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I am able to write the following main program:

int main(){
#include <stdio.h>
#include <stdlib.h>

FILE *fpointer = fopen("file.txt", "r+");

if (fpointer == NULL){
    printf("file does not exist!\n");
    return EXIT_FAILURE;

fclose(fpointer);
return 0;
}

However, I am not able to split this into a new function (open file and check for existince). I want to call the new function checkFileExistence(). This is how I tried to write it:

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

int checkFileExistence(char* filepath){

        FILE* file=fopen(filepath,"w");

        if (file == NULL){
                printf("File does not exist!");
                return EXIT_FAILURE;
        }

        return EXIT_SUCCESS;
}

int main(){

        char* filepath = "test.txt";
        FILE* new_file_pointer=checkFileExistence(filepath);

        fclose(new_file_pointer);

        return 0;
}

But I keep getting errors for hours now. The error at this point is:

file.c:19:8: warning: incompatible integer to pointer conversion initializing 'FILE *'
      (aka 'struct __sFILE *') with an expression of type 'int' [-Wint-conversion]
        FILE* new_file_pointer=checkFileExistence(filepath);

How do I code correctly? I just want to use an own function to check the existence of a file without putting everything into the main function. Thank you in advance for the answer. I looked into the theory of data types, but still aren't able to use this correctly, because the pointer are somehow confusing me. Thank you very much. I love this platform and always appreciate all answers!

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11
  • Why did you change the mode argument from "r+" in your first (successful) code to "w" in your second attempt? That would make a difference, for sure. Commented Mar 8, 2021 at 17:40
  • Also, this may be useful: stackoverflow.com/q/13508712/10871073 Commented Mar 8, 2021 at 17:43
  • Hello, thank you for your answer. I am pretty new to this topic and I just want to check the existence of a file. I want to check it regardless of the mode (sometimes I need to open it, sometimes I need to write to a file). If I change the mode, I keep getting the same error, did I do something wrong ? Commented Mar 8, 2021 at 17:44
  • 1
    You need to double check your return types and what you expect to be returned from your function call to checkFileExistence. Commented Mar 8, 2021 at 17:47
  • 1
    By the way, EXIT_SUCCESS and EXIT_FAILURE are meant to be used for exit codes, not normal function returns. In fact, you'd probably want 1 for true and 0 for false, yet EXIT_SUCCESS is defined as 0 and EXIT_FAILURE is defined as 1. Commented Mar 8, 2021 at 18:05

2 Answers 2

Reset to default
1

Hmmmm.... if you #include <stdio.h> inside the body of main you'll probably run into trouble, as that limits the scope of all definitions made in stdio.h to the body of main. So you'll have to include it again if you need the stdio.h definitions in a different function you write after main. But this will not work, as stdio.h defines a flag that avoids it to be included again in the same compilation unit, to prevent double definition errors. So, never #include <stdio.h> inside a function or a block delimited by { and }.

Anyway, you have several problems in your second snippet of code:

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

int checkFileExistence(char* filepath){

if you return int, you cannot assign it to a FILE * variable, as you have in your main routine. Better use FILE * checkFileExistence(char* filepath) as your prototype for the function.


        FILE* file=fopen(filepath,"w");

        if (file == NULL){
                printf("File does not exist!");
                return EXIT_FAILURE;

EXIT_FAILURE is an integer, but you want to return a pointer. It should be better to just exit(EXIT_FAILURE); so the program is finished (if you cannot open the file, what else you can do?) or return NULL, to indicate some trouble in the opening of the file to the calling code.

        }

        return EXIT_SUCCESS;

You should return the FILE * obtained from fopen, or you will have nothing to operate upon in main. Better write return file;.

}

int main(){

        char* filepath = "test.txt";
        FILE* new_file_pointer=checkFileExistence(filepath);

        fclose(new_file_pointer);

        return 0;
}

So your file, after editing, should be something like:

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

FILE *checkFileExistence(char* filepath){

        FILE* file=fopen(filepath,"w");

        if (file == NULL){
                printf("File does not exist!");
                exit(EXIT_FAILURE);
                /* or return NULL; */
        }

        return file;
}

int main(){

        char* filepath = "test.txt";
        FILE* new_file_pointer=checkFileExistence(filepath);

        /* it should be convenient you say something, as you'll get the file
         * opened and closed without you seeing anything at the terminal. */
        if (new_file_pointer != NULL) {
            printf("File opened successfully\n");
        }

        fclose(new_file_pointer);

        return 0;
}
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3 Comments

Since #include is a preprocessor directive, wouldn't there not really be any scope to it (though yes you wouldn't be able to use stuff in the header from before you #includeed)? Furthermore, IMHO returning a FILE * just seems weird, since checking for the NULL would probably just be as long as checking from fopen directly. Even if you exit instead of returning NULL, it seems like it could be done with a simple if statement outside any function again. Honestly, the more I look at OP's program and the function, the better it seems to just not use any function at all.
Actually, nevermind the #include thing, I just tested and putting a function declaration in another actually works. Hadn't known about that :P
Thank you, this solved lots of my questions !
1

Your return type from checkFileExistence() is int. But in main(), you are assinging it to a variable of datatype FILE.

Try changing

FILE* new_file_pointer=checkFileExistence(filepath);

to

int existance_status = checkFileExistence(filepath);

but this would cause a variable scope error, you might want to look out for that too!

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11 Comments

Hello, wow this looks like really clean code. Thank you, it looks like you found the cause of my issue. Is it possible to change more of my code fragments so that I don't get any errors? Thank you in advance
If I change existance_status to int type, I can't close the file, do I ? Just want a simple solution to get a working function, which checks for the existence of a file
@kingkonzgg is there any reason why you don't want to close the file inside checkFileExistance too?
@kingkonzgg I see. Personally, I would just do all of this outside any function unless I'm opening a lot of files at once. But if you really do want to keep it in a function and don't want to use any other functions (if you don't mind, this post may have some nice suggestions), I'd honestly just suggest you open it twice. It doesn't seem like your program would really suffer too much.
@kingkonzgg Are you talking about this "Personally, I would just do all of this outside any function unless I'm opening a lot of files at once"? If so, nothing much for me to code, really. I'm simply saying to do everything in your function directly in the main function, so open the file outside the function and do the if statement outside too (to check if it doesn't exist). If it doesn't exist, do something to handle the issue or exit. If it exists, all's good and you can continue onward. This way, it wouldn't possibly have to open twice.
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