Re: [PATCH] unix.7: add example

["Michael Kerrisk (man-pages)" <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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-Mmb7MZpHnFY@public.gmane.org>
> ---
>  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-h9bWGtP8wOw@public.gmane.org>.
> -.\" and Copyright (C) 2008-2014, Michael Kerrisk <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> +.\" This man page is Copyright (C) 1999 Andi Kleen <ak-h9bWGtP8wOw@public.gmane.org>,
> +.\" Copyright (C) 2008-2014, Michael Kerrisk <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>,
> +.\" and Copyright (C) 2016, Heinrich Schuchardt <xypron.glpk-Mmb7MZpHnFY@public.gmane.org>
>  .\"
>  .\" %%%LICENSE_START(VERBATIM_ONE_PARA)
>  .\" and Copyright (C) 2008, 2012 Michael Kerrisk <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> @@ -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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