system function not executed correctly

[Morris Dovey]

When i use system() it always get return code 11 and the command is not
executed regardless of the command used. Does anyone know what is with
it? Environment is Linux, glibc and gcc.

Can you post a minimal program that exhibits the problem
behavior, please?

 

[Anonymous User]

When i use system() it always get return code 11 and the command is not
executed regardless of the command used. Does anyone know what is with
it? Environment is Linux, glibc and gcc.

Regards,
Li Zhou

 

[santosh]

When i use system() it always get return code 11 and the command is
not executed regardless of the command used. Does anyone know what is
with it? Environment is Linux, glibc and gcc.

Can you show us the code?

 

[Anonymous User]

Can you show us the code?

if ( system() ) /* see if a shell exists */
{
int return_code = system("echo hello");
std::cout << "return code " << return_code << std::endl;
}

result is:

return code 11

and repeated lines of:

semop lock failure invalid argument
semop unlock failure invalid argument

 

[Mark Bluemel]

But not C - the code you've shown us is C++

if ( system() ) /* see if a shell exists */

That should be "system(NULL)" according to my man pages and compiler

{
int return_code = system("echo hello");
std::cout << "return code " << return_code << std::endl;
}

result is:

return code 11

and repeated lines of:

semop lock failure invalid argument
semop unlock failure invalid argument

I think you'd do better a) in a C++ group or b) in a forum relating to
the GNU tools you're using. The semop warning messages suggest there's
something awry in your installation, I'd guess.

The code below (which is in C, as far as I can tell works for me
and returns 0 from the second call to system().

#include <stdlib.h>
#include <stdio.h>
int main(void) {
if (system(NULL)) {
int result = system("echo hello world");
printf("Got %d\n",result);
}
}

 

[Richard Heathfield]

Anonymous User said:

if ( system() ) /* see if a shell exists */

That should be:

if ( system(NULL) ) /* see if a shell exists */

{
int return_code = system("echo hello");

That's fine, but there are one and a half reasons why it isn't doing what
you expect. Firstly, the command (*almost* certainly) is being executed,
but a shell is being created for the purpose, the echo is happening within
that shell, and the shell is then terminating - all too fast for you to
even notice (and there's no requirement on the shell, as far as I know, to
provide a visible terminal session, although on some systems it actually
will do that).

One-and-a-halfthly, and this is more of a heads-up in your case than an
actual problem, the return value is not necessarily the one that the
command returns (alas!), but an implementation-defined value. On my
system, however, the docs say that "The value returned is -1 on error
(e.g. fork failed), and the return status of the command otherwise." ISO C
doesn't guarantee this, but perhaps your implementation does.

std::cout << "return code " << return_code << std::endl;

This, however, will not compile. It's stuffed full with errors.

 

[Richard Tobin]

When i use system() it always get return code 11 and the command is not
executed regardless of the command used. Does anyone know what is with
it? Environment is Linux, glibc and gcc.

That usually indicates a segmentation fault in the called command.
It seems unlikely that you would get this for every command, so
you'll have to show us your exact program.

-- Richard

 

[Joachim Schmitz]

That usually indicates a segmentation fault in the called command.

You may be confusing signal 11, SISSEGV, with exit(11)

It seems unlikely that you would get this for every command, so
you'll have to show us your exact program.

-- Richard

Bye, Jojo

 

[Anonymous User]

Anonymous User said:


That should be:

if ( system(NULL) ) /* see if a shell exists */

yeah, otherwise it won't compile.

That's fine, but there are one and a half reasons why it isn't doing what
you expect. Firstly, the command (*almost* certainly) is being executed,
but a shell is being created for the purpose, the echo is happening within
that shell, and the shell is then terminating - all too fast for you to
even notice (and there's no requirement on the shell, as far as I know, to
provide a visible terminal session, although on some systems it actually
will do that).

Hmmm, that echo command was just an example. The real command is to
print something on the printer. When i issue the printing command from
the shell everything works as expected but when used inside system()
nothing happened.

