[C language] system function print output when not expected.


 
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# 1  
Old 05-04-2010
[C language] system function print output when not expected.

Hi,

I am new to C and have a little problem.
I am not planning to be a C expert, but this would be nice to understand.

The problem is that a 'system' call prints it output to stdout, when I do not expect this.

This is the program:

trial.c

Code:
#include <ctype.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>


int main() {

fprintf(stdout, "-> main()\n");

int return_value;
char passing_var[9];

strcpy(passing_var, "datum");

fprintf(stdout, "main() passing_var  = %s\n",passing_var);

return_value=date(passing_var);

fprintf(stdout, "main() return_value = %s\n",return_value);
fprintf(stdout, "<- main()\n");

}


int date(char *passing_var) {

fprintf(stdout, "-> date()\n");
fprintf(stdout, "date() passing_var  = %s\n",passing_var);

char command[128];
int date_return;

strcpy(command, "date");

fprintf(stdout, "date() command      = %s\n",command);

date_return=system(command);

fprintf(stdout, "date() date_return  = %s\n",date_return);
fprintf(stdout, "<- date()\n");

return date_return;

}

This is the output:

# ./trial

Code:
-> main()
main() passing_var  = datum
-> date()
date() passing_var  = datum
date() command      = date
Tue May  4 13:53:42 GMT 2010
date() date_return  =
<- date()
main() return_value =
<- main()

I do not understand why the 'date' output is printed in the 'date()' subroutine and not given to the 'date_value' variable.

Probably wrong declarations and other mistakes.

Can someone help and shine a light on this ?

Many thanks in advance,

E.J.
# 2  
Old 05-04-2010
Read the man page for system(). It does not return a string, it returns the status of the command, as an integer -- probably 0 most of the time, or nonzero on error. If you want to actually capture the output of the command, use popen, it opens a FILE stream you can read from.

Better yet, don't create external processes for things you can do inside C. See man gettimeofday and man ctime.
# 3  
Old 05-04-2010
Thanks.

That explained why I did not see the return values.
But the command output is still printed.

Code:
-> main()
main() passing_var  = datum
-> date()
date() passing_var  = datum
date() command      = date
Tue May  4 15:10:34 GMT 2010
date() date_return  = 0
<- date()
main() return_value = 0
<- main()

Probably the command function is not what I need.
I just read that the command function just executes the command and only delivers the exit value of the command executed.
Perhaps I can make use of this.
I do not need the date output, that was just as an example.
I only need an integer, so I will try to use the exit value for this.

---------- Post updated at 05:42 PM ---------- Previous update was at 05:19 PM ----------

As I have control over the exit value of the command executed, I can have the output go to "> /dev/null 2&>1".
Then with "WEXITSTATUS" I can parse the exit value to my variable.

Slowly getting there :-)
# 4  
Old 05-04-2010
Quote:
Originally Posted by ejdv
Thanks.

That explained why I did not see the return values.
But the command output is still printed.
Yes, it prints to standard output and standard error. When you create a new process it gets copies of your file descriptors.
Quote:
As I have control over the exit value of the command executed, I can have the output go to "> /dev/null 2&>1".
Then with "WEXITSTATUS" I can parse the exit value to my variable.

Slowly getting there :-)
Sure you can. But if that output's the thing you wanted to get, then you're not accomplishing much. Smilie By using system() you're executing a whole new shell and running things inside it. Handy sometimes but if you want to program C, not the way things are generally done.
# 5  
Old 05-04-2010
The problem is that I personally do not want to program C.
I need to adjust a program that is written in C by a colleague that left the company.
With minimum effort, if possible.

The program that is being executed actually only needs to return 0, 1 or -1, so that is quite simple.
If I could program C, I would not use a system command at all.
It is basically a C program calling a Perl program, which seems stupid.

Anyway, it is working now and serves its purpose.

Perhaps later I will have a go at it and integrate the Perl program into the C program.
# 6  
Old 05-04-2010
Quote:
Originally Posted by ejdv
Perhaps later I will have a go at it and integrate the Perl program into the C program.
Vice versa sounds a better idea if you want to avoid C entirely. C has a lot of "gotchas" that may surprise you if you're not familiar with the language.
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