How to create a file in a function and use it in other functions

Question

The file is created, but the function WriteFile can't write in the file, I need to create the file in a function not in the main.

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

    void CreateFile(FILE *file){
        file = fopen("file.txt","w");
        if(file == NULL){
            printf("Error : Could not create the file");
        }
    }
   void WriteFile(FILE *file){
        fprintf(file,"Hello !");
    }

    int main()
    {
        FILE *file = NULL;
        CreateFile(file);
        WriteFile(file);
        fclose(file);
        return 0;
    }

The value of `file` is "forgotten" right after `CreateFile` is returning. — Eugene Sh., Sep 21 '17 at 15:22
You should define the function like this (following @Eugene Sh.'s comment) `FILE *CreateFile(FILE *file)`. This function will return the file descriptor which you can use it later . — kiner_shah, Sep 21 '17 at 15:24
Pass it a `FILE **` parameter. Or return the file descriptor as a return value. — Eugene Sh., Sep 21 '17 at 15:25
@EugeneSh.: Why make a 2-star programmer where 1 is sufficient? — too honest for this site, Sep 21 '17 at 15:36
Possible duplicate of [Passing by reference in C](https://stackoverflow.com/questions/2229498/passing-by-reference-in-c) — Andrew Henle, Sep 21 '17 at 16:04

Hans Petter Taugbøl Kragset · Answer 1 · 2017-09-21T15:37:56.313

0

As mentioned by Eugene Sh, you don't keep the pointer to the file you create. This fixes it:

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

FILE *CreateFile(void){
    FILE *file = fopen("file.txt","w");
    if(file == NULL){
        printf("Error : Could not create the file");
        exit(1);
    }
    return file;
}

void WriteFile(FILE *file){
    fprintf(file, "Hello !");
}

int main(void)
{
    FILE *file = CreateFile();
    WriteFile(file);
    fclose(file);
    return 0;
}

Or, if you want to send the variable file from main into the function, use a double pointer:

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

void CreateFile(FILE **file){
    *file = fopen("file.txt","w");
    if(*file == NULL){
        printf("Error : Could not create the file");
    }
}

void WriteFile(FILE *file){
    fprintf(file, "Hello !");
}

int main(void)
{
    FILE *file = NULL;
    CreateFile(&file);
    WriteFile(file);
    fclose(file);
    return 0;
}

edited Sep 21 '17 at 15:37

answered Sep 21 '17 at 15:25

Hans Petter Taugbøl Kragset

959
6
17

*minor*: it should be `FILE *CreateFile(void)` – Eugene Sh. Sep 21 '17 at 15:26
OP didnt have void as return value on fucntions so I stuck with a "modern" C style and excluded void there... – Hans Petter Taugbøl Kragset Sep 21 '17 at 15:27
Not sure what you mean by "modern", but the "modern" standard does require `void` if a function takes no arguments. – Eugene Sh. Sep 21 '17 at 15:30
I've always compiled my C code without `void`... gcc 4.4.7 is the oldest I've used. – Hans Petter Taugbøl Kragset Sep 21 '17 at 15:32
Hehe thanks, I knew strictly that it is required, but I came to C from Java and like(d) the visuals of empty parentheses ;) – Hans Petter Taugbøl Kragset Sep 21 '17 at 15:36
Thanks @Olaf, I should avoid the use of "reference" as a literal English word in this context. – Hans Petter Taugbøl Kragset Sep 21 '17 at 15:38

score 0 · Answer 2 · answered Sep 21 '17 at 15:31

The problem is not the file, it's your use of the pointers. In C, function parameters are passed by value : It means that in your code when you call CreateFile(file); you are giving the value of file, which is NULL. You have two possibilities to fix this, you can use a double pointer :

void CreateFile(FILE **file);

Calling the function like this :

FILE *file;
CreateFile(&file);

Or you can return the value from CreateFile() :

FILE *CreateFile(void);
FILE *file = CreateFile();

(Edit : I did not see @Hans Petter Taugbøl Kragset's answer ...)

How to create a file in a function and use it in other functions

2 Answers2