Is select() + non-blocking write() possible on a blocking pipe or socket?

Question

The situation is that I have a blocking pipe or socket fd to which I want to write() without blocking, so I do a select() first, but that still doesn't guarantee that write() will not block.

Here is the data I have gathered. Even if select() indicates that writing is possible, writing more than PIPE_BUF bytes can block. However, writing at most PIPE_BUF bytes doesn't seem to block in practice, but it is not mandated by the POSIX spec.

That only specifies atomic behavior. Python(!) documentation states that:

Files reported as ready for writing by select(), poll() or similar interfaces in this module are guaranteed to not block on a write of up to PIPE_BUF bytes. This value is guaranteed by POSIX to be at least 512.

In the following test program, set BUF_BYTES to say 100000 to block in write() on Linux, FreeBSD or Solaris following a successful select. I assume that named pipes have similar behavior to anonymous pipes.

Unfortunately the same can happen with blocking sockets. Call test_socket() in main() and use a largish BUF_BYTES (100000 is good here too). It's unclear whether there is a safe buffer size like PIPE_BUF for sockets.

#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <limits.h>
#include <stdio.h>
#include <sys/select.h>
#include <unistd.h>

#define BUF_BYTES PIPE_BUF
char buf[BUF_BYTES];

int
probe_with_select(int nfds, fd_set *readfds, fd_set *writefds,
                  fd_set *exceptfds)
{
    struct timeval timeout = {0, 0};
    int n_found = select(nfds, readfds, writefds, exceptfds, &timeout);
    if (n_found == -1) {
        perror("select");
    }
    return n_found;
}

void
check_if_readable(int fd)
{
    fd_set fdset;
    FD_ZERO(&fdset);
    FD_SET(fd, &fdset);
    printf("select() for read on fd %d returned %d\n",
           fd, probe_with_select(fd + 1, &fdset, 0, 0));
}

void
check_if_writable(int fd)
{
    fd_set fdset;
    FD_ZERO(&fdset);
    FD_SET(fd, &fdset);
    int n_found = probe_with_select(fd + 1, 0, &fdset, 0);
    printf("select() for write on fd %d returned %d\n", fd, n_found);
    /* if (n_found == 0) { */
    /*     printf("sleeping\n"); */
    /*     sleep(2); */
    /*     int n_found = probe_with_select(fd + 1, 0, &fdset, 0); */
    /*     printf("retried select() for write on fd %d returned %d\n",  */
    /*            fd, n_found); */
    /* } */
}

void
test_pipe(void)
{
    int pipe_fds[2];
    size_t written;
    int i;
    if (pipe(pipe_fds)) {
        perror("pipe failed");
        _exit(1);
    }
    printf("read side pipe fd: %d\n", pipe_fds[0]);
    printf("write side pipe fd: %d\n", pipe_fds[1]);
    for (i = 0; ; i++) {
        printf("i = %d\n", i);
        check_if_readable(pipe_fds[0]);
        check_if_writable(pipe_fds[1]);
        written = write(pipe_fds[1], buf, BUF_BYTES);
        if (written == -1) {
            perror("write");
            _exit(-1);
        }
        printf("written %d bytes\n", written);
    }
}

void
serve()
{
    int listenfd = 0, connfd = 0;
    struct sockaddr_in serv_addr;

    listenfd = socket(AF_INET, SOCK_STREAM, 0);
    memset(&serv_addr, '0', sizeof(serv_addr));

    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    serv_addr.sin_port = htons(5000);

    bind(listenfd, (struct sockaddr*)&serv_addr, sizeof(serv_addr));

    listen(listenfd, 10);

    connfd = accept(listenfd, (struct sockaddr*)NULL, NULL);

    sleep(10);
}

int
connect_to_server()
{
    int sockfd = 0, n = 0;
    struct sockaddr_in serv_addr;

    if((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
        perror("socket");
        exit(-1);
    }

    memset(&serv_addr, '0', sizeof(serv_addr));

    serv_addr.sin_family = AF_INET;
    serv_addr.sin_port = htons(5000);

    if(inet_pton(AF_INET, "127.0.0.1", &serv_addr.sin_addr) <= 0) {
        perror("inet_pton");
        exit(-1);
    }

    if (connect(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0) {
        perror("connect");
        exit(-1);
    }

    return sockfd;
}

void
test_socket(void)
{
    if (fork() == 0)  {
        serve();
    } else {
        int fd;
        int i;
        int written;
        sleep(1);
        fd = connect_to_server();

        for (i = 0; ; i++) {
            printf("i = %d\n", i);
            check_if_readable(fd);
            check_if_writable(fd);
            written = write(fd, buf, BUF_BYTES);
            if (written == -1) {
                perror("write");
                _exit(-1);
            }
            printf("written %d bytes\n", written);
        }
    }
}

int
main(void)
{
    test_pipe();
    /* test_socket(); */
}

Answer 1

The answer by ckolivas is the correct one but, having read this post, I thought I could add some test data for interest's sake.

