Why wont this simple C program print?

Question

I'm trying to make a basic C program to read from a file, but for some reason when I run it with make Test1 and then ./Test1 test1.txt I get "error: 's' may be used uninitialized in this function".

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

int main (int argc, char** argv) {


    if (argc < 2) {
        printf("error\n");
        return 0;
    }

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

    if (fp == NULL)  {
        printf ("error\n");
        return 0;
    }

    char * s;

    int r = fscanf(fp, "%s", s);

    while (r != EOF) {
        printf("%s\n", s);
        r = fscanf(fp, "%s", s);
    }

    return 0;
}

How do you start your program after you compile it? – Andrew Henle Feb 07 '18 at 01:13 — Andrew Henle, Feb 07 '18 at 01:13
Try add breakpoint and debug. – Zamrony P. Juhara Feb 07 '18 at 01:15 — Zamrony P. Juhara, Feb 07 '18 at 01:15
char * s isn't pointing to anything right now. – Josh Abraham Feb 07 '18 at 01:27 — Josh Abraham, Feb 07 '18 at 01:27

Severin Brunner · Answer 1 · 2018-02-07T01:41:15.900

1

The problem is with fscanf. It won't allocate any memory, you have to do this by yourself, e.g. by doing

char *s = malloc(100);  //array s is stored in the heap memory

or

char s[100];  //array s is stored in the stack

To make sure that fscanf won't read more than 100 characters (because that's how much memory we got) you have to write

int r = fscanf(fp, "%99s", s);

edited Feb 07 '18 at 01:41

answered Feb 07 '18 at 01:22

Severin Brunner

19
3

1

You had to leave one character for the null terminator, so `fscanf(fp, "%99s", s)`. – Iharob Al Asimi Feb 07 '18 at 01:26
1

[Don't cast the return of `malloc`](https://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc) – lurker Feb 07 '18 at 01:28

score 0 · Answer 2 · answered Feb 07 '18 at 01:13

0

It should be crashing as you are reading into s which is set to NULL, meaning there is no memory you have set aside to read into and it should crash your program, depending on OS depends on error

doing char s[1000] at least gives it some memory to read into.

answered Feb 07 '18 at 01:13

Keith Nicholas

43,549
15
93
156

Yeah I was still testing code when I uploaded it. Removed the NULL, but I still get told that "s may be uninitialized". – AlanGhalan Feb 07 '18 at 01:18
@AlanGhalan, `s` IS uninitialized. You need to initialize it. – Iharob Al Asimi Feb 07 '18 at 01:26
@lurker for some reason the OP changed the code, if you check the history it was assigned NULL – Keith Nicholas Feb 07 '18 at 01:31
@lurker As it appears it was, and the OP changed it. Read [this comment](https://stackoverflow.com/questions/48654448/why-wont-this-simple-c-program-print#comment84307136_48654473). – Iharob Al Asimi Feb 07 '18 at 01:31
I see, thanks. I was a little late to the party. – lurker Feb 07 '18 at 01:53

Why wont this simple C program print?

2 Answers2