Linux sockets

Tutorials

• Socket programming in C on Linux – tutorial.

A thoough step-by-step example of client and socket code.

• C Socket Programming for Linux with a Server and Client Example Code

Simple example of client and server code using Linux sockets.

References

• Socket options SO_REUSEADDR and SO_REUSEPORT, how do they differ? Do they mean the same across all major operating systems?

A very detailed answer about the differences between SO_REUSEADDR and SO_REUSEPORT.

• Programmed Instruction Course

A course on internet technologies, including Domain Naming (DNS...), the IP protocol, the TCP protocol, ...

• Manpages:

• ip (7)
• socket (7)

C API

read(2) - Read from a file descriptor

#include <unistd.h>
ssize_t read(int fd, void *buf, size_t count);

• Mostly equivalent to recv except there is no specification for flags (e.g. to change blocking/non-blocking behaviour).

recv(2), recvfrom(2), recmmsg(2) - Receive a message from a socket

#include <sys/types.h>
#include <sys/socket.h>

ssize_t recv(int sockfd, void *buf, size_t len, int flags);
ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags, struct sockaddr *src_addr, socklen_t *addrlen);
ssize_t recvmsg(int sockfd, struct msghdr *msg, int flags);

write(2) - Write to a file descriptor

#include <unistd.h>

ssize_t write(int fd, const void *buf, size_t count);

• The main difference with send is that when wrote returns EPIPE, the writing process will also receive a SIGPIPE signal, terminating it unless it catches it or ignore it.

send(2), sendto(2), sendmsg(2) - Send a message on a socket

#include <sys/types.h>
#include <sys/socket.h>

ssize_t send(int sockfd, const void *buf, size_t len, int flags);
ssize_t sendto(int sockfd, const void *buf, size_t len, int flags, const struct sockaddr *dest_addr, socklen_t addrlen);
ssize_t sendmsg(int sockfd, const struct msghdr *msg, int flags);

• The main difference with write is the presence of flags, which among other can configured to prevent the generation of SIGPIPE signal.

Troubleshooting

Use errno / perror / strerror to get error

if( bind(socket_desc,(struct sockaddr *)&server , sizeof(server)) < 0)
{
    printf("bind failed: %s (errno %d)", strerror(errno), errno);
    return 1;
}

Or there is also the perror function:

if( bind(socket_desc,(struct sockaddr *)&server , sizeof(server)) < 0)
{
    perror("bind failed");
    return 1;
}

Use netstat to get socket status

./server
# bind done
# Waiting for incoming connections...
# Connection accepted
# Handler assigned
# ^C

./server
# bind failed: Address already in use (errno 98)

netstat -a | grep 8888
# tcp        0      0 zavcxl0005:58888        165.225.76.32:http      ESTABLISHED
# tcp        0      0 zavcxl0005:48888        10.75.126.1:https       ESTABLISHED
# tcp        0      0 localhost.localdom:8888 localhost.localdo:50310 TIME_WAIT  

# ... So port is still in use

bind failed: Address already in use (errno 98)

Call to bind fails with error

bind failed: Address already in use (errno 98)

On a server, this is probably due to an old instance of the server that tries to bind to the same port and address [1].

As a fix, try to bind with option SO_REUSEADDR and/or SO_REUSEPORT. From SO:

if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &(int){ 1 }, sizeof(int)) < 0)
    error("setsockopt(SO_REUSEADDR) failed");

For more information, see socket(7) and ip(7) manpages.

Program crashes on write with exit code -13

Exit code -13 means the program died because of SIGPIPE signal, meaning it tried to write to a closed socket.

Solutions:

• Ignore SIGPIPE in the program [2]:

signal (SIGPIPE, SIG_IGN);

• Use send instead of write. Send returns -1 and set errno to EPIPE rather than generating a fatal SIGPIPE like write does [3]:

n = send(newsockfd, data.c_str(), data.length()+1, MSG_NOSIGNAL);