second case? It should move from 0 to 1 right?

NOTE: the file can be opened, because then how would it print 0 in the first call and the first character from the file without being able to be opened?

Answer 1

 fscanf(f,"%s",&n);

is very wrong, since you declared char n; (of only one byte). You got undefined behavior. Be very scared (and next time, be ashamed).

I recommend:

Test that fopen don't fail:

FILE *f = fopen("unsorted.txt","r");
if (!f)  { perror("fopen unsorted.txt"); exit(EXIT_FAILURE); };

Declare a buffer of reasonable size (80 was the size of punched cards in the 1970s).

char buf[80];

clear it (you want defensive programming):

memset(buf, 0, sizeof(buf));

Then read carefully about fscanf. Read that documentation several times. Use it with a fixed size and test its result:

if (fscanf(f, "%72s", buf) > 0) {

^{(72 was the usable size in PL/1 programs of punched cards; it is less than 80)}

Don't forget to read documentation of other functions, including ftell.

Important hint:

compile with all warnings and debug info (gcc -Wall -Wextra -g with GCC), improve the code to get no warnings, use the debugger gdb to run it step by step.

^{PS. As an exercise, find the possible content of unsorted.txt which made your initial program run correctly. Could you in that case predict its output? If not, why??}

Answer 2

There are multiple problems in your code:

• You do not test the return value of fopen(). Calling ftell() with a NULL pointer has undefined behavior. You cannot draw conclusions from observed behavior.

• printf("%d\n", ftell(f)); is incorrect because the return value of ftell() is a long. You should use the format %ld.

• fscanf(f, "%s", &n); is incorrect because you pass the address of a single char for fscanf() to store a null-terminated string. fscanf() will access memory beyond the size of the char, which has undefined behavior. Define an array of char such as char buf[80]; and pass the maximum number of characters to store as: fscanf(f, "%79s", buf); and check the return value, or use %c to read a single byte.

• int l = n - '0'; is not strictly incorrect, but it is error prone: avoid naming a variable l as it looks confusingly similar to 1.

• printf("%d %d\n", l, ftell(f)); is incorrect as the previous call to printf: use the conversion specifier %ld for the return value of ftell().

Note also that the return value of ftell() on a text stream is not necessarily the byte offset in the file.

Here is a corrected version:

#include <stdio.h>

int main(void) {
    FILE *f = fopen("unsorted.txt", "r");
    char c;

    if (f != NULL) {
        printf("%ld\n", ftell(f));
        if (fscanf(f, "%c", &c) == 1) {
            int diff = c - '0';
            printf("%d %ld\n", diff, ftell(f));
        }
    }
    return 0;
}

Output:

0
3 1