Hello Heinrich,
On 01/05/2016 04:17 PM, Heinrich Schuchardt wrote:
> A complete example demonstrating the usage of sockets for local interprocess
> communication is added.
Thanks for doing this. I'm happy to take such a piece for the page, but
I think things could be improved a little.
> Signed-off-by: Heinrich Schuchardt <xypron.glpk@xxxxxx>
> ---
> man7/unix.7 | 261 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 257 insertions(+), 4 deletions(-)
>
> diff --git a/man7/unix.7 b/man7/unix.7
> index e6e311f..dd630ba 100644
> --- a/man7/unix.7
> +++ b/man7/unix.7
> @@ -1,5 +1,6 @@
> -.\" This man page is Copyright (C) 1999 Andi Kleen <ak@xxxxxx>.
> -.\" and Copyright (C) 2008-2014, Michael Kerrisk <mtk.manpages@xxxxxxxxx>
> +.\" This man page is Copyright (C) 1999 Andi Kleen <ak@xxxxxx>,
> +.\" Copyright (C) 2008-2014, Michael Kerrisk <mtk.manpages@xxxxxxxxx>,
> +.\" and Copyright (C) 2016, Heinrich Schuchardt <xypron.glpk@xxxxxx>
> .\"
> .\" %%%LICENSE_START(VERBATIM_ONE_PARA)
> .\" and Copyright (C) 2008, 2012 Michael Kerrisk <mtk.manpages@xxxxxxxxx>
> @@ -618,9 +619,261 @@ that the applications that
> pathname sockets follow the rules outlined above under
> .IR "Pathname sockets" .
> .SH EXAMPLE
> -See
> -.BR bind (2).
> +The following code demonstrates the usage of sockets for local interprocess
> +communication.
> +It comprises two programs.
> +The server program waits for a connection from the client program.
> +The client sends all of its command line arguments.
> +The server treats them as integers and adds them up.
> +The client sends the command string END.
> +The server returns the result.
> +The client prints the sum of the received integers and exits.
> +The server waits for the next client to connect.
> +To stop the server the client is called with the command line argument
> +DOWN.
> +.PP
> +The following output was recorded while running the server in the background
> +and repeatedly calling the client.
> +Execution of the server program ended when receiving the DOWN command.
> +.SS Example output
> +.in +4n
> +.nf
> +$ ./server &
> +[1] 25887
> +$ ./client 3 4
> +Result = 7
> +$ ./client 11 \-5
> +Result = 6
> +$ ./client DOWN
> +Result = 0
> +[1]+ Done ./server
> +$
> +.fi
> +.in
> +.SS Program source
> +.nf
> +/*
> + * File connection.h
> + */
> +
> +#define SOCKET_NAME "/tmp/9Lq7BNBnBycd6nxy.socket"
> +#define BUFFER_SIZE 12
> +
> +/*
> + * File server.c
> + */
> +
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <string.h>
> +#include <sys/socket.h>
> +#include <sys/un.h>
> +#include <unistd.h>
> +#include "connection.h"
> +
> +int
> +main(int argc, char *argv)
char *argv[]
> +{
> + struct sockaddr_un name;
> + int down = 0;
> + int ret;
> + int sock;
> + int sock2;
I think these names could be more meaningful. One is the listening socket,
the other is the connected socket. So, how about
lfd cfd
or
listen_sock conn_sock
or
lsock csock
or
something else suitable
> + int result;
> + char buf[BUFFER_SIZE];
> +
> + /*
> + * In case the program exited inadvertently on the last run
> + * remove the socket.
> + */
> +
> + unlink(SOCKET_NAME);
> +
> + /* Create local socket. */
> +
> + sock = socket(PF_LOCAL, SOCK_SEQPACKET, 0);
s/PF_LOCAL/AF_UNIX/
(POSIX does not specify the PF_* constants; they're historical
baggage, and for my money, AF_UNIX beats AF_LOCAL.)
> + if (sock == \-1) {
> + perror("socket");
> + exit(EXIT_FAILURE);
> + }
> +
> + /* Bind socket to socket name. */
> +
> + name.sun_family = AF_UNIX;
> + strcpy (name.sun_path, SOCKET_NAME);
make this:
memset(name, 0, sizeof(struct sockaddr_un));
strncpy (name.sun_path, SOCKET_NAME, sizeof(name.sun_path) \- 1);
memset() really is needed, for portability, since some implementations
have additional (nonstandard) fields in the structure.
(I won't insist on strncpy(), but I think it's marginally better.)
