if(fp != EOF) -> Warning: Comparison between pointer and integer

Question

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

int main()
{
    FILE *fp;

   char written[]= "This is not as easy as I thought it would be.";
    fp = fopen("Aufgabe 3.txt", "w");
    if(fp != EOF)
    {
        fprintf(fp, "%s", written);
        printf("Text was written!\n");
    }
    else
    {
        printf("File can not be found!");
    }
    fclose(fp);

    return 0;
}

Hi, I am new to coding and need a little help. :D

Does anyone know how to get rid of the warning, I simply want to write a sentence into a .txt .

Warning in line: 10 if(fp != EOF):

comparison between pointer and integer

Try using `if(fp != NULL)` to check whether opening the file was successful — JCWasmx86, Jul 27 '20 at 07:52

score 2 · Answer 1 · answered Jul 27 '20 at 07:53

The fopen function returns a pointer to the newly opened file stream or, on error, a NULL pointer (which is not the same as the EOF integer constant). Use this, instead:

    if(fp != NULL)
    {
        fprintf(fp, "%s", written);
        printf("Text was written!\n");
    }
    else
    {
        printf("File can not be found!");
    }

August Karlstrom · Answer 2 · 2020-07-27T09:27:32.403

The variable fp is a pointer so you should compare it with NULL, not EOF which is an integer (see the reference documentation for fopen: https://pubs.opengroup.org/onlinepubs/9699919799/). Also

you should write a newline character to the file (in order for it to be a text file)
errors should be printed to the standard error stream
it's a good idea to print the cause of why opening the file failed
the program should exit with an error code upon failure
the call to fclose should only be made if opening the file was successful
since the variable written is not modified it can be turned into a pointer constant.

Here is a corrected version:

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

int main()
{
    const char *filename = "Aufgabe 3.txt";
    const char *written = "This is not as easy as I thought it would be.";
    FILE *fp;

    fp = fopen(filename, "w");
    if (fp != NULL) {
        fprintf(fp, "%s\n", written);
        printf("Text was written!\n");
        fclose(fp);
    } else {
        fprintf(stderr, "Opening file '%s' failed: %s:\n", filename, strerror(errno));
        exit(EXIT_FAILURE);
    }
    return 0;
}

RobertS supports Monica Cellio · Accepted Answer · 2020-07-27T09:20:14.330

EOF is a macro for a negative integer constant. fp is a pointer to FILE.

If you use

if (fp != EOF)

you compare a pointer with an integer of a non-zero value, which isn't permissible.

Replace EOF with NULL which is a macro to check for errors with pointers:

if (fp != NULL)

Side Notes:

Also you should only use fclose() on a pointer to a stream which was successfully opened. If you use flose(fp) if fp is NULL, the program has undefined behavior.
Furthermore with if (fp != NULL) you check if the opening of the stream was without errors and continue then into the ifs body when the opening of the stream to the file was successful. Else if the opening was not successful you go into the else body.

This code can be simplified.

Simply check whether an error occurred and if yes flow through an error routine in the if's body. If not, The control flow immediately continues to the code in main() after the if statement. No need for an else.
Your error routine should be enhanced in production code. Just printing out that the opening of the file wasn't successful isn't enough.

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

int main (void)
{
    FILE *fp;

    char written[] = "This is not as easy as I thought it would be.";

    fp = fopen("Aufgabe 3.txt", "w");
    if (fp == NULL)
    {
        perror("fopen: ");
        exit(EXIT_FAILURE);
    }

    fprintf(fp, "%s", written);
    printf("Text was written!\n");

    fclose(fp);

    return 0;
}

if(fp != EOF) -> Warning: Comparison between pointer and integer

3 Answers3