TCP sockets in c

Question

I'm attempting to write a TCP socket interface for my program and I'm pulling my hair out with an accept() error (I think). For this I've created some boiled down test code.
First I do a little set up

int server_socket = 0;
server_socket = socket(AF_INET, SOCK_STREAM, 0);

int accepted_connection = 0;

struct sockaddr_in server_address;
server_address.sin_port = htons(9001);
server_address.sin_family = AF_INET;
server_address.sin_addr.s_addr = INADDR_ANY;

struct sockaddr_in client_address;
client_address.sin_port = 0;
client_address.sin_family = AF_INET;

char * server_socket_read_buffer[100] = {0};
int server_socket_read_length = 0;

All pretty simple stuff. Just allocate some variables. Next I bind and listen

if (bind(server_socket,(struct sockaddr *)&server_address, sizeof(server_address)) < 0)
{
    perror("Bind() on server_socket has failed\n");
}

if (listen(server_socket, 10) < 0)
{
    perror("Listen() on server_socket has failed\n");
}

Next is the part where I believe I have my problem

printf("Attempting accept!\n");
if (accepted_connection = accept(server_socket, (struct sockaddr *)NULL, NULL) < 0)
{
    perror("Accept failed\n");
}
sleep(10);
if (server_socket_read_length = read(accepted_connection, &server_socket_read_buffer, server_socket_read_length) < 0)
{
    perror("Read failed\n");
}
printf("Read %d bytes from socket\n", server_socket_read_length);
for (int i = 0; i<server_socket_read_length;i++)
{
    printf("%x\n",server_socket_read_buffer[i]);
}

This compiles and runs. When I use nc with the command 'nc 127.0.0.1 9001' I get a connection, but no data is read. In particular I get 0 bytes of data. I thought this might be due to the NULLs in the accept line, but changing those to a proper struct and length prevent my code from compiling.

If anyone can shed some light on what I'm doing wrong I would be very grateful.

are you on a machine where htons() is necessary? that's byte swapping, and you may very well be setting port 0x2923 (10531) instead of 0x2329 (9001). — Marc B, Jul 06 '15 at 18:55
You should always use `htons()` on a constant value when setting the `sin_port` field. On little-endian machines the bytes will be reversed, while on big-endian machines it is a no-op. — dbush, Jul 06 '15 at 19:02

score 1 · Answer 1 · answered Jul 06 '15 at 18:57

1

You are passing in server_socket_read_length = 0 which causes a maximum read length of zero. Pass the buffer size. The declaration of server_socket_read_buffer is incorrect as well. Probably you should allocate a bigger buffer (like 4KB) on the heap.

Also remove the sleep.

The rest is probably working because nc obtains a connection and you are able to accept and read without error.

answered Jul 06 '15 at 18:57

usr

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And then I felt dumb. Thanks for pointing out the zero length to me. The issue isn't fully resolved, but this helps. – Darakian Jul 06 '15 at 19:15

Maxim Egorushkin · Accepted Answer · 2015-07-06T19:02:57.923

1

There are a couple of errors:

INADDR_ANY is in host byte order and needs to be converted to network one like htonl(INADDR_ANY). But it does not matter since constant INADDR_ANY is defined as 0.

This

char * server_socket_read_buffer[100]

should be

char server_socket_read_buffer[100]

This

read(accepted_connection, &server_socket_read_buffer, server_socket_read_length)

should be

read(accepted_connection, server_socket_read_buffer, sizeof server_socket_read_buffer)

edited Jul 06 '15 at 19:02

answered Jul 06 '15 at 18:57

Maxim Egorushkin

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I've made the changes you suggested and it looks like the program is blocking on the read line. I've sent well over 100 bytes of data. I can't find anything in the man pages on read() about it being a blocking function. Thought? – Darakian Jul 06 '15 at 19:13
@Darakian Why is the `sleep` in there? – David Schwartz Jul 06 '15 at 19:29
@DavidSchwartz It's not any more. The sleep was in there to ensure I had enough time to smash my keyboard in another terminal to send data. – Darakian Jul 06 '15 at 19:47
@Darakian Test is with `echo abc | nc 127.0.0.1 9001`. And post the complete code if that does not work. – Maxim Egorushkin Jul 07 '15 at 07:49

score 0 · Answer 3 · answered Jul 06 '15 at 20:57

So after more struggle I found my final answer. The block

    if (accepted_connection = accept(server_socket, (struct sockaddr *)NULL, NULL) < 0)
    {
    //code
    }

Doesn't work. My read later on was blocking because accepted_connection wasn't a valid socket. Changing to

    (accepted_connection = accept(server_socket, (struct sockaddr *)NULL, NULL);
    if accepted_connection < 0)
    {
    //code
    }

resolved my issue. As far as I can gather the file descriptor wasn't being created inline with the if() and reading data from an integer isn't very helpful.
Thanks for the input everyone.

TCP sockets in c

3 Answers3

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