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<concept id="GUID-AF07AD54-86F1-5DB7-80FF-633A559DA4BD" xml:lang="en"><title>Creating

Processes and Threads </title><prolog><metadata><keywords/></metadata></prolog><conbody>

<section id="GUID-6E0A2832-98BA-54AF-B7A7-28DFFD5FEC76"><title>Introduction</title> <p>Process

creation on Unix-like machines is normally done in two stages. Firstly, <codeph>fork()</codeph> 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 <codeph>exec()</codeph> family functions

is invoked by the child which changes the program being executed by that process.

For more information about <codeph>fork()</codeph> or <codeph>exec()</codeph>,

see <xref href="http://www.opengroup.org" scope="external">Open Group</xref>. </p> <p>P.I.P.S.

does not support <codeph>fork()</codeph> and <codeph>exec()</codeph> 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: </p> <ul>

<li><p><xref href="GUID-ADB9AD91-37A2-4FE6-B378-C43CD0A7B513.dita">Alternatives

to <codeph>fork()</codeph> and <codeph>exec()</codeph> for creating child

processes</xref></p></li>

<li><p><xref href="GUID-545DA961-217E-49CA-A90E-7A41E2C03A99.dita">Forking without <codeph>exec()</codeph></xref></p></li>

<li><p><xref href="GUID-B58D282E-E6EC-4AE1-96DB-30A5FB850563.dita"><codeph>exec()</codeph> without

forking using <apiname>posix_spawn()</apiname></xref></p></li>

</ul> <p><b>Omission of fork()</b> </p> <p>A Unix-like kernel

is designed with the two stage process creation in mind. On Symbian platform

(and Microsoft Windows®) the kernel was designed to create a process, which

runs a single executable, in a single step. </p> <p>P.I.P.S. does not implement

the <codeph>fork()</codeph> 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 <codeph>fork()</codeph> function, especially

in the provision of Pthreads. For more information about Pthreads, see the <xref href="http://en.wikipedia.org/wiki/POSIX_Threads" scope="external">Pthread

Wikipage</xref>. </p> <p><b>Omission of exec()</b> </p> <p>On Unix-like systems, <codeph>exec()</codeph> 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. </p> <p>On 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 <codeph>exec()</codeph> function were deemed to be

too risky. </p> <p><b>Generic IPC</b> </p> <p>P.I.P.S. supports two types

of Inter-Process Communication (IPC) between processes or threads: </p> <ul>

<li><p> <b>Pipes (named and unnamed):</b> Unnamed pipes are created using <xref href="GUID-EC7F85FF-FD4E-324C-8D01-EF6F7E7A79FE.dita"><apiname>pipe()</apiname></xref> and

named pipes are created using <xref href="GUID-F4749DAA-1B29-3D1D-A3AA-0D52B851E501.dita"><apiname>mkfifo()</apiname></xref>. Pipe-based communication

is also possible between a parent and a child process when the child is created

using <xref href="GUID-A9DB6E7C-B8D6-377A-BBE6-39E0A7A09E5D.dita"><apiname>popen()</apiname></xref> or <xref href="GUID-A1E2E172-C4E4-30FB-99CF-E4140E31DF59.dita"><apiname>popen3()</apiname></xref>. </p></li>

<li><p> <b>Local file sockets:</b> These correspond to sockets created with <codeph>AF_LOCAL/PF_LOCAL/AF_UNIX</codeph> as

the address family. The semantics of their use are similar to those of their

Unix equivalents. </p></li>

</ul> </section>

</conbody><related-links>

<link href="GUID-6590B534-D976-5305-BE95-48DD05120DFB.dita#GUID-6590B534-D976-5305-BE95-48DD05120DFB/GUID-3CA8FF29-79B4-5D43-8A23-EDB136BF7A8F">

<linktext>One Definition Rule - warning</linktext></link>

</related-links></concept>