[ale] Accessing /proc from C

[cfowler]

Here is where a discussion may get started.  Some people like binary
some like ASCII.  I prefer Binary data whenever possible because it
allows for fewer cycles to go to parsing.  However it is not as portable
as ASCII.  So in /proc all that data is ASCII.  You must parse them out
like you would any file but the code is not that much of a killer.  Use
buffered i/o fgets() and friends to read the data in.  Then use a
sscanf() function to parse the data into char arrays (strings).  You can
eaisly display the data in that file.

I *heavily* modified the Comtrol Rocket Port driver for Linux.  It now
includes instant reporting in /proc/rocketport/  of that status of each
port.  I have a program that needs to retrieve DCD state.  Now I can
eaisly get DCD stat by using an ioctl() on a open file but this would
assert DTR and RTS on that port.  This is not the behavior that I
wanted.  So I needed a interface (/proc is a good one) that would allow
me to get data about each individual port without open()ing that port. 
So that is why I had to do some modifications.  But, below is some code
that eaisly parses the status of the port from this:

[cfowler at badass]# pwd
/proc/rocketport
[cfowler at badass]# cat 1
Port: 1
OUT: 0  IN: 7698
AOUT: 0  AIN: 1089
Refcount: 1
LastCharIn: 40839886
DTR: 1
DCD: 1
CTS: 0
RTS: 1
DSR: 0
DCDChanges: 3
[cfowler at badass]# 



/*
 * get_dcd_state() -    Return a pointer to a (static) string containing
the
 *                      status of the DCD pin of the specified port
(0-based)
 *
 *                      Essentially a stripped down version of
 *                      snmp-agent/outpostUtils.c:getTermios()
 */
static char *
get_dcd_state(int port)
{
    char pName[64] ;
    FILE *fp ;
    char *status = "unknown" ;

    snprintf(pName, sizeof(pName), "/proc/rocketport/%d", port) ;

    /*
     * Read the /proc entry, looking for a line that says:
     * "DCD: x", where x = 0 or 1. Failsafe to "unknown"
     * if we can't find anything useful or even open the
     * file.
     */
    if ((fp = fopen(pName, "r")) != NULL) {
        char buf[100] ;
        int i ;
        while (fgets(buf, sizeof(buf), fp) != NULL) {
            if (sscanf(buf, "DCD: %d\n", &i) == 1) {
                if (i == 0) {
                    status = "false" ;
                } else if (i == 1) {
                    status = "true" ;
                }
                break ;
            }
        }
        fclose(fp) ;
    }

    return(status) ;
}

So see, it really is not that hard to parse the data from that file. 
You'll need to do this for all files in proc.


Here is code that parses the data from /proc/<pid>/status

static void parseproc(char *S, proc_t *P)
{
        char *tmp;

        memset(P->cmd, 0, sizeof(P->cmd));
        P->pid = P->mem = P->data = 0;

        sscanf(S, "Name:\t%15c", P->cmd);
        tmp = strchr(P->cmd, '\n');
        if (tmp)
                *tmp= '\0';

        tmp = strstr(S, "State:");
        if(tmp)
                sscanf(tmp, "State:\t%c\n", &P->state);
        tmp = strstr(S,"Pid:");
        if (tmp)
                sscanf(tmp, "Pid:\t%d\n", &P->pid);
        tmp = strstr(S,"PPid:");
        if (tmp)
                sscanf(tmp, "PPid:\t%d\n", &P->ppid);

        tmp = strstr(S, "VmRSS:");
        if (tmp)
                sscanf(tmp, "VmRSS:\t%d\n", &P->mem);


        tmp = strstr(S, "VmData:");
        if(tmp)
                sscanf(tmp, "VmData:\r%d\n", &P->data);

        tmp = strstr(S, "Uid:");
        if(tmp)
                sscanf(tmp, "Uid:\r%d\n", &P->ruid);

}





On Sat, 2002-12-28 at 21:09, Benjamin Dixon wrote:
> 
> Does anyone know if its possible to access information stored in the /proc
> filesystem from C without resorting to reading the file and parsing the
> strings? Any references or links? I've found plenty of stuff on how to
> write modules that use /proc but nothing on how to interact with it other
> than from the command line.
> 
> Ben
> 
> 
> _______________________________________________
> Ale mailing list
> Ale at ale.org
> http://www.ale.org/mailman/listinfo/ale


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