| modified | Friday 21 February 2025 |
|---|
nowinch, a tiny wrapper to protect a program from receiving WINCH signals.
Back in 2012 I was running a gunicorn project inside a tmux, and every time I resized my tmux a SIGWINCH would get sent to gunicorn which would immediately shut down.
SIGWINCH is the signal to notify a terminal process that its window that it runs in has been resized. Normally a program ignores this specific signal but Gunicorn decided to interpret it as a clean shutdown signal. Which is fair: the idea behind specific signals are defined but it’s up to developers to decide how to actually deal with them, and there are only so many of them you can use.
See man 7 signal for more details about signals in general.
The entire code for nowinch can be found below (download nowinch.c / view nowinch.c)
1/*
2 * Compile instructions (your choice):
3 * gcc nowinch.c -o nowinch
4 * clang nowinch.c -o nowinch
5 *
6 * Originally written to deal with gunicorn inside tmux: gunicorn kills worker
7 * processes when it receives a SIGWINCH, and tmux sends these whenever the
8 * window size changes.
9 *
10 */
11
12#include <signal.h>
13#include <stdbool.h>
14#include <stdio.h>
15#include <stdlib.h>
16#include <string.h>
17#include <sys/errno.h>
18#include <sys/wait.h>
19#include <unistd.h>
20
21void usage() {
22 fprintf(stderr, "Usage: [--help | -h] [--quiet | -q] <command> [arg [ arg ... ]\n\n");
23 fprintf(stderr, "This is a wrapper command that suppresses SIGWINCH signals.\n\n");
24 fprintf(stderr, "When you run 'nowinch somecommand arg1 arg2' the wrapper will launch\n");
25 fprintf(stderr, "`somecommand` with all the given argments while intercepting SIGWINCH\n");
26 fprintf(stderr, "signals: they will not reach your program.\n");
27}
28
29void terminate_with_missing_command() {
30 usage();
31 fprintf(stderr, "\nProblem: Missing command\n");
32 exit(1);
33}
34
35int main(int argc, char **argv)
36{
37 if (argc <= 1) terminate_with_missing_command();
38
39 if (strcmp(argv[1], "-h") == 0 || strcmp(argv[1], "--help") == 0) {
40 usage();
41 exit(0);
42 }
43 bool quiet = false;
44 int command_offset = 1;
45 if (strcmp(argv[1], "--quiet") == 0 || strcmp(argv[1], "-q") == 0) {
46 quiet = true;
47 command_offset++;
48 }
49 if (command_offset >= argc) terminate_with_missing_command();
50
51 if (!quiet) fprintf(stderr, "<< SIGWINCH will be ignored from now on >>\n");
52 char **cmd = argv + command_offset;
53 sigset_t sigset;
54 sigemptyset (&sigset);
55 sigaddset(&sigset, SIGWINCH);
56 sigprocmask(SIG_BLOCK, &sigset, NULL);
57 if (execvp(*cmd, cmd) == -1) {
58 fprintf(stderr, "There was a problem spawning your program: error code %i\n", errno);
59 exit(1);
60 }
61}