============= Syscall Layer
This page discusses supports a syscall layer from communication between a monolithic, kernel-mode NuttX kernel and a separately built, user-mode application set.
With most MCUs, NuttX is built as a flat, single executable image containing the NuttX RTOS along with all application code. The RTOS code and the application run in the same address space and at the same kernel- mode privileges. In order to exploit security features of certain processors, an alternative build model is also supported: NuttX can be built separately as a monolithic, kernel-mode module and the applications can be added as a separately built, user-mode module.
The syscall layer provided in this directory serves as the communication layer from the user-mode application into the kernel-mode RTOS. The switch from user-mode to kernel-mode is accomplished using software interrupts (SWIs). SWIs are implemented differently and named differently by different manufacturers but all work essentially the same: A special instruction is executed in user-mode that causes a software generated interrupt. The software generated interrupt is caught within the kernel and handle in kernel-mode.
Header Files
include/syscall.h
This header file supports general access to SWI facilities. It is simply
a wrapper file that includes ``include/sys/syscall.h`` and
``include/arch/syscall.h``.
``include/sys/syscall.h``
The SWIs received by the kernel are distinguish by a code that identifies how to process the SWI. This header file defines all such codes understood by the NuttX kernel.
include/arch/syscall.h (or arch/<cpu>/include/syscall.h)
This header file is provided by the platform-specific logic and declares
(or defines) the mechanism for providing software interrupts on this
platform. The following functions must be declared (or defined) in this
header file:
- ``SWI`` with ``SYS_`` call number only:
.. code-block:: C
uintptr_t sys_call0(unsigned int nbr);
- ``SWI`` with ``SYS_`` call number and one parameter:
.. code-block:: C
uintptr_t sys_call1(unsigned int nbr, uintptr_t parm1);
- ``SWI`` with ``SYS_`` call number and two parameters:
.. code-block:: C
uintptr_t sys_call2(unsigned int nbr, uintptr_t parm1, uintptr_t parm2);
- ``SWI`` with ``SYS_`` call number and three parameters:
.. code-block:: C
uintptr_t sys_call3(unsigned int nbr, uintptr_t parm1,
uintptr_t parm2, uintptr_t parm3);
- ``SWI`` with ``SYS_`` call number and four parameters:
.. code-block:: C
uintptr_t sys_call4(unsigned int nbr, uintptr_t parm1, uintptr_t parm2,
uintptr_t parm3, uintptr_t parm4);
- ``SWI`` with ``SYS_`` call number and five parameters:
.. code-block:: C
uintptr_t sys_call5(unsigned int nbr, uintptr_t parm1, uintptr_t parm2,
uintptr_t parm3, uintptr_t parm4, uintptr_t parm5);
- ``SWI`` with ``SYS_`` call number and six parameters:
.. code-block:: C
uintptr_t sys_call6(unsigned int nbr, uintptr_t parm1, uintptr_t parm2,
uintptr_t parm3, uintptr_t parm4, uintptr_t parm5,
uintptr_t parm6);
Syscall Database
~~~~~~~~~~~~~~~~
Sycall information is maintained in a database. That "database" is
implemented as a simple comma-separated-value file, ``syscall.csv``. Most
spreadsheets programs will accept this format and can be used to maintain
the syscall database.
The format of the CSV file for each line is:
* Field 1: Function name
* Field 2: The header file that contains the function prototype
* Field 3: Condition for compilation
* Field 4: The type of function return value.
* Field 5 - N+5: The type of each of the N formal parameters of the function
* Fields N+5 - : If the last parameter is "...", then the following fields
provide the type and number of of possible optional parameters.
See note below about variadic functions
Each type field has a format as follows:
* type name:
For all simpler types
* formal type | actual type:
For array types where the form of the formal (eg. ``int parm[2]``)
differs from the type of actual passed parameter (eg. ``int*``).
This is necessary because you cannot do simple casts to array types.
* formal type | union member actual type | union member fieldname:
A similar situation exists for unions. For example, the formal
parameter type union sigval -- You cannot cast a uintptr_t to
a union sigval, but you can cast to the type of one of the union
member types when passing the actual parameter. Similarly, we
cannot cast a union sigval to a uinptr_t either. Rather, we need
to cast a specific union member fieldname to ``uintptr_t``.
Variadic Functions
------------------
General variadic functions which may have an arbitrary number of argument
or arbitrary types cannot be represented as system calls.
``syslog()`` is a good example. Normally you would work around this by
using the non- variadic form of the OS interface that accepts a ``va_list``
as an argument, ``vsyslog()`` in this case.
There are many functions that have a variadic form but take only
one or two optional arguments. There can be handled as system
calls, but only by treating them as though they had a fixed number of
arguments.
These are handled in ``syscall.csv`` by appending the number and type of
optional arguments. For example, consider the ``open()`` OS interface. Its
prototype is:
.. code-block:: C
int open(const char *path, int oflag, ...);
In reality, open may take only a single optional argument of type ``mode_t``
and is represented in ``syscall.csv`` like this::
"open","fcntl.h","","int","const char*","int","...","mode_t"
The existence of the ``mode_t`` tells ``tools/mksyscall`` that there is at most
one optional parameter and, if present, it is of type ``mode_t``.
NOTE: This CSV file is used not only to support the generate of trap information,
but also for the generation of symbol tables. See ``Documentation/components/tools/``
and ``Documentation/components/libs/`` for further information.
Auto-Generated Files
--------------------
Stubs and proxies for the sycalls are automatically generated from this CSV
database. Here the following definition is used:
* Proxy - A tiny bit of code that executes in the user space. A proxy
has exactly the same function prototype as does the "real" function
for which it proxies. However, it only serves to map the function
call into a syscall, marshaling all of the system call parameters
as necessary.
* Stub - Another tiny bit of code that executes within the NuttX kernel
that is used to map a software interrupt received by the kernel to
a kernel function call. The stubs receive the marshaled system
call data, and perform the actually kernel function call (in
kernel-mode) on behalf of the proxy function.
Sub-Directories
---------------
* ``stubs`` - Autogenerated stub files are placed in this directory.
* ``proxies`` - Autogenerated proxy files are placed in this directory.
mksyscall
---------
mksyscall is C program that is used during the initial NuttX build
by the logic in the top-level ``syscall/`` directory. Information about the
stubs and proxies is maintained in a comma separated value (CSV) file
in the ``syscall/`` directory. The mksyscall program will accept this CSV
file as input and generate all of the required proxy or stub files as
output. See ``Documentation/components/tools/`` for additional information.