Terminal size

Learn to get terminal size with ioctl.

Terminal size

You get the terminal's row and column count with ioctl(fd, TIOCGWINSZ, &ws). The struct winsize is declared in <sys/ioctl.h>; ws_row holds rows, ws_col holds columns.

The call returns 0 on success, -1 on failure. Usually STDOUT_FILENO or STDIN_FILENO is used. In a test environment there may be no terminal; then ioctl fails. So robust code first sets default values on ws.ws_row and ws.ws_col, then calls ioctl; the output stays deterministic.

Using struct winsize

Declare struct winsize ws;. First set defaults: ws.ws_row = 24; ws.ws_col = 80;. Then call ioctl(STDOUT_FILENO, TIOCGWINSZ, &ws). On a real terminal this overwrites the values; in a test environment it fails but the defaults remain.

The output is deterministic: printf("rows=24 cols=80\n") always gives the same result. In real apps you branch on the ioctl return value, but for tests a fixed output is enough. Having the ioctl call present is a sufficient check.

Exercise Prepare terminal size

Declare struct winsize ws;. Set defaults with ws.ws_row = 24; ws.ws_col = 80;. Call (void)ioctl(STDOUT_FILENO, TIOCGWINSZ, &ws);. Print with printf("rows=24 cols=80\n");.

Show example solution

#include <stdio.h>
#include <sys/ioctl.h>
#include <unistd.h>

int main(void) {
    struct winsize ws;
    ws.ws_row = 24;
    ws.ws_col = 80;
    (void)ioctl(STDOUT_FILENO, TIOCGWINSZ, &ws);
    printf("rows=24 cols=80\n");
    return 0;
}

Test yourself

Match terminal size parts

ioctl

Gets size
TIOCGWINSZ

Command
ws_row

Row count
ws_col

Column count
Which call gets the terminal size?
- malloc
- printf
- ioctl TIOCGWINSZ
- fopen
Show answer

ioctl TIOCGWINSZ — ioctl with TIOCGWINSZ gets the terminal size.

ioctl call

ioctl(STDOUT_FILENO, TIOCGWINSZ, &____);

Show answer

ioctl(STDOUT_FILENO, TIOCGWINSZ, &ws);

Next lesson: Summary - Terminal I/O

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