> + ret = bind(sock, (const struct sockaddr *) &name,
> + sizeof(struct sockaddr_un));
> + if (ret == \-1) {
> + perror("bind");
> + exit(EXIT_FAILURE);
> + }
> +
> + /*
> + * Prepare for accepting connections. The backlog size is set to 2. So
> + * while one request is being processed other requests can be waiting.
> + */
> +
> + ret = listen(sock, 2);
2 is unusually low. Why that number?
> + if (ret == \-1) {
> + perror("listen");
> + exit(EXIT_FAILURE);
> + }
> +
> + /* This is the main loop for handling connections. */
> +
> + for (;;) {
> +
> +
> + /* Wait for incoming connection. */
> +
> + sock2 = accept(sock, NULL, NULL);
> + if (ret == \-1) {
> + perror("accept");
> + exit(EXIT_FAILURE);
> + }
> +
> + result = 0;
> + for(;;) {
> +
> + /* Wait for next datagram. */
> +
> + ret = recv(sock2, buf, BUFFER_SIZE, 0);
> + if (ret == \-1) {
> + perror("recv");
> + exit(EXIT_FAILURE);
> + }
Maybe it's simpler just to use read() here, since
the 'flags' argument is unused.
> +
> + /* Ensure buffer is 0\-terminated. */
> +
> + buf[BUFFER_SIZE \- 1] = 0;
> +
> + /* Handle commands. */
> +
> + if (!strncmp(buf, "DOWN", BUFFER_SIZE)) {
> + down = 1;
> + break;
> + }
> +
> + if (!strncmp(buf, "END", BUFFER_SIZE)) {
> + break;
> + }
> +
> + /* Add received summand. */
>
> + result += atoi(buf);
> + }
> +
> + /* Send result. */
> +
> + sprintf(buf, "%d", result);
> + ret = send(sock2, buf, BUFFER_SIZE, 0);
> +
> + if (ret == \-1) {
> + perror("send");
> + exit(EXIT_FAILURE);
> + }
Maybe it's simpler just to use read() here, since
the 'flags' argument is unused.
> + /* Close socket. */
> +
> + close(sock2);
> +
> + /* Quit on DOWN command. */
> +
> + if (down) {
> + break;
> + }
> + }
> +
> + close(sock);
> +
> + /* Unlink the socket. */
> +
> + unlink(SOCKET_NAME);
> +
> + exit(EXIT_SUCCESS);
> +}
> +
> +/*
> + * File client.c
> + */
> +
> +#include <errno.h>
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <string.h>
> +#include <sys/socket.h>
> +#include <sys/un.h>
> +#include <unistd.h>
> +#include "connection.h"
> +
> +int
> +main(int argc, char *argv[])
> +{
> + struct sockaddr_un name;
> + int i;
> + int ret;
> + int sock;
> + char buf[BUFFER_SIZE];
> +
> + /* Create local socket. */
> +
> + sock = socket(PF_LOCAL, SOCK_SEQPACKET, 0);
PF_LOCAL ==> AF_UNIX
> + if (sock == \-1) {
> + perror("socket");
> + exit(EXIT_FAILURE);
> + }
> +
> + /* Connect socket to socket name. */
> +
> + name.sun_family = AF_UNIX;
> + strcpy (name.sun_path, SOCKET_NAME);
See comments above re memset() and strncpy().
> + ret = connect (sock, (const struct sockaddr *) &name,
> + sizeof(struct sockaddr_un));
> + if (ret == \-1) {
> + fprintf(stderr, "The server is down.\\n");
> + exit(EXIT_FAILURE);
> + }
> +
> + /* Send arguments. */
> +
> + for (i = 1; i < argc; ++i) {
> + ret = send(sock, argv[i], strlen(argv[i]) + 1, 0);
> + if (ret == \-1) {
> + perror("send");
> + break;
> + }
write() instead of send()?
> + }
> +
> + /* Request result. */
> +
> + strcpy (buf, "END");
> + ret = send(sock, buf, strlen(buf) + 1, 0);
> + if (ret == \-1) {
> + perror("send");
> + exit(EXIT_FAILURE);
> + }
> +
> +
> + /* Receive result. */
> +
> + ret = recv(sock, buf, BUFFER_SIZE, 0);
read() instead of recv()
> + if (ret == \-1) {
> + perror("recv");
> + exit(EXIT_FAILURE);
> + }
> +
> + buf[BUFFER_SIZE \- 1] = 0;
> +
> + printf("Result = %s\\n", buf);
> +
> + /* Close socket. */
> +
> + close(sock);
> +
> + exit(EXIT_SUCCESS);
> +}
> +.fi
> +.PP
> For an example of the use of
> .BR SCM_RIGHTS
> see
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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