Program hangs after opening file with open and fdopen [duplicate]

Question

I'm using open() with O_RDWR and then using the descriptor in fdopen() with "r+". I check that the file exists first with access(), and check for open() returning -1 then check that the FILE *fp set from fdopen() isn't NULL.

Now whenever I use any function such as fgets() or fgetc() with fp the program hangs in my terminal. I get no compiler warnings. Any ideas?

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

int main(int argc, char **argv) {
    char buffer[512];
    int file_descriptor;
    FILE *file;

    if (argc != 2) {
        puts("ERROR: Missing argument");
        exit(0);
    }
    else if (access(argv[1], F_OK) == -1) { // Check if file exists
        puts("ERROR: File does not exist");
        exit(0);
    } else if (file_descriptor = open(argv[1], O_RDWR) == -1) { // Check for open() failure
        puts("ERROR: Unable to open file using open()");
        exit(0);
    }

    file = fdopen(file_descriptor, "r+"); // Use descriptor

    if (file == NULL) {
        puts("ERROR: Unable to open file using fdopen()");
        fclose(file);
        exit(0);
    }

    // fscanf(file, "%c", stdout); // Hangs
    // fgets(buffer, sizeof(buffer), file); // Hangs
    printf("%c", fgetc(file)); // Hangs

    fclose(file);

    return 0;
}

Answer 1

The expression

file_descriptor = open(argv[1], O_RDWR) == -1

does not work as you expect it to, because the equality operator == have higher precedence than the assignment operator. That means the expression is more like

file_descriptor = (open(argv[1], O_RDWR) == -1)

which means your file_descriptor will be either 0 or 1 depending on how the comparison went.

You need to use parentheses for the assignment:

(file_descriptor = open(argv[1], O_RDWR)) == -1

Answer 2

To add to the other answer. It's "hanging" because the bug causes it actually to read standard input (file descriptor 0). Try typing something and it will read it.