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:
```
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 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:

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.