system call as variabe

[rudra]

i have written a code::
#include <stdio.h>
#include <stdlib.h>
int main()
{
int sys;
int i686;
sys=system("/bin/uname -m");
//printf ("%d",sys);
if (sys==i686){
printf("the syst. 32 bit");
}
else
printf("64but") ;
}
where i want the output of "system("/bin/uname -m")" as the variable
sys.....to find the machine structure.this procedure is not quite
working. can you people suggest me some way?

[Richard Heathfield]

Re: system call as variabe

rudra said:

i have written a code::
#include <stdio.h>
#include <stdlib.h>
int main()
{
int sys;
int i686;
sys=system("/bin/uname -m");
//printf ("%d",sys);
if (sys==i686){

Since the defined-but-not-initialised i686 int object doesn't have a value,
how can this comparison be meaningful? I think what you're really trying
to do is find out whether the output of uname is the string "i686", but
the return value of the system() function won't tell you this.

One possibility for achieving your objective is to redirect the output of
the command to a file, and then open and read the file. This would keep
the code reasonably portable. An alternative that you might wish to
explore if you're into system-specific stuff is a pipe (and
comp.unix.progr ammer knows all about pipes).

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Keith Thompson]

Re: system call as variabe

Richard Heathfield <rjh@see.sig.in validwrites:

rudra said:

>i have written a code::
> #include <stdio.h>
> #include <stdlib.h>
> int main()
> {
> int sys;
> int i686;
> sys=system("/bin/uname -m");
> //printf ("%d",sys);
> if (sys==i686){

>
Since the defined-but-not-initialised i686 int object doesn't have a value,
how can this comparison be meaningful? I think what you're really trying
to do is find out whether the output of uname is the string "i686", but
the return value of the system() function won't tell you this.
>
One possibility for achieving your objective is to redirect the output of
the command to a file, and then open and read the file. This would keep
the code reasonably portable. An alternative that you might wish to
explore if you're into system-specific stuff is a pipe (and
comp.unix.progr ammer knows all about pipes).

Portability is a tricky thing. The call system("/bin/uname -m") is
portable in the sense that it uses a function defined by the C
standard, but it will fail unless the string "/bin/uname -m" happens
to be meaningful as a command.

As it happens, any system on which "/bin/uname -m" is a valid command
will have other methods to achieve the same result, using non-portable
functions. <OT>Using a pipe should work, but invoking the underlying
uname() function directly is cleaner and really no less portable.</OT>

Let me repeat the excellent advice to post to comp.unix.progr ammer.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Richard Heathfield]

Re: system call as variabe

Keith Thompson said:

Richard Heathfield <rjh@see.sig.in validwrites:

<snip>

>>
>One possibility for achieving your objective is to redirect the output
>of the command to a file, and then open and read the file. This would
>keep the code reasonably portable. An alternative that you might wish to
>explore if you're into system-specific stuff is a pipe (and
>comp.unix.prog rammer knows all about pipes).

>
Portability is a tricky thing. The call system("/bin/uname -m") is
portable in the sense that it uses a function defined by the C
standard, but it will fail unless the string "/bin/uname -m" happens
to be meaningful as a command.

Yes, which is why I said "reasonably portable" rather than "maximally
portable". One obvious improvement is to have the command string read in
at runtime, allowing the user to provide a command that is relevant to the
host system. This presupposes, alas, that the user knows what he's doing,
and also trusts that he is not a malicious exploiter.

<snip>

Let me repeat the excellent advice to post to comp.unix.progr ammer.

By all means.

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Antoninus Twink]

Re: system call as variabe

On 9 Jul 2008 at 21:50, Keith Thompson wrote:

As it happens, any system on which "/bin/uname -m" is a valid command
will have other methods to achieve the same result, using non-portable
functions. Using a pipe should work, but invoking the underlying
uname() function directly is cleaner and really no less portable.

Cleaner and safer.

Here's a simple example:

#include <stdio.h>
#include <sys/utsname.h>

int main(void)
{
struct utsname buf;
if(uname(&buf)= =0)
printf("sysname : %s\nnodename: %s\nrelease: %s\nversion: %s\nmachine: %s\n",
buf.sysname, buf.nodename, buf.release, buf.version, buf.machine);
return 0;
}

[Antoninus Twink]

Re: system call as variabe

On 9 Jul 2008 at 22:00, Richard Heathfield wrote:

One obvious improvement is to have the command string read in at
runtime, allowing the user to provide a command that is relevant to
the host system.

An improvement, eh? Let's all pray we never have to rely on the security
of your software. Do you also like your programs to run with root
privileges for good measure?