+++ /dev/null
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-><DIV
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->Name</H2
->cyg_hal_sys_xyz -- Access Linux system facilities</DIV
-><DIV
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->Synopsis</H2
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-CLASS="FUNCSYNOPSIS"
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-><TR
-><TD
-><PRE
-CLASS="FUNCSYNOPSISINFO"
->#include <cyg/hal/hal_io.h>
- </PRE
-></TD
-></TR
-></TABLE
-><P
-><CODE
-><CODE
-CLASS="FUNCDEF"
->int cyg_hal_sys_xyzzy</CODE
->(...);</CODE
-></P
-><P
-></P
-></DIV
-></DIV
-><DIV
-CLASS="REFSECT1"
-><A
-NAME="SYNTH-SYSCALLS-DESCRIPTION"
-></A
-><H2
->Description</H2
-><P
->On a real embedded target eCos interacts with the hardware by peeking
-and poking various registers, manipulating special regions of memory,
-and so on. The synthetic target does not access hardware directly.
-Instead I/O and other operations are emulated by making appropriate
-Linux system calls. The HAL package exports a number of functions
-which allow other packages, or even application code, to make these
-same system calls. However this facility must be used with care: any
-code which calls, for example, <TT
-CLASS="FUNCTION"
->cyg_hal_sys_write</TT
->
-will only ever run on the synthetic target; that functionality is
-obviously not provided on any real hardware because there is no
-underlying Linux kernel to implement it.
- </P
-><P
->The synthetic target only provides a subset of the available system
-calls, specifically those calls which have proved useful to implement
-I/O emulation. This subset can be extended fairly easily if necessary.
-All of the available calls, plus associated data structures and
-macros, are defined in the header file <TT
-CLASS="FILENAME"
->cyg/hal/hal_io.h</TT
->. There is a simple
-convention: given a Linux system call such as
-<TT
-CLASS="FUNCTION"
->open</TT
->, the synthetic target will prefix
-<TT
-CLASS="LITERAL"
->cyg_hal_sys</TT
-> and provide a function with that name.
-The second argument to the <TT
-CLASS="FUNCTION"
->open</TT
-> system call is
-a set of flags such as <TT
-CLASS="CONSTANT"
->O_RDONLY</TT
->, and the header
-file will define a matching constant
-<TT
-CLASS="CONSTANT"
->CYG_HAL_SYS_O_RDONLY</TT
->. There are also data
-structures such as <SPAN
-CLASS="STRUCTNAME"
->cyg_hal_sys_sigset_t</SPAN
->,
-matching the Linux data structure <SPAN
-CLASS="STRUCTNAME"
->sigset_t</SPAN
->.
- </P
-><P
->In most cases the functions provided by the synthetic target behave as
-per the documentation for the Linux system calls, and section 2 of the
-Linux man pages can be consulted for more information. There is one
-important difference: typically the documentation will say that a
-function returns <TT
-CLASS="LITERAL"
->-1</TT
-> to indicate an error, with the
-actual error code held in <TT
-CLASS="VARNAME"
->errno</TT
->; the actual
-underlying system call and hence the
-<TT
-CLASS="FUNCTION"
->cyg_hal_sys_xyz</TT
-> provided by eCos instead returns
-a negative number to indicate an error, with the absolute value of
-that number corresponding to the error code; usually it is the C
-library which handles this and manipulates errno, but of course
-synthetic target applications are not linked with that Linux library.
- </P
-><P
->However, there are some exceptions. The Linux kernel has evolved over
-the years, and some of the original system call interfaces are no
-longer appropriate. For example the original
-<TT
-CLASS="FUNCTION"
->select</TT
-> system call has been superseded by
-<TT
-CLASS="FUNCTION"
->_newselect</TT
->, and that is what the
-<TT
-CLASS="FUNCTION"
->select</TT
-> function in the C library actually uses.
-The old call is still available to preserve binary compatibility but,
-like the C library, eCos makes use of the new one because it provides
-the appropriate functionality. In an attempt to reduce confusion the
-eCos function is called <TT
-CLASS="FUNCTION"
->cyg_hal_sys__newselect</TT
->,
-in other words it matches the official system call naming scheme. The
-authoritive source of information on such matters is the Linux kernel
-sources themselves, and especially its header files.
- </P
-><P
->eCos packages and applications should never
-<TT
-CLASS="LITERAL"
->#include</TT
-> Linux header files directly. For example,
-doing a <TT
-CLASS="LITERAL"
->#include </usr/include/fcntl.h></TT
->
-to access additional macros or structure definitions, or alternatively
-manipulating the header file search path, will lead to problems
-because the Linux header files are likely to duplicate and clash with
-definitions in the eCos headers. Instead the appropriate functionality
-should be extracted from the Linux headers and moved into either
-<TT
-CLASS="FILENAME"
->cyg/hal/hal_io.h</TT
-> or into
-application code, with suitable renaming to avoid clashes with eCos
-names. Users should be aware that large-scale copying may involve
-licensing complications.
- </P
-><P
->Adding more system calls is usually straightforward and involves
-adding one or more lines to the platform-specific file in the
-appropriate platform HAL, for example
-<TT
-CLASS="FILENAME"
->syscall-i386-linux-1.0.S</TT
->. However it is necessary
-to do some research first about the exact interface implemented by the
-system call, because of issues such as old system calls that have been
-superseded. The required information can usually be found fairly
-easily by searching through the Linux kernel sources and possibly the
-GNU C library sources.
- </P
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