HTTP Push Client in Posix C

Have you ever wanted to push a value to a HTTP page and don’t care about the result? Well, you *can* do it in pure C, but be prepared – it is not as trivial as you might think. Here is how to do it.

# Includes #

#include 
#include 
#include 
#include 

## Distinguish Windows from Unix/Linux ##

We need to include **winsock** if we compile under Windows – otherwise use Unix/Linux headers for networking operations.

#ifdef _WIN32
#include 
#include 
#else
#include 
#include 
#include 
#include 
#define closesocket(s) close(s)
#endif

# Defining the port to connect to #

The default port to connect to is 80. You can change it here if you need to.

#define HTTP_PORT 80

# The *push_value* Function #

void push_value( char *host, char *path, char *value ) {

## Variable Declarations ##

    time_t tt;

This is the place to save the actual date string to. It’s length is limited to 15 characters – 14 for the date string and one for the terminating at the end.

    char datum[15];

The actual HTTP Request is being written to this variable.

    char request[300];

These are needed to create the socket/connection.

    struct sockaddr_in server;
    struct hostent *host_info;
    unsigned long addr;
    int sock;

## Windows needs Winsock initialisation ##

Windows TCP initialisation has to be done only if compiled under Windows.

#ifdef _WIN32
    short wVersReq;
    WSADATA wsaData;
    wVersReq = MAKEWORD( 1, 1 );
    if( WSAStartup( wVersReq, &wsaData ) != 0 ) {
        fprintf( stderr,
                 "Failed to init windows socketsn" );
        return;
    }
#endif

## Get actual date ##

`strftime` creates the actual date into *datum*, it uses the format given (*”%Y%m%d%H%M%S”*) and the current time from *time(NULL)*.

    tt = time(NULL);
    strftime( datum,
              sizeof( datum ),
              "%Y%m%d%H%M%S",
              localtime(&tt) );

## Create a socket ##

        sock = socket( PF_INET, SOCK_STREAM, 0 );
        if( sock < 0 ) {
            perror( "failed to create socket" );
            return;
        }

## Create struct for connection partner ##

    memset( &server, 0, sizeof( server ) );
    if( ( addr = inet_addr( host ) ) != INADDR_NONE ) {

*host* is a numerical IP Address. Nothing special has to be done.

        memcpy( (char *) &server.sin_addr,
                &addr, sizeof( addr ) );
    } else {

*host* is a domain name. Convert this domain name into a numerical IP Address.

        host_info = gethostbyname( host );
        if( NULL == host_info ) {
            fprintf( stderr,
                     "unknown server: %sn",
                     host );
            return;
        }
        memcpy( (char *) &server.sin_addr,
                host_info->h_addr,
                host_info->h_length );
    }

Set *server* parameters.

    server.sin_family = AF_INET;
    server.sin_port = htons( HTTP_PORT );

## Create connection with partner ##

Time to get together. Creating connection to server.

    if( connect( sock, (struct sockaddr*) &server,
                 sizeof( server ) ) < 0 ) {
        perror( "can't connect to server" );
        return;
    }

Create HTTP 1.0 request with given value *value* and current time *datum*.

    sprintf( request,
             "GET %s?value=%s&time=%s HTTP/1.0nn",
             path,
             value,
             datum);

Send created HTTP request to server. Ignore response, as it is not needed (see specification).

    send( sock, request, sizeof( request ), 0 );

Close socket, we don’t need it anymore – “fire and forget”.

    closesocket( sock );
}

# Call push_value #

This is a small main function that gets the *host*, *path* and *value* from the command line and calls the *push_value* function. Originally for testing purpose only, but I left it in for you to test it on your own.

int main( int argc, char **argv ) {
    if( argc < 3 ) {
        fprintf( stderr, "usage: %s host path value",
                 argv[0] );
        return -1;
    }
    push_value( argv[1], argv[2], argv[3] );
    return 0;
}

# Conclusion #

With a shell script it would have been a lot easier. Given the above C programme we could create an equivalent bash shell script to do that:

#!/bin/sh
if [ $# -lt 3 ]; then
    echo "usage: $0 host path value"
    exit -1
fi
TIME=$(date +%Y%m%d%H%M%S)
wget -q -O /dev/null "http://$1:80$2?value=$3&time=$TIME"

On the other hand this shell script takes about twice the time to complete than the C programme. Decide for yourself if it’s worth the cost.

If you like you can download the [C Source Code](http://www.sdm-net.org/data/dev/pushclient.c) or the [Bash Script](http://www.sdm-net.org/data/dev/pushclient.sh). Compile the C programme with `gcc -o pushclient pushclient.c`.