picoc

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Originally forked from https://github.com/zsaleeba/picoc

Description

PicoC is a very small C interpreter for scripting. It was originally written as a script language for a UAV on-board flight system. It’s also very suitable for other robotic, embedded and non-embedded applications.

The core C source code is around 3500 lines of code. It’s not intended to be a complete implementation of ISO C but it has all the essentials. When compiled it only takes a few k of code space and is also very sparing of data space. This means it can work well in small embedded devices. It’s also a fun example of how to create a very small language implementation while still keeping the code readable.

It’s been tested on x86-32, x86-64, powerpc, arm, ultrasparc, HP-PA and blackfin processors and is easy to port to new targets.

Running files from the command line

You can run standard C programs straight from the command line:

$ picoc file.c

If your program is split into multiple files you can list them all on the command line.

$ picoc file1.c file2.c file3.c

If your program takes arguments you add them after a ‘-’ character.

$ picoc file.c - arg1 arg2

Running script files

Scripts are slightly simpler than standard C programs because, A) all the system headers are included automatically for you so you don’t need to include them in your file/s and B) scripts don’t require a main() function; they have statements that are run directly from the top of a file to the bottom.

$ picoc -s file.c

Here’s an example script:

printf("Starting my script\n");

int i;
int total = 0;
for (i = 0; i < 10; i++) {
    printf("i = %d\n", i);
    total += i;
}

printf("The total is %d\n", total);

Here’s the output from this script:

$ ./picoc -s script.c
Starting my script
i = 0
i = 1
i = 2
i = 3
i = 4
i = 5
i = 6
i = 7
i = 8
i = 9
The total is 45

Interactive mode

> picoc -i

Here’s an example session:

$ ./picoc -i
starting picoc v2.1
picoc> char inbuf[80];
picoc> gets(inbuf);
hello!
picoc> printf("I got: %s\n", inbuf);
I got: hello!

Deleting variables and functions.

Sometimes in interactive mode you want to change a function or redeclare a variable. You can do this using the “delete” statement:

$ ./picoc -i
starting picoc v2.1
picoc> int fred = 1234;
picoc> printf("fred = %d\n", fred);
fred = 1234
picoc> delete fred;
picoc> char *fred = "hello";
picoc> printf("fred = '%s'\n", fred);
fred = 'hello'

Note, you can quit picoc’s interactive mode using control-D.

Environment variables

In some cases you may want to change the picoc stack space. The default stack size is 512KB (see PICOC_STACK_SIZE in picoc.c) which should be large enough for most programs.

To change the stack size you can set the STACKSIZE environment variable to a different value. The value is in bytes.

Compiling PicoC

picoc can be compiled for a UNIX/Linux/POSIX host by typing “make”.

The test suite can be run by typing “make test”.

On Windows, use the MSVC++ sln file in the msvc/picoc folder.

Porting PicoC

platform.h is where you select your platform type and specify the includes etc. for your platform.

platform_XXX.c contains support functions so the compiler can work on your platform, such as how to write characters to the console etc..

platform_library.c contains your library of functions you want to make available to user programs.

There’s also a clibrary.c which contains user library functions like printf() which are platform-independent.

Porting the system will involve setting up suitable includes and defines in platform.h, writing some I/O routines in platform_XXX.c, putting whatever user functions you want in platform_library.c and then changing the main program in picoc.c to whatever you need to do to get programs into the system.

platform.h is set to UNIX_HOST by default so tests can be easily run on a UNIX system. You’ll need to specify your own host setup dependent on your target platform.

Copyright

PicoC is published under the “New BSD License”, see the LICENSE file.

Adding native C functions

Introduction

picoc allows you to define your own library functions. These functions are written in C using your system’s native C compiler. Since the native C compiler can access the hardware this means you can add functions which give picoc control of your hardware.

