Get system() error message

[jose luis fernandez diaz]

Hi,

In the program below:

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

using namespace std;
int main()
{
system("ls asdf");

printf("%d\n", errno);

return 0;
}


the output is:

ls: asdf not found
0


Two questions:

1) How can I get the error message(ls: asdf not found) in the program?

2) Why is errno 0 ?

Thanks,
Jose Luis.

 

[Joona I Palaste]

jose luis fernandez diaz <[email protected]> scribbled the following

Hi,

In the program below:

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

using namespace std;
int main()
{
system("ls asdf");

printf("%d\n", errno);

return 0;
}

the output is:

ls: asdf not found
0

Two questions:

1) How can I get the error message(ls: asdf not found) in the program?

By redirecting it to a file and reading from there. It's the only way
supported by ISO standard C or C++.

2) Why is errno 0 ?

Because the program was correctly able to call the command ls. The fact
that the command ls itself failed is none of C's or C++'s concern.

 

[Darrell Grainger]

Hi,

In the program below:

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

using namespace std;
int main()
{
system("ls asdf");
printf("%d\n", errno);
return 0;
}

the output is:

ls: asdf not found
0

Two questions:

1) How can I get the error message(ls: asdf not found) in the program?

Cannot give a definitive answer using just ISO/ANSI C. You need to get
into system specific code. Something like:

system("ls asdf > error.txt");

might work. But not if > only captures stdout and the message is sent to
stderr. The type of shell that system is using will make a difference. You
might need to use:

system("ls asdf 2> error.txt");
or
system("ls asdf >& error.txt");

The possibilities are many. It will depend on so many factors. Namely, how
is system() implemented? How is the command implemented? how is the system
configured?

2) Why is errno 0 ?

Why not? There is nothing in the system() command that indicates it will
set errno for any reason. I could run a program as a user where system()
fails and it still won't set errno to anything different.

You are going to have to restrict your solution to a specific operating
system. In which case you need to ask a newsgroup that deals with your
operating system to find out the best solution.

 

[Dan Pop]

In the program below:

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

using namespace std;

This is a syntax error in C. If you cross-post, make sure that your code
is correct in both languages!

int main()
{
system("ls asdf");

printf("%d\n", errno);

return 0;
}

the output is:

ls: asdf not found
0


Two questions:

1) How can I get the error message(ls: asdf not found) in the program?

By not using system() in the first place. Your platform provides a better
alternative. It's even documented in the c.l.c FAQ, which you didn't
bother to check *before* posting.

2) Why is errno 0 ?

Why not? The C standard doesn't require system() to touch errno at all
and your platform's documentation says:

ERRORS

The system() function may set errno values as described by fork().
In addition, system() may fail if:

[ECHILD]
The status of the child process created by system()
is no longer available.

The implied fork() call obviously succeeded, so it had no reason to touch
errno, either.

Before posting such questions, try to understand how things work and
check the documentation. Sure, there *was* an errno set somewhere,
but this somewhere was the process executing the ls command, not *your*
program. That errno was lost without trace by the time that process
terminated, before your system() call returned.

Dan

 

[Kieran Simkin]

The way I understand it, piping output to a file via system() isn't easily
do-able portably. You might want to look into popen() or a pipe() - fork() -
exec()

--


~Kieran Simkin
Digital Crocus
http://digital-crocus.com/

 

[Dan Pop]

The way I understand it, piping output to a file via system() isn't easily
do-able portably. You might want to look into popen() or a pipe() - fork() -
exec()

Redirecting output to a file via system() is a lot more portable than
popen() or pipe-fork-exec. So, if portability is an overriding issue,
output redirection is the winning approch.

OTOH, portability is seldom an issue when system() is used, considering
that system's argument is highly non-portable (when it's not a null
pointer).

Dan