Fscanf failiure

Question

I need to get name from the file (string).

But certainly it gets (null) value.

void dealer__init(){
    FILE * dealer = fopen(DEALER_L, "r");
    fscanf(dealer, "%d  %d  %255s", &my_og.money, &my_og.check_ok, my_og.name);
    printf("%255s\n", my_og.name);
    printf("%d\n", my_og.money);
    printf("%d\n", my_og.check_ok);
    
    if(money_at_beg == true && my_og.check_ok == 0){
        my_og.money = MONEY_AT_BEG_INT;
        my_og.check_ok = 1;
        dealer__save();
    }
    printf("My name is set to: %s", &my_og.name);
    fclose(dealer);
}

My file:

3000  1  dimitar

My struct:

struct dealer{
    int money;
    int check_ok;
    char *name;
}my_og;

What does your `char* name` structure element point to? Where do you initialize it to point to some *actual* memory? — Adrian Mole, Jun 04 '22 at 17:17
i don't allocate it, because when i scanf it it everything works - the problem comes when u fscan it — Димитър Милчев, Jun 04 '22 at 17:21
We don' t know what `my_og` is and how it is initialised, so at this oint we can only play guessing games. Please post a [mcve]. — n. m. could be an AI, Jun 04 '22 at 17:32
If you know what the problem is, why did you post the question? @AdrianMole has given you an answer. Please review [ask] and post an [mcve]. — jwdonahue, Jun 04 '22 at 18:25

score 3 · Answer 1 · answered Jun 04 '22 at 17:27

You can't use the fscanf function (or most other functions)¹ to read data into an uninitialized – or invalid – pointer. That causes undefined behaviour which, in your case, appears to go unnoticed until you try to print out the value after the attempted read. (This is one of the nastier aspects of UB: that the code appears to work.)

You need to either explicitly allocate some memory to which your pointer refers to, like this:

my_og.name = malloc(256); // Allocate some memory

Or, perhaps simpler, declare the name member as a fixed-size array, rather than a pointer:

struct dealer{
    int money;
    int check_ok;
    char name[256]; // 'allocated' when the structure is declared
}my_og;

¹ There is, I believe a non-standard (POSIX) version of the scanf formatting option (%ms) that automatically allocates memory, but I would personally avoid such things like the plague. See: scanf dynamic allocation.

Appreciate it ! – Димитър Милчев Jun 06 '22 at 17:07 — Димитър Милчев, Jun 06 '22 at 17:07

Fscanf failiure

1 Answers1