redirect stderr

[wega007]

I understand I can use freopen to redirect stderror to a file; I can get my program working, however I do not want to be doing that.

I am running on Ubuntu. doing some googling I understand os.popen4 require me to install libs in my system. I cannot be doing that as this program eventually run in an embeded system.

I found pstream (PStreams - POSIX Process Control in C++), but I am unable to use it according to their examples.

string str;
ipstream host("hostname");
getline(host, str); <=== always error as getline expects (char**, size_t*, FILE*), changed the syntax and I am still getting error as host isn't FILE *

did a quick Ubuntu and pstream search and I found I need to install the libs as well.

I am using popen to run some command, and wish to capture the stderr without directing it to a file. Is there a way to cleanly and easily do it?
eg:
FILE *fp = popen(command, "r");

while (fgets(buf, 511, fp))
{
...
}

[gerard4143]

if your just trying to send or redirect stderr why don't you try dup2(int old, int new)

[wega007]

dup2 duplicate a file descriptor, how will that help in redirecting stderr?
have an example or link to help? I am not getting the use at this time.

[gerard4143]

stderr file descriptor is 2....I hope this helps

stdin = 0
stdout = 1
stderr = 2

[wega007]

Thanx, I read up on this thread on dup but somehow I am not able to get it working. The stderr is not redirected. what could be the issue here.
Yahoo! Groups


FILE *fp = popen(command, "r");
int d = fileno(fp);
dup2(d,fileno(stdout));
dup2(d,fileno(stderr);

while (fgets(buf, 511, fp))
{
....
}

[wega007]

I can get popen( "command1 2>&1", "r" ) to redirect stderr
however if there is a string of commands
eg:
"command1;command2;command3" I will need to perform a insert of " 2>&1" according.

Is there a easier to get dup2, freopen etc so that I don't have to do " 2>&1"?

[gerard4143]

I never used popen before but if your using pipes and redirection this is a good example of redirection....just compile this program then run it:

./program ps wc

and it will redirect the bash command ps into wc

#include <unistd.h>
#include <stdlib.h>

enum PIPES {READ, WRITE};

int main(int argc, char**argv)
{
	if (argv[2])
	{
		int hpipe[2];
		pipe(hpipe);

		if (fork())
		{
			close(hpipe[WRITE]);
			dup2(hpipe[READ], 0);//stdin = 0
			close(hpipe[READ]);
			execlp(argv[2],argv[2],NULL);
		}
		else
		{
			close(hpipe[READ]);
			dup2(hpipe[WRITE], 1);//stdout = 1
			close(hpipe[WRITE]);
			execlp(argv[1],argv[1],NULL);
		}
	}
	exit(EXIT_SUCCESS);
}