w+ not working when trying to read file content

Question

Code:

#include <stdio.h>

void main() {
    FILE *ptr;
    char buff[255];
    ptr = fopen("Test.txt", "w+");

    if (ptr != NULL) {
        printf("Success\n");
    }

    fputs("Hello", ptr);

    fgets(buff, 255, (FILE *)ptr);
    printf("%s", buff);
    fclose(ptr);
}

The file "Text.txt" has the content "Hello" when I opened it, but I just can't print it out with fgets. What did I do wrong here?

Answer 1

There are multiple problems in your code:

• You must issue a call to fseek(), fsetpos() or rewind() to switch between writing to and reading from the stream, and vice versa.

• The prototype for main without arguments is int main(void).

• There is no need to cast ptr in fgets(buff, 255, (FILE *)ptr);. Useless casts can hide type mismatches and other similar bugs.

• You do not test the return value of fgets() before passing buff to printf(). This has undefined behavior if fgets() fails, a it does in your case.

• You do test the return value of fopen(), but still pass a potentially null ptr to other stream functions, invoking undefined behavior.

Here is a corrected version:

#include <stdio.h>

int main(void) {
    FILE *ptr;
    char buff[255];

    ptr = fopen("Test.txt", "w+");
    if (ptr == NULL) {
        printf("Cannot open Test.txt\n");
        return 1;
    }
    printf("Success\n");

    fputs("Hello", ptr);
    rewind(ptr);
    if (fgets(buff, sizeof buff, ptr)) {
        printf("%s", buff);
    } else {
        printf("Cannot read from stream\n");
    }
    fclose(ptr);
    return 0;
}