One-and-a-halfthly, and this is more of a heads-up in your case than an
actual problem, the return value is not necessarily the one that the
command returns (alas!), but an implementation-defined value. On my
system, however, the docs say that "The value returned is -1 on error
(e.g. fork failed), and the return status of the command otherwise." ISO C
doesn't guarantee this, but perhaps your implementation does.


This, however, will not compile. It's stuffed full with errors.

This is some c++ code for debugging.

 

[Richard Tobin]

Can you show us the code?

[/QUOTE]

We really need the whole code, not just the bit where you think
the error is.

if ( system() ) /* see if a shell exists */

This is wrong, and shouldn't compile. Have you forgotten to
include stdlib.h?

You should be passing a null argument to it.

{
int return_code = system("echo hello");
std::cout << "return code " << return_code << std::endl;

Um, that looks like C++, not C.

-- Richard

 

[Richard Tobin]

That usually indicates a segmentation fault in the called command.

[/QUOTE]

You may be confusing signal 11, SISSEGV, with exit(11)

No. Under Linux, the return code from system() is the signal number
if the program dies from a signal. If the command exited with status
11, system() would return 11*256.

-- Richard

 

[Joachim Schmitz]

You may be confusing signal 11, SISSEGV, with exit(11)

No. Under Linux, the return code from system() is the signal number
if the program dies from a signal. If the command exited with status
11, system() would return 11*256.[/QUOTE]
Right, sorry, didn't read the man-page properly

int status = system("whatever")
if ( status = -1)
perror("system()");
else if WISIGNALED(status)
fprintf(stderr, "killed by signal %d\n", WTERMSIG(status));
else if WIFEXITED(status)
if (WEXITSTATUS(status))
fprintf(stderr, "exited with\n", WEXITSTATUS(status));
else
printf("all is well!\n");
....

Bye, Jojo

 

[Richard]

Right, sorry, didn't read the man-page properly

int status = system("whatever")
if ( status = -1)
perror("system()");

You might want to use a debugger on your code and examine the condition
above very closely. Hint : keep an eye on "status".

 

[Anonymous User]

But not C - the code you've shown us is C++


That should be "system(NULL)" according to my man pages and compiler


I think you'd do better a) in a C++ group or b) in a forum relating to
the GNU tools you're using. The semop warning messages suggest there's
something awry in your installation, I'd guess.

The code below (which is in C, as far as I can tell works for me
and returns 0 from the second call to system().

#include <stdlib.h>
#include <stdio.h>
int main(void) {
if (system(NULL)) {
int result = system("echo hello world");
printf("Got %d\n",result);
}
}

Yeap, your code works even on Cygwin.

 

[Joachim Schmitz]

You might want to use a debugger on your code and examine the
condition above very closely. Hint : keep an eye on "status".

No debugger needed, it should be status == -1.
and here's a %s missing...

 

[Joachim Schmitz]

No debugger needed, it should be status == -1.

and here's a %s missing...

Rubbish, %d. It's been a long day...

 

[Richard]

No debugger needed, it should be status == -1.

There are many people in CLC who think debuggers are evil. Having worked
on numerous huge multi team projects I think their view is distorted to
say the least. We are not all Linus Torwalds.

I run ALL my code through a debugger watching the locals and parameter
panels as I step through for any unexpected blinks to occur.

This would have immediately alerted you to the change in status.

So my point was more "use a debugger" than "in this case". Well, it was
intended to be.

Littering code with printfs in amateurish at best unless there is no
good HW debugger for your platform.

Read this and good luck!

http://heather.cs.ucdavis.edu/~matloff/UnixAndC/CLanguage/Debug.html#tth_sEc2

 

[Kenneth Brody]

Anonymous User wrote:
[...]

Hmmm, that echo command was just an example. The real command is to
print something on the printer. When i issue the printing command from
the shell everything works as expected but when used inside system()
nothing happened.

[...]

Does the system("echo hello") also return 11? (If not, then why
post code that doesn't fail?)

See Richard Tobin's reply elsethread regarding SEGV crashes.

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