incubator-nuttx/syscall/README.txt

syscall/README.txt
==================

This directory 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 and one parameter

    uintptr_t sys_call0(unsigned int nbr);

  - SWI with SYS_ call number and one parameter

    uintptr_t sys_call1(unsigned int nbr, uintptr_t parm1);

  - SWI with SYS_ call number and two parameters

    uintptr_t sys_call2(unsigned int nbr, uintptr_t parm1, uintptr_t parm2);

  - SWI with SYS_ call number and three parameters

    uintptr_t sys_call3(unsigned int nbr, uintptr_t parm1,
                        uintptr_t parm2, uintptr_t parm3);

  - SWI with SYS_ call number and four parameters

    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

    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

    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 there are many functions that have a variadic form but take only
   one or two arguments 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 are handled in syscall.csv by appending the number and type of
   optional arguments.  For example, consider the open() OS interface.  Its
   prototype is:

      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.cvs 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 both to support the generate of trap information,
but also for the generation of symbol tables.  See nuttx/tools/README.txt
and nuttx/lib/README.txt 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 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 CVS
  file as input and generate all of the required proxy or stub files as
  output.  See tools/README.txt for additional information.