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<H2><A NAME="SECTION001143000000000000000">10.4.3 Process control under Linux</A></H2>
<P>
Under Linux, the <EM>ptrace</EM> system call is supported for process
control, and it works as in 4.3BSD. To obtain process and system
information, Linux also provides a <EM>/proc</EM> filesystem, but with
very different semantics. Under Linux, <EM>/proc</EM> consists of a
number of files providing general system information, such as
memory usage, load average, loaded module statistics, and network
statistics. These files are generally accessed using <EM>read</EM>
and <EM>write</EM> and their contents can be parsed using <EM>scanf</EM>.
The <EM>/proc</EM> filesystem under Linux also provides a directory
entry for each running process, named by process ID, which
contains file entries for information such as the command line, links
to the current working directory and executable file, open file
descriptors, and so forth. The kernel provides all of this information
on the fly in response to <EM>read</EM> requests. This implementation
is not unlike the <EM>/proc</EM> filesystem found in Plan 9, but it
does have its drawbacks--for example, for a tool such as <EM>ps</EM>
to list a table of information on all running processes, many
directories must be traversed and many files opened and read.
By comparison, the <EM>kvm</EM> routines used on other UNIX systems
read kernel data structures directly with only a few system calls.
<P>
Obviously, each implementation is so vastly different that porting
applications which use them can prove to be a real task. It should
be pointed out that the SVR4 <EM>/proc</EM> filesystem is a very
different beast than that found in Linux, and they may not be used
in the same context. Arguably, any program which uses the <EM>kvm</EM>
routines or SVR4 <EM>/proc</EM> filesystem is not really portable, and
those sections of code should be rewritten for each operating system.
<P>
The Linux <EM>ptrace</EM> call is nearly identical to that found in BSD,
but there are a few differences:
<UL><LI> The requests <TT>PTRACE_PEEKUSER</TT> and <TT>PTRACE_POKEUSER</TT> under
BSD are named <TT>PTRACE_PEEKUSR</TT> and <TT>PTRACE_POKEUSR</TT>, respectively,
under Linux.<LI> Process registers can be set using the <TT>PTRACE_POKEUSR</TT> request
with offsets found in <TT>/usr/include/linux/ptrace.h</TT>.<LI> The SunOS requests <TT>PTRACE_{READ,WRITE}{TEXT,DATA}</TT> are
not supported, nor are <TT>PTRACE_SETACBKPT</TT>, <TT>PTRACE_SETWRBKPT</TT>,
<TT>PTRACE_CLRBKPT</TT>, or <TT>PTRACE_DUMPCORE</TT>.
These missing requests should only affect a small number of existing
programs.
</UL>
<P>
Linux does <EM>not</EM> provide the <EM>kvm</EM> routines for reading the
kernel address space from a user program, but some programs (most
notably <EM>kmem_ps</EM>) implement their own versions of these routines.
In general, these are not portable, and any code which uses the
<EM>kvm</EM> routines is probably depending upon the availability of
certain symbols or data structures in the kernel--not a safe assumption
to make. Use of <EM>kvm</EM> routines should be considered architecture-specific.
<P>
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