Sockets in C

Caution

• The sockets API is idiomatic C
• Be careful
• Check errors

Connection Lifecycle

• Get an IP address (usually by looking up a hostname in DNS)
• Create a socket using the address, protocol, and port number
• Bind the socket to the remote host
• Loop over open sockets
  • Write data
  • Read data on ready sockets
• Deallocate dynamic resources

Lookup a hostname

#include < netdb.h>
main() {
    struct addrinfo hints;
    hints.ai_family = AF_INET;      // IPv4
    hints.ai_socktype = SOCK_DGRAM; // datagram socket
    hints.ai_flags = 0;
    hints.ai_protocol = 0;          // any protocol

    struct addrinfo *results;       // array, allocated in gai()
    int e = getaddrinfo("vlab00.cs.usfca.edu", "2221", &hints, 
        &results);
    if (e != 0) {
        printf("getaddrinfo: %s\n", gai_strerror(e));
        exit(-1);
    }
}

Make a socket File Descriptor

#include < sys/socket.h>
struct addrinfo *r;
int fd;
for (r = results; r != NULL; r = r->ai_next) {
    fd = socket(r->ai_family, r->ai_socktype, r->ai_protocol);
    if (fd == -1)
        continue;
    if (connect(fd, r->ai_addr, r->ai_addrlen) != -1)
        break;                  // success
    close(fd);
}
freeaddrinfo(results);         // dealloc array
if (r == NULL) {               // no address succeeded
    printf("Could not connect\n");
    exit(-1);
}

Connect socket and addrinfo

if (connect(fd, rp->ai_addr, rp->ai_addrlen) != -1)
    // connected!

Write on the connected socket

if (write(fd, "hello", 6) == 6)
    // success!

Read on the connected socket

char buf[32];
if (read(fd, buf, 32) != -1)
    // success!

Deallocate resources

close(fd);
freeaddrinfo(results);

getaddrinfo, UDP demos

Key Concepts

1. A socket file descriptor is just an int
2. File descriptors can be used with read() and write() system calls
3. Clients: lookup, socket, connect, read/write
4. Servers: socket, bind, recvfrom (UDP)
5. Check errors. Use perror()