CGI C output correct when executing from the command line, but not from web-request

Question

Code:

#include <unistd.h>
#include <stdio.h>

typedef struct hddinfo
{
    char szDevice[4];
    char szMfg[64];
    char szModel[64];
    int nTemp;
} hddinfo;

int main( int argc, char *argv[] )
{

    setuid( 0 );

    hddinfo hdi[4] = {0};

    FILE *fp = popen("/usr/sbin/hddtemp /dev/sd[abcd]","r");
    if( NULL != fp )
    {

        for( int i = 0; i < 4; i++ )
            fscanf( fp,"/dev/%3[^:]: %64s %64[^:]: %d°C\n",&hdi[i].szDevice, &hdi[i].szMfg, &hdi[i].szModel, &hdi[i].nTemp );

        pclose( fp );

        printf( "Content-type: text/html\n\n" );
        printf( "<table id=\"hddtemp\">\n\t<tr>\n\t\t<th>Device</th>\n\t\t<th>Mfg</th>\n\t\t<th>Model</th>\n\t\t<th>Temp</th>\n\t</tr>\n" );
        for( int i = 0; i < 4; i++ )
            printf( "\t<tr>\n\t\t<td>%s</td>\n\t\t<td>%s</td>\n\t\t<td>%s</td>\n\t\t<td>%d&deg;C</td>\n\t</tr>\n", hdi[i].szDevice, hdi[i].szMfg, hdi[i].szModel, hdi[i].nTemp );
        printf( "</table>" );

    }
    else
        printf( "Status: 400 Bad Request\nContent-type: text/html\n\n" );

    return 0;
}

Output when run from command line as www-data user:

<table id="hddtemp">
    <tr>
        <th>Device</th>
        <th>Mfg</th>
        <th>Model</th>
        <th>Temp</th>
    </tr>
    <tr>
        <td>sda</td>
        <td>WDC</td>
        <td>WD2500AAKX-001CA0</td>
        <td>29&deg;C</td>
    </tr>
    <tr>
        <td>sdb</td>
        <td>WDC</td>
        <td>WD20EURS-63S48Y0</td>
        <td>30&deg;C</td>
    </tr>
    <tr>
        <td>sdc</td>
        <td>WDC</td>
        <td>WD20EURS-63S48Y0</td>
        <td>30&deg;C</td>
    </tr>
    <tr>
        <td>sdd</td>
        <td>WDC</td>
        <td>WD20EURS-63S48Y0</td>
        <td>30&deg;C</td>
    </tr>
</table>

Output when accessed through the web-browser:

<table id="hddtemp">
    <tr>
        <th>Device</th>
        <th>Mfg</th>
        <th>Model</th>
        <th>Temp</th>
    </tr>

    <tr>
        <td>sda</td>
        <td>WDC</td>
        <td>WD2500AAKX-001CA0</td>
        <td>29&deg;C</td>
    </tr>

    <tr>
        <td></td>
        <td></td>
        <td></td>
        <td>0&deg;C</td>
    </tr>
    <tr>
        <td></td>

        <td></td>
        <td></td>
        <td>0&deg;C</td>
    </tr>
    <tr>
        <td></td>
        <td></td>
        <td></td>

        <td>0&deg;C</td>
    </tr>
</table>

Employed Russian · Accepted Answer · 2011-12-28 04:51:41Z

2

It looks like you have non-ASCII format in your fscanf call (for the degree sign).

It is likely that that's where your parsing stops, due to LANG=C while executing from within CGI, while it is set to something else when executing on command line.

answered Dec 28, 2011 at 4:51

Employed Russian

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1 Comment

Drew Chapin Over a year ago

Ah!!!! Now I understand why it's putting the degree sign when run from the command line, and not when it's piped, or called from the web-service. It seem that hddtemp is checking the LANG environment variable and determining whether to put the degree symbol or not. So, if I had changed my command call to LANG=C /user/sbin/hddtemp /dev/sd[abcd]", and removed the degree symbol, it would have worked for both!

Drew Chapin · Accepted Answer · 2011-12-28 04:45:37Z

1

Okay, so I found the problem... Apparently whenever apache runs the program for a web-request, the degree character is not included, and a space character is include instead. I can also replicate that issue from the command line by piping the output to another application like so:

./hddtemp.cgi | tee

Removing the degree character fixes the output for web-requests, but then the problem occurs on command line calls. Looks like I'm stuck with one or the other working.

answered Dec 28, 2011 at 4:45

Drew Chapin

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2 Comments

Drew Chapin Over a year ago

A better options seems to be for me to specify the --numeric flag on hddtemp, but that does mean I lose getting the mfg and sku/model

dicaprio Over a year ago

appreciate your dedication in solving your own problem and also posting your thoughts for others.

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