Can't I use fscanf() for infinite time?

Question

#include <stdio.h>

main() {
    int n;
    FILE *file;
    printf("We are here to create a file!\n");
    file = fopen("demo.txt", "w+");
    if (file != NULL)
        printf("Succesfully opened!");

    printf("Enter number\n");
    while (scanf("%d", &n)) {
        fprintf(file, "%d", n);
    }
    fclose(file);
}

why fscanf() is not working here? Here scanf is working properly but fscanf() is not responding or working here. Can anyone explain what the problem is?

Answer 1

Your code has some problems:

• the prototype for main without arguments is int main(void)
• you do not exit the program if the file cannot be opened. You will have undefined behavior if fopen returns NULL because you later pass this null pointer to fprintf .
• the loop iterates until scanf() returns 0 . You should instead iterate while scanf() returns 1 . scanf() will return EOF if it fails at end of file, causing an infinite loop.
• you should probably output a separator after the number in fprintf() otherwise all numbers are going to be clumped together forming a long sequence of digits.
• main() should return 0 or an error status

Here is a corrected version:

#include <stdio.h>

int main(void) {
    int n;
    FILE *file;

    printf("We are here to create a file\n");
    file = fopen("demo.txt", "w");
    if (file != NULL) {
        printf("Successfully opened\n");
    } else {
        printf("Cannot open demo.txt\n");
        return 1;
    }
    printf("Enter numbers\n");
    while (scanf("%d", &n) == 1) {
        fprintf(file, "%d\n", n);
    }
    fclose(file);
    return 0;
}

Regarding your question: why can't I use fscanf() instead of scanf() ?

• you can use fscanf() as long as you give it a stream pointer opened for reading: if you write while (fscanf(stdin, "%d", &n) == 1) the program will behave the same way.
• if you want fscanf() to read from file , you need to perform a file positioning command between read and write operations, such as rewind() of fseek() . Yet fscanf() will fail if there is no number to read at the current position in the file and since you open file with "w+" mode, fopen() will be truncated it.

You could cause an infinite loop by writing a number to the file, rewinding it to the beginning and re-reading the same number, etc.

Here is some code for illustration:

#include <stdio.h>

int main(void) {
    int n;
    FILE *file;

    printf("We are here to create a file\n");
    file = fopen("demo.txt", "w+");
    if (file != NULL) {
        printf("Successfully opened\n");
    } else {
        printf("Cannot open demo.txt\n");
        return 1;
    }
    printf("Enter a number: ");
    if (scanf("%d", &n) == 1) {
        fprintf(file, "%d\n", n);
        rewind(file);
        while (fscanf(file, "%d", &n) == 1) {
            printf("read %d from the file\n", n);
            if (n == 0)
                break;
            rewind(file);
            fprintf(file, "%d\n", n >> 1);
            rewind(file);
        }
    }
    fclose(file);
    return 0;
}

Interaction:

We are here to create a file
Successfully opened
Enter a number: 10
read 10 from the file
read 5 from the file
read 2 from the file
read 1 from the file
read 0 from the file