I quickly wrote a slow reading tcp server (sleeping 100ms between reads) which did a read of 4KB on each cycle. Then a fast writing client which I used for testing various scenarios on write. Both were using select before read (server) or write (client).

This was on Linux Mint 18 running under a Windows 7 VM (VirtualBox) with 1GB of memory assigned.

For the blocking case

If a write of a "certain number of bytes" became possible, select returned and the write either completed in total immediately or blocked until it completed. On my system, this "certain number of bytes" was at least 1MB. On the OP's system, this was clearly much less (less than 100,000).

So select did not return until a write of at least 1MB was possible. There was never a case (that I saw) where select would return if a smaller write would subsequently block. Thus select + write(x) where x was 4K or 8K or 128K never write blocked on this system.

This is all very well of course but this was an unloaded VM with 1GB of memory. Other systems would be expected to be different. However, I would expect that writes below a certain magic number (PIPE_BUF perhaps), issued subsequent to a select, would never block on all POSIX compliant systems. However (again) I don't see any documentation to that effect so one can't rely on that behaviour (even though the Python documentation clearly does). As the OP says, it's unclear whether there is a safe buffer size like PIPE_BUF for sockets. Which is a pity.

Which is what ckolivas' post says even though I'd argue that no rational system would return from a select when only a single byte was available!

Extra information:

At no point (in normal operation) did write return anything other than the full amount requested (or an error).

If the server was killed (ctrl-c), the client side write would immediately return a value (usually less than was requested - no normal operation!) with no other indication of error. The next select call would return immediately and the subsequent write would return -1 with errno saying "Connection reset by peer". Which is what one would expect - write as much as you can this time, fail the next time.

This (and EINTR) appears to be the only time write returns a number > 0 but less than requested.

If the server side was reading and the client was killed, the server continued to read all available data until it ran out. Then it read a zero and closed the socket.

For the non-blocking case:

The behaviour below some magic value is the same as above. select returns, write doesn't block (of course) and the write completes in its totality.

My issue was what happens otherwise. The send(2) man page says that in non-blocking mode, send fails with EAGAIN or EWOULDBLOCK. Which might imply (depending on how you read it) that it's all or nothing. Except that it also says select may be used to determine when it is possible to send more data. So it can't be all or nothing.

Write (which is the same as send with no flags), says it can return less than requested. This nitpicking seems pedantic but the man pages are the gospel so I read them as such.

In testing, a non-blocking write with a value larger than some particular value returned less than requested. This value wasn't constant, it changed from write to write but it was always pretty large (> 1 to 2MB).

Answer 2

Unless you wish to send one byte at a time whenever select() says the fd is ready for writes, there is really no way to know how much you will be able to send and even then it is theoretically possible (at least in the documentation, if not in the real world) for select to say it's ready for writes and then the condition to change in the time between select() and write().

Non blocking sends are the solution here and you don't need to change your file descriptor to non blocking mode to send one message in non-blocking form if you change from using write() to send(). The only thing you need to change is to add the MSG_DONTWAIT flag to the send call and that will make the one send non-blocking without altering your socket's properties. You don't even need to use select() at all in this case either since the send() call will give you all the information you need in the return code - if you get a return code of -1 and the errno is EAGAIN or EWOULDBLOCK then you know you can't send any more.

Answer 3

The Posix section you cite clearly states:

[for pipes] If the O_NONBLOCK flag is clear, a write request may cause the thread to block, but on normal completion it shall return nbyte.

[for streams, which presumably includes streaming sockets] If O_NONBLOCK is clear, and the STREAM cannot accept data (the STREAM write queue is full due to internal flow control conditions), write() shall block until data can be accepted.

The Python documentation you quoted can therefore only apply to non-blocking mode only. But as you're not using Python it has no relevance anyway.