Re: How to execute a cmd by C++ ?

From:

Ian Collins <ian-news@hotmail.com>

Newsgroups:

comp.lang.c++

Date:

Thu, 26 Jun 2014 09:53:29 +1200

Message-ID:

<c10umpFi725U1@mid.individual.net>

Scott Lurndal wrote:

Jorgen Grahn <grahn+nntp@snipabacken.se> writes:

On Wed, 2014-06-25, Chris Vine wrote:
...

It can be assumed that on a POSIX conforming implementation, and
presumably in windows (although I don't use it), that system() will
work.

However, system() is usually the wrong choice, since it calls up a
command processor (a shell) to parse the command, which adds significant
overhead.

IMO it's not usually so much about efficiency as it is about control.
You're blocked while the shell executes, you cannot feed it input or
capture its output, and if you build the string from user input you
might execute something you didn't expect, the same way that SQL
injection attacks work.

On the other hand, getting everything right around the fork/exec
is more problematic than just using system() in most cases.

If it's something you have to do often (which I do!), writing a wrapper
library for fork/exec is a one off dose of pain. My code is scattered
with the likes of

     Caller slappasswd;

     const std::string command("/usr/sbin/slappasswd -h {SSHA} -s ");

     std::string hash = slappasswd( command+password );

And
the performance issues are generally a red herring for intermittent
use.

I agree, it's often more convenient to call a system utility with well
defined output than it is to use the underlying system libraries. More
so if the library in question isn't stable. In the example above I
could use the SSL hashing library calls directly and I would if I was
generating millions of hashes, but for a few convenience wins.

And you often want variable substitution and other shell-like
capabilities in the command provided to system().

You do? Yikes!

--
Ian Collins