Creating -Processes and Threads

Introduction

Process -creation on Unix-like machines is normally done in two stages. Firstly, fork() is -called. This creates a child process, which is a clone of the calling (parent) -process. The child process is identical (except for a few details) to the -parent. Secondly, a call to one of the exec() family functions -is invoked by the child which changes the program being executed by that process. -For more information about fork() or exec(), -see Open Group.

P.I.P.S. -does not support fork() and exec() and the -reasons for the same are described in the following sections. However, there -are industry standard alternatives supported by P.I.P.S. and are detailed -in the following sections:

Alternatives -to fork() and exec() for creating child -processes
Forking without exec()
exec() without -forking using posix_spawn()

Omission of fork()

A Unix-like kernel is designed -with the two stage process creation in mind. On the Symbian platform (and -Microsoft Windows®), the kernel was designed to create a process, which runs -a single executable, in a single step.

P.I.P.S. does not implement -the fork() operation because this would require an entirely -new way of dealing with processes in the kernel and memory model. For example, -duplicating the address space of an existing process was not something that -was considered in the Symbian platform kernel's design. Symbian took this -decision as the effort involved in customising this feature would be more -than the benefit of providing the fork() function, especially -in the provision of Pthreads. For more information about Pthreads, see the Pthread -Wikipage.

Omission of exec()

On Unix-like systems, exec() is -the standard way to load a process image into memory. Only the initialisation -process is loaded directly into a new address space (special case code in -the kernel), all other processes are loaded into an existing address space -which was initially created by forking.

On the Symbian -platform, the only way to load a process image into memory is using the loader. -It assumes that the image is being loaded as a new process - one that owns -no resources, has no open handles and so on. The change to the loader and -any associated kernel changes, to support the exec() function -were deemed to be too risky.

Generic IPC

P.I.P.S. supports -two types of Inter-Process Communication (IPC) between processes or threads:

Pipes (named and unnamed): Unnamed pipes are created using pipe() and -named pipes are created using mkfifo(). Pipe-based communication -is also possible between a parent and a child process when the child is created -using popen() or popen3().
Local file sockets: These correspond to sockets created with AF_LOCAL/PF_LOCAL/AF_UNIX as -the address family. The semantics of their use are similar to those of their -Unix equivalents.