Re: Making stdio writes robust/recoverable under errors

[Rich Felker <dalias@libc.org>]

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On Fri, May 29, 2015 at 09:53:00AM -0400, Rich Felker wrote:
> An attached test program demonstrates the issue. It assumes Linux 64k
> pipe buffers so it's not appropriate for inclusion in tests in its
> current form, but it gets the job done where it works. It attempts to
> send 128k over a pipe via write followed by a series of fwrite retries
> that are interrupted by signals that cause EINTR; thanks to the
> kernel's pipe buffer semantics, there's basically no timing/race
> aspect involved. With current musl, I get only 130560 across the pipe;
> with the above line removed, I get all 131072.

And here is the missing attachment. :-)

Rich

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#include <pthread.h>
#include <stdio.h>
#include <signal.h>
#include <string.h>
#include <unistd.h>
#include <sys/time.h>

char bigbuf[65536];

void *thr(void *ptr)
{
	int *p = ptr;
	char tmp[256];
	size_t l, t=0;
	for (;;) {
		l = read(p[0], tmp, sizeof tmp);
		if (l <= 0) break;
		t += l;
	}
	printf("read %zu\n", t);
}

void dummy(int sig)
{
}

int main()
{
	int testpipe[2];
	pipe(testpipe);
	FILE *f = fdopen(testpipe[1], "w");
	struct sigaction sa = { .sa_handler = dummy };
	sigaction(SIGALRM, &sa, 0);
	//alarm(1);
	setitimer(ITIMER_REAL, &(struct itimerval){ .it_interval.tv_usec = 10000, .it_value.tv_usec = 10000 }, 0);
	char *p = bigbuf;
	size_t n = sizeof bigbuf, l;
	memset(p, 42, n);

	write(testpipe[1], p, n);
	l = fwrite(p, 1, 512, f);
	p+=l, n-=l;
	l = fwrite(p, 1, n, f);

	pthread_t td;
	pthread_create(&td, 0, thr, testpipe);

	while (p+=l, n-=l)
		l = fwrite(p, 1, n, f);

	fclose(f);

	pthread_join(td, 0);
}