fscanf not scanning any numbers

Question

I am currently developing a simple C application. It takes a single file as a command line argument, which is formatted like:

1,2,3
4,5,6
7,8,9
etc.

However, for whatever reason, fscanf never scans the numbers! Here is an example:

#include <stdio.h>

int main(int argc, char **argv) {
    FILE *file = fopen(*argv, "r");
    int i1, i2, i3;
    while (fscanf(file, "%d,%d,%d", &i1, &i2, &i3) == 3) {
        printf("Doing stuff with %d, %d, and %d...\n", i1, i2, i3);
    }
    fclose(file);
    return 0;
}

If you run it with the filename as an argument, then it immediately exits, due to fscanf returning 0. I've tried several variations of this, to no avail. How do I make fscanf read the numbers correctly?

`fopen(*argv, "r")` --> `fopen(*++argv, "r")` or `fopen(argv[1], "r")` — BLUEPIXY, Jan 27 '16 at 13:29
@SouravGhosh For my purposes, the file can be assumed to exist. — LegionMammal978, Jan 27 '16 at 13:30
It is simply a misunderstanding. (it contains the command in C.) It is often the case. — BLUEPIXY, Jan 27 '16 at 13:32
@SouravGhosh While you have a general point -- for chrissake, always check return values! --, it wouldn't help here! The program can open `argv[0]` alright, alas it will only be able to scan three comma separated numbers from those bytes under exceptional circumstances ;-). — Peter - Reinstate Monica, Jan 27 '16 at 13:35

score 2 · Answer 1 · answered Jan 27 '16 at 14:33

2

As noted by BLUEPIXY, you should use the second element of the argv array: argv[1]:

FILE *file = fopen(argv[1], "r");

The first element (argv[0] or *argv) is the name of the program being executed - it's not the right file to open.

answered Jan 27 '16 at 14:33

anatolyg

26,506
9
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chux - Reinstate Monica · Accepted Answer · 2016-01-27T15:15:15.167

Superficial answer: wrong file was opened as code should have used argv[1] rather than *argv.

Let us look deeper.

Code had troubles and lacked error checking in at least 2 places.

FILE *file = fopen(*argv, "r"); was not followed by a test on file. This classic check would not have detected OP's problem as the file, the executable, was open-able.
The return value from fscanf(file, "%d,%d,%d", &i1, &i2, &i3) was only lightly tested. Return values of EOF 0 1 2 3 were possible yet only EOF 3 were expected. Had code tested for non-EOF 3, the problem would have rapidly been found.

Lesson to be learned: Insure code, especially mis-behaving code, has adequate error checking. Saves the coder time in the long run.

#include <stdio.h>

int main(int argc, char **argv) {
  if (argc != 2) {
    fprintf(stderr, "Unexpected argument count %d.\n", argc);
    return 1;
  } 
  FILE *file = fopen(argv[1], "r");
  if (file == NULL) {
    fprintf(stderr, "Unable to open file: \"%s\"", argv[1]);
    return 1;
  } 
  int i1, i2, i3;
  int n;
  while ((n = fscanf(file, "%d,%d,%d", &i1, &i2, &i3)) == 3) {
    printf("Doing stuff with %d, %d, and %d...\n", i1, i2, i3);
  }
  if (n != EOF) {
    fprintf(stderr, "Unexpected scan failure count %d\n", n);
    return 1;
  }
  fclose(file);
  return 0;
}

Most of this was checked in earlier iterations. I checked the file (it wasn't `NULL`), I looked at the return values of `fscanf` (it kept returning `0`), and checking the values of the `in` (they never changed.) — LegionMammal978, Jan 27 '16 at 22:11

fscanf not scanning any numbers

2 Answers2