Working with the 'system' call

HarryBoy · 12-10-2010, 06:10 AM

I have the following code:

Code:

#include <stdio.h>
#include <stdlib.h>

int main()
{
    system("ps -ef | grep myprocessname");

    return 0;
}

When I run this program it outputs the following list of running processes:

Code:

root     10279 10275  0 13:02 ?        00:00:00 myprocessname myvar1=value1 myvar2=value2
root     10341 10337  1 13:02 ?        00:00:00 myprocessname myvar1=value1 myvar2=value2
blah..
blah..

What I want to really do is instead of writing the output to screen I want to read the output and parse the various values value1, value2 etc.

What is the best way to do this?

HarryBoy · 12-10-2010, 08:37 AM

When I run the following:

Quote:

int data_processed = 0;
int file_pipes[2];
char buffer[BUFSIZ -1];
pid_t child;
memset(buffer, '\0', sizeof(buffer));

if(pipe(file_pipes) == 0)
{
child = fork();
if(child < 0)
{
printf("Failed to fork child process\n");
return 1;
}
else if(child == 0)
{
printf("Child process forked\n");
(void)execl("/bin/ps", "ps -ef | grep myProgram ", (char *)0);
return 1;
}
else
{
printf("Parent process\n");
data_processed = read(1, buffer, BUFSIZ);
printf("%d bytes read: %s\n",data_processed, buffer);
}

}

I get:

Quote:

root@MyLinuxMachine myFiles ./myProgram
Parent process
Child process forked
PID TTY TIME CMD
3637 ttyp0 00:00:00 bash
11003 ttyp0 00:00:00 myProgram
11004 ttyp0 00:00:00 ps

1 bytes read:

What am I doing wrong here? I want to read from the stdout of the ps program.

thanks

firstfire · 12-10-2010, 09:30 AM

Hi.
I'd suggest to use `popen()' instead of `system()', e.g.

Code:

#include <stdio.h>

int main()
{
	FILE *data = popen("ps -Af | grep udev","r");
	char c;
	while( (c=fgetc(data)) !=EOF )
		putchar(c);

	pclose(data);
	return 0;
}

You can read and parse the `data' stream as you wish. For example:

Code:

#include <stdio.h>
#include <string.h>

#define BUFLEN 500

void parse(char * line)
{
	char *p1, *p2, *e;
begin:
	e = index(line, '=');
	if(e) { p2=e+1; *e='\0'; }
	else return;
	
	p1 = rindex(line, ' ');
	if(!p1) p1=line;
	else p1++;

	e=p2;
	while(!(strchr(" \t\n\0", *e))) e++;
	if(*e != '\0') { line = e+1; *e='\0'; }

	printf("var: [%s]\nval: [%s]\n", p1, p2);

	goto begin;
}
int main()
{
	FILE *data = popen("ps -Af | grep =","r");
	char buf[BUFLEN]; 

	while(! feof(data) )
	{
		fgets(buf, BUFLEN, data);
		puts(buf);
		parse(buf);
	}
	pclose(data);
	return 0;
}

HarryBoy · 12-10-2010, 09:46 AM

Hi firstfire, thanks for your reply. I have to use the system due to my project.
Your parse function will come in useful thanks.
I have got the following working:

Code:

 int data_processed = 0;
    int pipes[2];

    char buffer[BUFSIZ -1];
    pid_t child;
    memset(buffer, '\0', sizeof(buffer));

    if(pipe(pipes) == 0)
    {
        child = fork();

        if(child < 0)
        {
            printf("Failed to fork child process\n");
            return 1;
        }
        else if (child == 0)//Child process
        {
            dup2( pipes[1], STDOUT_FILENO );    
            close( pipes[0] );
            execl( "/bin/ls", "ls",(char *)0);  
        }
        else
        {
            dup2( pipes[0], STDIN_FILENO );
            close( pipes[1] );
            data_processed = read(pipes[0], buffer, BUFSIZ);
            printf("%d bytes read: %s\n",data_processed, buffer);

        }

    }

Not that this using the 'ls' command. I used this jsut to get it working. My Parent now prints out the ls output.
I cannot for the life of me get the ps -ef command working.
I have used the following:

Quote:

execl("/bin/ps", "ps -ef | grep myprocess", (char *)0);
execl("/bin/ps -ef","ps", (char *)0);
execl("/bin/ps -ef | grep myprocess","ps", (char *)0);
execl("/bin/ps", "ps", buffer, (char *)0);

But none gives me what I expect.

Do you know what I'm doing wrong?

Thanks

firstfire · 12-10-2010, 05:03 PM

Hi.
Try this:

Code:

execl( "/bin/ps", "ps", "-ef",(char *)0);

You should supply execl() with a path to the program binary and a list of arguments to that program. Inside of `ps' this list is accessed via `char **argv'.
Here is an excerpt from `man exec':

Code:

       int execl(const char *path, const char *arg, ...);
                . . . . . . . . . . . . . . . . . .
       The  initial  argument for these functions is the pathname of a file
       which is to be executed.

       The const char *arg and subsequent ellipses  in  the  execl(),  exe‐
       clp(),  and execle() functions can be thought of as arg0, arg1, ...,
       argn.  Together they describe a list of  one  or  more  pointers  to
       null-terminated  strings  that represent the argument list available
       to the executed program.  The first argument, by convention,  should
       point  to the filename associated with the file being executed.  The
       list of arguments must be terminated by a NULL pointer,  and,  since
       these  are  variadic  functions,  this pointer must be cast (char *)
       NULL.

Note that you can not just "pipe" the data from ps to grep using exec because this involves running two processes and establishing a data channel between them, while exec is a low-level function only able to "run" one process. That's why

Code:

execl("/bin/ps", "ps -ef | grep myprocess", (char *)0);
execl("/bin/ps -ef | grep myprocess","ps", (char *)0);

did not work.

On the other hand `system(char *command)' and `popen()' are higher-level functions which pass given command to the shell, where actual processing is done (including redirections):

Code:

system("command") is equivalent to /bin/sh -c "command"