Handling an array of FILE in C

Question

I need to modify a program written in C, which I have very little experience with and I would like to handle an array of files. However, I am experiencing a problem of handling with pointers when I want to write into these files...

Here's the minimal code to see what I want to do:

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

void func(FILE *f){
    fprintf(f, "test\n");
}

void func_1(FILE *f){
    fprintf(f[0], "test\n");
}

void func_2(FILE *f){
    fprintf(f[0][0], "test\n");
}

int main() {
    FILE *f = fopen("test.txt", "w");
    func_1(f);
    fclose(f);

    FILE *f_1[2];
    f_1[0] = fopen("test.txt", "w");
    f_1[1] = fopen("test_2.txt", "w");
    func_1(f_1);
    fclose(f_1[0]);
    fclose(f_1[1]);

    FILE *f_2[2][2];
    fprintf(f[0][0], "test\n");
    f_2[0][0] = fopen("test.txt", "w");
    f_2[1][1] = fopen("test_2.txt", "w");
    func_2(f_2);
    fclose(f_2[0][0]);
    fclose(f_2[1][1]);
}

Basically, the first part with f is working but not when I am creating f_1 or f_2 which are arrays of files. How to handle them to write into some of them specifically.

Sincerely, Paul

EDIT: As noted my minimal code is not running and is confusing. It is my bad, I continue on trying other things and forgot to remove the test modifications.

f[0,0] is the same as f[0]. Please enable compiler warnings and listen to them. — KamilCuk
– KamilCuk, Commented Feb 29, 2020 at 22:07
Can you elaborate on "not working"? What exactly is the expected behavior and actual behavior? — kaylum
– kaylum, Commented Feb 29, 2020 at 22:08
func_1(f_1); and fprintf(f[0,0], "test\n"); and func_2(f_2); are not correct. your program will not compile — Landstalker
– Landstalker, Commented Feb 29, 2020 at 22:10

selbie · Accepted Answer · 2020-03-01 04:49:46Z

0

Instead of multiple func functions, You just need a single function that takes a FILE* array and an index. Individual files are passed as FILE*, so an array of files is passed as FILE**.

void func(FILE** file_array, unsigned int index) {
    FILE* f = file_array[index];
    if (f)
    {
        fprintf(f, "test\n");
    }
}

Then to invoke:

FILE *files[2];
files[0] = fopen("test.txt", "w");
files[1] = fopen("test_2.txt", "w");
func(files,0);
func(files,1);
fclose(files[0]);
fclose(files[1]);

You can also invoke func when all you have is a single FILE* instance. That is:

FILE* f = fopen("test.txt", "w");
func(&f, 0);

For your 2-d array, I'm not sure what you are wanting. But this:

FILE *f_2[2][2];
fprintf(f[0][0], "test\n");

Is undefined behavior. You are invoking fprintf with an uninitialized file handle.

edited Mar 1, 2020 at 4:49

answered Mar 1, 2020 at 4:43

selbie

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