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


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.


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 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.


  • stubs - Autogenerated stub files are placed in this directory.

  • proxies - Autogenerated proxy files are placed in this directory.


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 CSV file as input and generate all of the required proxy or stub files as output. See Documentation/components/tools/ for additional information.