read_timeout.c

#include <unistd.h>
#include <sys/time.h>
#include <sys/select.h>
#include <errno.h>

/* this function does not do much error checking. be warned! */

ssize_t read_timeout(int fd, char *buf, size_t len, int timeout)
{
	ssize_t sz, rsz;
	fd_set r;
	struct timeval now, end, tv;

	FD_ZERO(&r);

	gettimeofday(&end, NULL);
	end.tv_sec += timeout;

	sz = 0;
	for (;;) {
		gettimeofday(&now, NULL);
		if (!timercmp(&end, &now, >))
			break;
		timersub(&end, &now, &tv);

		FD_SET(fd, &r);

		if (select(fd+1, &r, NULL, NULL, &tv) < 0) {
			if (errno == EAGAIN)
				continue;
			if (errno == EINTR)
				break;
			return -1;
		}

		if (FD_ISSET(fd, &r)) {
			rsz = read(fd, buf, len);
			if (rsz == 0)
				break;
			if (rsz < 0)
				return rsz;
			sz += rsz;
			buf += rsz;
			len -= rsz;
		}
	}

	return sz;
}

test.c

#include <stdio.h>
#include <unistd.h>
#include <sys/time.h>
#include <sys/select.h>

extern ssize_t read_timeout(int fd, char *buf, size_t len, int timeout);

int main(int argc, char *argv[])
{
	char buf[301];
	ssize_t sz;

	sz = read_timeout(0, buf, 300, 5);
	if (sz >= 0) buf[sz] = '\0';
	printf("read %d bytes: %s", sz, buf);

	return 0;
}