SIGALRM is not interrupting a blocking write to a pipe

[Takashi Yano]

On Mon, 6 May 2024 23:01:49 +0300
ilya Basin wrote:
> I need your help with troubleshooting an issue with "pv": https://codeberg.org/a-j-wood/pv/issues/87
> 
> This app uses SIGALRM to interrupt a blocking write to STDOUT and read more data into the buffer.
> On Linuxes write() returns 0 after the signal, but on Cygwin even though the signal handler is called, the write call does not return, at least when writing to a pipe.

What Linux environment you assume? I run the STC below on Debuan GNU/Linux,

#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <errno.h>
#include <signal.h>
#include <string.h>

void handler(int sig)
{
	printf("sig=%d\n", sig);
}

int main()
{
	int fd[2];

	signal(SIGALRM, handler);
	pipe(fd);
	for (;;) {
		int l = write(fd[1], "A", 1);
		if (l == 1) {
			printf("."); fflush(stdout); /* Normal */
		} else {
			printf("%d: %s\n", l, strerror(errno)); /* Interrupt */
		}
	}
}

but,
kill -ALRM <pid of STC>
does not make output /* Interrupt */ line, but only /* Normal */ line.

uname -a:
Linux debian2 6.1.0-21-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.1.90-1 (2024-05-03) x86_64 GNU/Linux

The behaviour is same with cygwin.

-- 
Takashi Yano <takashi.yano@nifty.ne.jp>