Comtrol eCos - Manual (Page 767)

System Calls
Name
cyg_hal_sys_xyz — Access Linux system facilities
Synopsis
#include < cyg/hal/hal_io.h >
int cyg_hal_sys_xyzzy(...);
Description
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,
cyg_hal_sys_write 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.
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
cyg/hal/hal_io.h . There is a simple
convention: given a Linux system call such as open , the synthetic target will prefix cyg_hal_sys and provide a
function with that name. The second argument to the open system call is a set of flags such as O_RDONLY , and
the header file will define a matching constant CYG_HAL_SYS_O_RDONLY . There are also data structures such as
cyg_hal_sys_sigset_t, matching the Linux data structure sigset_t.
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 dif-
ference: typically the documentation will say that a function returns
-1 to indicate an error, with the actual error
code held in errno ; the actual underlying system call and hence the cyg_hal_sys_xyz 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.
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
select system call has been superseded by
_newselect , and that is what the select 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 ap-
propriate functionality. In an attempt to reduce confusion the eCos function is called
cyg_hal_sys__newselect ,
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.
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