sockets/inet_sockets.c

This is sockets/inet_sockets.c (Listing 59-9, page 1228), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.

  Cover of The Linux Programming Interface
+/* inet_sockets.c
+
+   A package of useful routines for Internet domain sockets.
+*/
 #define _BSD_SOURCE             /* To get NI_MAXHOST and NI_MAXSERV
                                    definitions from <netdb.h> */
 #include <sys/socket.h>
 #include <netinet/in.h>
 #include <arpa/inet.h>
 #include <netdb.h>
 #include "inet_sockets.h"       /* Declares functions defined here */
 #include "tlpi_hdr.h"
 
+/* The following arguments are common to several of the routines
+   below:
+
+        'host':         NULL for loopback IP address, or
+                        a host name or numeric IP address
+        'service':      either a name or a port number
+        'type':         either SOCK_STREAM or SOCK_DGRAM
+*/
+
+/* Create socket and connect it to the address specified by
+  'host' + 'service'/'type'. Return socket descriptor on success,
+  or -1 on error */
+
 int
 inetConnect(const char *host, const char *service, int type)
 {
     struct addrinfo hints;
     struct addrinfo *result, *rp;
     int sfd, s;
 
     memset(&hints, 0, sizeof(struct addrinfo));
     hints.ai_canonname = NULL;
     hints.ai_addr = NULL;
     hints.ai_next = NULL;
     hints.ai_family = AF_UNSPEC;        /* Allows IPv4 or IPv6 */
     hints.ai_socktype = type;
 
     s = getaddrinfo(host, service, &hints, &result);
     if (s != 0) {
         errno = ENOSYS;
         return -1;
     }
 
     /* Walk through returned list until we find an address structure
        that can be used to successfully connect a socket */
 
     for (rp = result; rp != NULL; rp = rp->ai_next) {
         sfd = socket(rp->ai_family, rp->ai_socktype, rp->ai_protocol);
         if (sfd == -1)
             continue;                   /* On error, try next address */
 
         if (connect(sfd, rp->ai_addr, rp->ai_addrlen) != -1)
             break;                      /* Success */
 
         /* Connect failed: close this socket and try next address */
 
         close(sfd);
     }
 
     freeaddrinfo(result);
 
     return (rp == NULL) ? -1 : sfd;
 }
 
+/* Create an Internet domain socket and bind it to the address
+   { wildcard-IP-address + 'service'/'type' }.
+   If 'doListen' is TRUE, then make this a listening socket (by
+   calling listen() with 'backlog'), with the SO_REUSEADDR option set.
+   If 'addrLen' is not NULL, then use it to return the size of the
+   address structure for the address family for this socket.
+   Return the socket descriptor on success, or -1 on error. */
+
 static int              /* Public interfaces: inetBind() and inetListen() */
 inetPassiveSocket(const char *service, int type, socklen_t *addrlen,
                   Boolean doListen, int backlog)
 {
     struct addrinfo hints;
     struct addrinfo *result, *rp;
     int sfd, optval, s;
 
     memset(&hints, 0, sizeof(struct addrinfo));
     hints.ai_canonname = NULL;
     hints.ai_addr = NULL;
     hints.ai_next = NULL;
     hints.ai_socktype = type;
     hints.ai_family = AF_UNSPEC;        /* Allows IPv4 or IPv6 */
     hints.ai_flags = AI_PASSIVE;        /* Use wildcard IP address */
 
     s = getaddrinfo(NULL, service, &hints, &result);
     if (s != 0)
         return -1;
 
     /* Walk through returned list until we find an address structure
        that can be used to successfully create and bind a socket */
 
     optval = 1;
     for (rp = result; rp != NULL; rp = rp->ai_next) {
         sfd = socket(rp->ai_family, rp->ai_socktype, rp->ai_protocol);
         if (sfd == -1)
             continue;                   /* On error, try next address */
 
         if (doListen) {
             if (setsockopt(sfd, SOL_SOCKET, SO_REUSEADDR, &optval,
                     sizeof(optval)) == -1) {
                 close(sfd);
                 freeaddrinfo(result);
                 return -1;
             }
         }
 
         if (bind(sfd, rp->ai_addr, rp->ai_addrlen) == 0)
             break;                      /* Success */
 
         /* bind() failed: close this socket and try next address */
 
         close(sfd);
     }
 
     if (rp != NULL && doListen) {
         if (listen(sfd, backlog) == -1) {
             freeaddrinfo(result);
             return -1;
         }
     }
 
     if (rp != NULL && addrlen != NULL)
         *addrlen = rp->ai_addrlen;      /* Return address structure size */
 
     freeaddrinfo(result);
 
     return (rp == NULL) ? -1 : sfd;
 }
 
+/* Create stream socket, bound to wildcard IP address + port given in
+  'service'. Make the socket a listening socket, with the specified
+  'backlog'. Return socket descriptor on success, or -1 on error. */
+
 int
 inetListen(const char *service, int backlog, socklen_t *addrlen)
 {
     return inetPassiveSocket(service, SOCK_STREAM, addrlen, TRUE, backlog);
 }
 
+/* Create socket bound to wildcard IP address + port given in
+   'service'. Return socket descriptor on success, or -1 on error. */
+
 int
 inetBind(const char *service, int type, socklen_t *addrlen)
 {
     return inetPassiveSocket(service, type, addrlen, FALSE, 0);
 }
 
+/* Given a socket address in 'addr', whose length is specified in
+   'addrlen', return a null-terminated string containing the host and
+   service names in the form "(hostname, port#)". The string is
+   returned in the buffer pointed to by 'addrStr', and this value is
+   also returned as the function result. The caller must specify the
+   size of the 'addrStr' buffer in 'addrStrLen'. */
+
 char *
 inetAddressStr(const struct sockaddr *addr, socklen_t addrlen,
                char *addrStr, int addrStrLen)
 {
     char host[NI_MAXHOST], service[NI_MAXSERV];
 
     if (getnameinfo(addr, addrlen, host, NI_MAXHOST,
                     service, NI_MAXSERV, NI_NUMERICSERV) == 0)
         snprintf(addrStr, addrStrLen, "(%s, %s)", host, service);
     else
         snprintf(addrStr, addrStrLen, "(?UNKNOWN?)");
 
     return addrStr;
 }

Note that, in most cases, the programs rendered in these web pages are not free standing: you'll typically also need a few other source files (mostly in the lib/ subdirectory) as well. Generally, it's easier to just download the entire source tarball and build the programs with make(1). By hovering your mouse over the various hyperlinked include files and function calls above, you can see which other source files this file depends on.

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