File descriptors
- C course
- Advanced
- Lesson 3/83
- 5 min
Learn the file descriptor concept and management.
File descriptors
A file descriptor (fd) is a small non-negative integer representing an open file. The operating system keeps a descriptor table for each process.
Every process starts with three standard descriptors open: 0 for stdin, 1 for stdout and 2 for stderr. The first newly opened file gets 3. open returns a new descriptor; close releases it.
STDIN_FILENO and friends
unistd.h defines symbolic constants for standard descriptors: STDIN_FILENO (0), STDOUT_FILENO (1) and STDERR_FILENO (2).
These constants replace magic numbers and make code more readable. For example, write(STDOUT_FILENO, msg, n) and write(1, msg, n) are the same, but the first is clearer. New files opened with open get descriptors 3 and above.
Practice Print standard descriptors
Print the values of STDIN_FILENO, STDOUT_FILENO and STDERR_FILENO with printf("%d %d %d\n", ...), separated by spaces. The output must be 0 1 2.
Starter code
#include <stdio.h>
#include <unistd.h>
// Print the three standard descriptors
int main(void) {
return 0;
}Sample solution
#include <stdio.h>
#include <unistd.h>
int main(void) {
printf("%d %d %d\n", STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO);
return 0;
}Expected output
0 1 2
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Print the standard descriptor
int fd = STDIN_FILENO;
printf("%d\n", ____);int fd = STDIN_FILENO;
printf("%d\n", fd);Explanation: You printed the fd variable.
2 Match descriptor values
| Term | Meaning |
|---|---|
0 |
stdin |
1 |
stdout |
2 |
stderr |
3 |
First open file |
3 What is the file descriptor of standard error?
- 0
- 1
- 2 Correct answer
- 3
Explanation: The stderr descriptor is 2.