Non-blocking input

Learn to read non-blocking input.

Non-blocking input

Non-blocking input prevents read from waiting when there is no data. You put the descriptor into non-blocking mode with fcntl(fd, F_SETFL, O_NONBLOCK). After this, read returns -1 if there is no data and errno becomes EAGAIN.

This is a foundational pattern for games and interactive apps. Once STDIN_FILENO is non-blocking, you can read in a loop; if there is no data, you do other work. For testing, printf("nonblock: 1\n") is enough; it is deterministic. In real apps, read is called in a loop and EAGAIN is checked.

Handling EAGAIN

A non-blocking read returns -1 if there is no data. The errno value becomes EAGAIN (or EWOULDBLOCK). This is not an error; it just signals that there is no data right now.

The correct pattern: ssize_t n = read(fd, buf, sizeof(buf)); if (n < 0 && errno == EAGAIN) { /* no data, continue */ }. So the program does other work instead of waiting when there is no data. For testing, just calling fcntl and printf is enough; the real read loop is environment-dependent. The output stays deterministic.

Exercise Set non-blocking mode

Set the descriptor to non-blocking mode with fcntl(STDIN_FILENO, F_SETFL, O_NONBLOCK);. Print the state with printf("nonblock: 1\n");.

Show example solution

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

int main(void) {
    fcntl(STDIN_FILENO, F_SETFL, O_NONBLOCK);
    printf("nonblock: 1\n");
    return 0;
}

Test yourself

fcntl call

fcntl(STDIN_FILENO, F_SETFL, ____);

Show answer

fcntl(STDIN_FILENO, F_SETFL, O_NONBLOCK);

Match non-blocking concepts

fcntl

Sets flag
O_NONBLOCK

Non-blocking
EAGAIN

No data
F_SETFL

Flag command
What does non-blocking read return when there is no data?
- -1 with errno EAGAIN
- Waits forever
- 0
- EOF
Show answer

-1 with errno EAGAIN — -1 is returned and errno is EAGAIN; not an error.

Next lesson: ANSI escape

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