Getting Started with QNX Neutrino - QNX Software Systems

© 2009, QNX Software Systems GmbH & Co. KG. Threads and processes
In fact, system() actually starts up a shell to handle the command that you want to
perform.
The editor that I’m using to write this book makes use of the system() call. When I’m
editing, I may need to “shell out,” check out some samples, and then come back into
the editor, all without losing my place. In this editor, I may issue the command :!pwd
for example, to display the current working directory. The editor runs this code for the
:!pwd command:
system ("pwd");
Is system() suited for everything under the sun? Of course not, but it’s useful for a lot
of your process-creation requirements.
Starting a process with the exec() and spawn() calls
Let’s look at some of the other process-creation functions.
The next process-creation functions we should look at are the exec() and spawn()
families. Before we go into the details, let’s see what the differences are between these
two groups of functions.
The exec() family transforms the current process into another one. What I mean by
that is that when a process issues an exec() function call, that process ceases to run the
current program and begins to run another program. The process ID doesn’t change —
that process changed into another program. What happened to all the threads in the
process? We’ll come back to that when we look at fork().
The spawn() family, on the other hand, doesn’t do that. Calling a member of the
spawn() family creates another process (with a new process ID) that corresponds to
the program specified in the function’s arguments.
Let’s look at the different variants of the spawn() and exec() functions. In the table that
follows, you’ll see which ones are POSIX and which aren’t. Of course, for maximum
portability, you’ll want to use only the POSIX functions.
Spawn POSIX? Exec POSIX?
spawn() No
spawnl() No execl() Yes
spawnle() No execle() Yes
spawnlp() No execlp() Yes
spawnlpe() No execlpe() No
spawnp() No
spawnv() No execv() Yes
continued. . .
April 30, 2009 Chapter 1 • Processes and Threads 29