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><A
NAME="SYNTH-SYSCALLS">System Calls</H1
><DIV
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><A
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><H2
>Name</H2
>cyg_hal_sys_xyz -- Access Linux system facilities</DIV
><DIV
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><A
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>Synopsis</H2
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><PRE
CLASS="FUNCSYNOPSISINFO"
>#include <cyg/hal/hal_io.h>
</PRE
></TD
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><P
><CODE
><CODE
CLASS="FUNCDEF"
>int cyg_hal_sys_xyzzy</CODE
>(...);</CODE
></P
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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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