Understanding the Linux Kernel, Second Edition

System Calls Related to Networking

We won’t be able to discuss all system calls related to networking. However, we shall examine the basic ones, namely those needed to send a UDP datagram.

In most Unix-like systems, the User Mode code fragment that sends a datagram looks like the following:

int sockfd; /* socket descriptor */
struct sockaddr_in addr_local, addr_remote; /* IPv4 address descriptors */
const char *mesg[] = "Hello, how are you?";
  
sockfd = socket(PF_INET, SOCK_DGRAM, 0);
  
addr_local.sin_family = AF_INET;
addr.sin_port = htons(50000);
addr.sin_addr.s_addr = htonl(0xc0a050f0); /* 192.160.80.240 */
bind(sockfd, (struct sockaddr *) & addr_local, sizeof(struct sockaddr_in));

addr_remote.sin_family = AF_INET;
addr_remote.sin_port = htons(49152);
inet_pton(AF_INET, "192.160.80.110", &addr_remote.sin_addr);
connect(sockfd, (struct sockaddr *) &addr_remote, sizeof(struct sockaddr_in));
  
write(sockfd, mesg, strlen(mesg)+1);

Obviously, this listing does not represent the complete source code of the program. For instance, we have not defined a main( ) function, we have omitted the proper #include directives for loading the header files, and we have not checked the return values of the system calls. However, the listing includes all network-related system calls issued by the program to send a UDP datagram.

Let’s describe the system calls in the order the program uses them.

The socket( ) System Call

The socket( ) system call creates a new endpoint for a communication between two or ...