how to capture result from system() in C/C++ [duplicate]

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Closed 15 years ago.

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How can I run an external program from C and parse its output?

Hi,

Could someone please tell us how to capture a result when executing system() function ?

Actually I wrote a c++ program that displays the machine's IP address, called "ipdisp" and I want when a sever program executes this ipdisp program, the server captes the display IP address. So, is this possible? if yes, how?

thanks for your replies

• dmckee --- ex-moderator kitten: Been asked and answered many time. One example: stackoverflow.com/questions/43116/…
• make: Sorry I didn't know as when I wrote the question, I didn't see related questions . thankS!

Answer 1

Yes, you can do this but you can't use system(), you'll have to use popen() instead. Something like:

FILE *f = popen("ipdisp", "r");
while (!feof(f)) {
 // ... read lines from f using regular stdio functions
}
pclose(f);

• make: thanks a lot! just to clarify please . the variable that display is e.g. (char *xx1;) so how can we get xx1? thanks again!
• Greg Hewgill: Assuming ipdisp writes its output to stdout, you'll be able to read its stdout through the file f above. Use fgets or similar.
• make: thanks! Actually I have two variables: hostname and ipaddress and I want to capte the ipaddress. is the stdout takes the last displayed one? thanks again
• make: I wrote a small program as you said, but when I read stdout I get this : Server IP-Address is: 192.168.0.113 and it doesn't display anything. So is it possible to get only this "192.168.0.113". thanks!

Answer 2

Greg is not entirely correct. You can use system, but it's a really bad idea. You can use system by writing the output of the command to a temporary file and then reading the file...but popen() is a much better approach. For example:

#include <stdlib.h>
#include <stdio.h>
void
die( char *msg ) {
 perror( msg );
 exit( EXIT_FAILURE );
}
int
main( void )
{
 size_t len;
 FILE *f;
 int c;
 char *buf;
 char *cmd = "echo foo";
 char *path = "/tmp/output"; /* Should really use mkstemp() */
 len = (size_t) snprintf( buf, 0, "%s> %s", cmd, path ) + 1;
 buf = malloc( len );
 if( buf == NULL ) die( "malloc");
 snprintf( buf, len, "%s> %s", cmd, path );
 if( system( buf )) die( buf );
 f = fopen( path, "r" );
 if( f == NULL ) die( path );
 printf( "output of command: %s\n", buf );
 while(( c = getc( f )) != EOF )
 fputc( c, stdout );
 return EXIT_SUCCESS;
}

There are lots of problems with this approach...(portability of the syntax for redirection, leaving the file on the filesystem, security issues with other processes reading the temporary file, etc, etc.)