C Language, unix shell script execution

Helmut Wenisc
#1 / 8

unix shell script execution

I have an unix shell script, which executes a commercial program. The shell
script is called with two parameters and returns a value to stdout.
I want to call this shell script inside a C-Function definition. The
C-Function should have two parameters (passed to the shell script). The
shell script's standard output should be the return value of the C-Function.
Any suggestions welcome!

Helmut Wenisch

Tue, 27 Jan 2004 14:11:29 GMT

Duncan Bayn
#2 / 8

unix shell script execution

Quote:

> I have an unix shell script, which executes a commercial program. The shell
> script is called with two parameters and returns a value to stdout.
> I want to call this shell script inside a C-Function definition. The
> C-Function should have two parameters (passed to the shell script). The
> shell script's standard output should be the return value of the C-Function.
> Any suggestions welcome!

AFAIK, snprintf() isn't ISO C, so my answer is strictly off-topic.
Generally, you'd be best off trying a UNIX programming newsgroup for
questions like yours.

However, I suggest trying something like the snippet below (untested).
I suggest also checking your systems manpage for the system() function.

int call_program(char *param1, char *param2)
{
char command[1024];
int result;

if ( (param1 == NULL) || (param2 == NULL) )
{
return -1;
}

snprintf(command, 1023, "/usr/bin/foo %s %s", param1, param2);
result = system(command);

return result;

Quote:

}

--
Duncan Bayne
1974 Yamaha RD350

+---------------------------+---------------------------------------+

+---------------------------+---------------------------------------+
| "Retreat Hell! We're just attacking in another direction." |
| -- Major General Oliver P. Smith |
+-------------------------------------------------------------------+

Tue, 27 Jan 2004 14:32:00 GMT

Daniel Fische
#3 / 8

unix shell script execution

Hi,

Quote:

> AFAIK, snprintf() isn't ISO C, so my answer is strictly off-topic.

snprintf is part of the current C standard.

Daniel

(followup-to comp.lang.c)

--
IMO, anyway.

acllc-c++ FAQ: http://snurse-l.org/acllc-c++/faq.html
08/10 Independence Day in Ecuador

Tue, 27 Jan 2004 16:40:24 GMT

Jens.Toerr..
#4 / 8

unix shell script execution

Quote:

>> I have an unix shell script, which executes a commercial program. The shell
>> script is called with two parameters and returns a value to stdout.
>> I want to call this shell script inside a C-Function definition. The
>> C-Function should have two parameters (passed to the shell script). The
>> shell script's standard output should be the return value of the C-Function.
>> Any suggestions welcome!
> AFAIK, snprintf() isn't ISO C, so my answer is strictly off-topic.
> Generally, you'd be best off trying a UNIX programming newsgroup for
> questions like yours.
> However, I suggest trying something like the snippet below (untested).
> I suggest also checking your systems manpage for the system() function.
> int call_program(char *param1, char *param2)
> {
> char command[1024];
> int result;
> if ( (param1 == NULL) || (param2 == NULL) )
> {
> return -1;
> }
> snprintf(command, 1023, "/usr/bin/foo %s %s", param1, param2);
> result = system(command);
> return result;
> }

That's not exactly what was asked for, because the standard output of
the script, not it's return status is needed. But, unfortunately, the
full answer gets really off topic for comp.lang.c because this needs
the popen()/pclose() function which definitely is not ISO C. So it
should better have been asked and answered in comp.unix.programmer...

But anyway, you may try a slightly changed version of Duncans function:

const char *call_program( const char *param1, const char *param2 )
{
FILE *fp;
char command[ 1024 ];
static char result[ 1024 ]; /* make this long enough ! */
int chars_read;

if ( param1 == NULL || param2 == NULL )
return NULL;

snprintf( command, 1023, "/usr/bin/foo %s %s", param1, param2 );

if ( ( fp = popen( command, "r" ) ) == NULL )
return NULL;

chars_read = fscanf( fp, "%1023c", result );
pclose( fp );

if ( chars_read == EOF )
return NULL;
else
result[ chars_read ] = '\0';

return result;

Quote:

}

Some comments: 1. Make sure the 'result' buffer is long enough to hold
the output of the script (and, if necessary also change the format
string in fscanf()) . 2. Make sure to strcpy the string received
from the function before calling it again. 3. If you're catching SIGCHLD
signals in your program don't wait() for the return status - pclose()
will already have reaped it - if this is a problem you'll have to roll
your own popen() using pipe()/fork()/exec(). 4 I did *NOT* test it.

Tue, 27 Jan 2004 17:32:09 GMT

James Dennet
#5 / 8

unix shell script execution

Quote:

> AFAIK, snprintf() isn't ISO C, so my answer is strictly off-topic.

You're in luck. snprintf made it into the latest C Standard,
C99.

-- James Dennett

Tue, 27 Jan 2004 17:50:43 GMT

Richard B
#6 / 8

unix shell script execution

Quote:

> > I want to call this shell script inside a C-Function definition. The
> > C-Function should have two parameters (passed to the shell script).

> AFAIK, snprintf() isn't ISO C, so my answer is strictly off-topic.

It is in C99.

Quote:

> int call_program(char *param1, char *param2)
> {
> char command[1024];
> int result;
> snprintf(command, 1023, "/usr/bin/foo %s %s", param1, param2);

Even better, you can use snprintf() with a size of 0; it will then write
nothing, but it will still return the number of characters that _would_
have been written (but not counting the null terminator) had the size
been large enough. Then, if your array is too small, you can try to
malloc() a sufficiently large one, and snprintf() to that instead.

Do I have to tell you to be careful what you pass to the shell? No?
Thought not.

Richard

Tue, 27 Jan 2004 18:03:04 GMT

Greg Come
#7 / 8

unix shell script execution

Quote:

>AFAIK, snprintf() isn't ISO C, so my answer is strictly off-topic.

It's not C90, but it is C99.
--
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Tue, 27 Jan 2004 22:02:27 GMT

Gael Trinqua
#8 / 8

unix shell script execution

Quote:

> I have an unix shell script, which executes a commercial program. The shell
> script is called with two parameters and returns a value to stdout.
> I want to call this shell script inside a C-Function definition. The
> C-Function should have two parameters (passed to the shell script). The
> shell script's standard output should be the return value of the C-Function.
> Any suggestions welcome!

> Helmut Wenisch

There's a function like system() in stdlib.h that does the same job
but where you can get back stdout. Have a look at the header.

Gael.

Tue, 27 Jan 2004 22:15:51 GMT