open and close
- C course
- Advanced
- Lesson 4/83
- 5 min
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.
Practice 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").
Starter code
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
// Open /dev/null
// Check for errors
// Close and print tamam
int main(void) {
return 0;
}Sample 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;
}Expected output
tamam
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match open flags
| Term | Meaning |
|---|---|
O_RDONLY |
Read only |
O_WRONLY |
Write only |
O_RDWR |
Read and write |
O_CREAT |
Create file |
2 What does open return on failure?
- 0
- 1
- A file descriptor
- -1 Correct answer
Explanation: open returns -1 on error and sets errno.
3 File permission
int fd = open("/dev/null", O_WRONLY | O_CREAT, ____);int fd = open("/dev/null", O_WRONLY | O_CREAT, 0644);Explanation: 0644 gives the owner read-write and others read.