How many bytes can I write at once on a TCP socket?

Question

As the title says, is there a limit to the number of bytes that can be written at once on a connection-oriented socket?

If I want to send a buffer of, for example, 1024 bytes, can I use a

write(tcp_socket, buffer, 1024);

or should I use multiple write() calls with a lower amount of bytes for each one?

Answer 1

write() does not guarantee that all bytes will be written so multiple calls to write() are required. From man write :

The number of bytes written may be less than count if, for example, there is insufficient space on the underlying physical medium, or the RLIMIT_FSIZE resource limit is encountered (see setrlimit(2)), or the call was interrupted by a signal handler after having written less than count bytes. (See also pipe(7).)

write() returns the number of bytes written so a running total of the bytes written must be maintained and used as an index into buffer and to calculate the number of remaining bytes to be written:

ssize_t total_bytes_written = 0;
while (total_bytes_written != 1024)
{
    assert(total_bytes_written < 1024);
    ssize_t bytes_written = write(tcp_socket,
                                  &buffer[total_bytes_written],
                                  1024 - total_bytes_written);
    if (bytes_written == -1)
    {
        /* Report failure and exit. */
        break;
    }
    total_bytes_written += bytes_written;
}

Answer 2

There's no inherent limit. TCP/IP will fragment and reassemble packets as required. Your system may impose (a possibly tunable) upper limit, but it is likely to be in the multi-MB range. See your manpage for setsockopt() and always check write()'s return value.

Answer 3

The actual amount you can write will depend on the type of socket. In general, you need to check the return value to see how many bytes were actually written. The number of bytes written can vary depend on whether the socket is in blocking mode or not.

Also, if the socket is blocking, you may not want to wait for all the data to be written in one go. You may want to write some at a time in order to be able to something else in between write operations.

Answer 4

根据我的经验，最好保持1024字节的限制

Answer 5

如您所见， 写入套接字的最大缓冲区大小为1048576字节。