open and close

Learn file management with open and close.

The open function

open opens a file and returns a file descriptor. It has two forms: open(path, flags) and open(path, flags, mode). The second form is used with O_CREAT when creating a new file.

Flags specify how the file is opened: O_RDONLY read only, O_WRONLY write only, O_RDWR read and write. O_CREAT creates the file if it does not exist, O_TRUNC truncates it if it does. Multiple flags are combined with |.

close and error checking

close(fd) releases an open file descriptor. Always close when done; otherwise you leak descriptors and may hit the process limit.

open returns -1 on error and sets errno. On success it gives a non-negative descriptor. Always check the return value: catch errors with if (fd < 0). close can also fail; checking it is a good habit.

Exercise Apply open and close

Get a descriptor with open("/dev/null", O_WRONLY). Check for errors with if (fd < 0); on error print printf("hata\n") and return 1. On success close(fd) and print printf("tamam\n").

Show example solution

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

int main(void) {
    int fd = open("/dev/null", O_WRONLY);
    if (fd < 0) {
        printf("hata\n");
        return 1;
    }
    close(fd);
    printf("tamam\n");
    return 0;
}

Test yourself

Match open flags

O_RDONLY

Read only
O_WRONLY

Write only
O_RDWR

Read and write
O_CREAT

Create file
What does open return on failure?
- 0
- 1
- A file descriptor
- -1
Show answer

-1 — open returns -1 on error and sets errno.

File permission

int fd = open("/dev/null", O_WRONLY | O_CREAT, ____);

Show answer

int fd = open("/dev/null", O_WRONLY | O_CREAT, 0644);

Next lesson: read and write

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