How libraries work

Your picoc distribution contains two files which are used to define library functions for your system. If your system is called “foobar” you’ll be using:

  • library_foobar.c - this is where the foobar-specific library functions go
  • clibrary.c - this is where standard C library functions like printf() are defined

We’ll start by defining a simple function in library_foobar.c. We need to do two things:

  • add the function prototype to our list of picoc library functions
  • define the native C implementation of the function

The prototype list

Each of the library_XXX.c files defines a list of picoc prototypes for each of the functions it defines. For example:

struct LibraryFunction PlatformLibrary[] =
{
     {ShowComplex,  "void ShowComplex(struct complex *)"},
     {Cpeek,        "int peek(int, int)"},
     {Cpoke,        "void poke(int, int, int)"},
     {Crandom,      "int random(int)"},
     {NULL,         NULL}
};

The first column is the name of the C function. The second column is the function prototype. The “{ NULL, NULL }” line at the end is required.

The native C function

The native C function is called with these parameters:

void MyCFunc(struct ParseState *Parser,
			 struct Value *ReturnValue,
			 struct Value **Param,
			 int NumArgs);
  • struct ParseState *Parser - this contains internal information about the progress of parsing. It’s mostly used here so error messages from your function can report the line number where an error occurred.
  • struct Value *ReturnValue - this points to the place you can put your return value. This is pre-created as a value of the correct return type so all you have to do is store your result here.
  • struct Value **Param - this points to an array of parameters. These are all pre-checked as being the correct type.
  • int NumArgs - this is the number of parameters. Normally this will already have been checked and will be exactly what you’ve defined in your function prototype. It is however possible to define functions with variable numbers of arguments using a stdarg-like “…” method and this is where you find out how many parameters were passed in if you’re doing that.

Here’s an example function definition of “random” (as defined above):

void Crandom(struct ParseState *Parser,
			 struct Value *ReturnValue,
			 struct Value **Param,
			 int NumArgs)
{
    ReturnValue->Val->Integer = random() % Param[0]->Val->Integer;
}

This function calls “random()” from the C standard library. It accesses an integer parameter and returns an integer value.

Passing parameters

We’ve seen how to pass integers into functions. What about passing other data types?

Type Method Comment
int Param[x]->Val->Integer
char Param[x]->Val->Integer Treated as ‘int’ here
double Param[x]->Val->FP Only available on some systems
float Param[x]->Val->FP Same as ‘double’
enum Param[x]->Val->Integer Gives integer value of enum
pointers See section below Slightly more complicated
char * See section below Slightly more complicated
arrays See section below Slightly more complicated
struct See section below Slightly more complicated
union See section below Slightly more complicated

Passing pointers

Pointer parameters are slighty more complicated to access since you have to dereference the pointer to get at the underlying data.

Here’s how we dereference a pointer parameter. In this example I’ll be reading an ‘int *’ parameter:

int IntValue = *(int*)Param[0]->Val->NativePointer;

Passing strings/char*

In this example I’ll be reading a ‘char *’ parameter. It’s pretty similar to the ‘int *’ example above:

char *CharPtr = (char*)Param[0]->Val->NativePointer;

picoc strings work like C strings - they’re pointers to arrays of characters, terminated by a null character. Once you have the C char * you can use it just like a normal C string.

Pointers to arrays of other data types work the same way.

Passing pointers to structures and unions

If you’re defining library functions which take structures as parameters you’ll have to do a little more work. You need to pre-define the structure so the function prototype can refer to it.

In library_XXX.c you’ll find a function called PlatformLibraryInit(). This is called before the library prototypes are defined. Here’s a quick way to define a complex number structure as if it was defined in an include file:

IncludeRegister("win32.h",
				&win32SetupFunc,
				&win32Functions[0],
				"struct complex {int i; int j;};");

Or you could just parse the structure directly:

const char *definition = "struct complex {int i; int j;};";
PicocParse("my lib", definition, strlen(definition), true, false, false);

The same method works for defining macros too:

const char *definition = "#define ABS(a) ((a) < (0) ? -(a) : (a))";
PicocParse("my lib", definition, strlen(definition), true, false, false);

Here’s a more sophisticated method, using the internal functions of picoc directly:

void PlatformLibraryInit()
{
    struct ParseState Parser;
    char *Identifier;
    struct ValueType *ParsedType;
    void *Tokens;
    char *IntrinsicName = TableStrRegister("complex library");
    const char *StructDefinition = "struct complex { int i; int j; }";

    /* define an example structure */
    Tokens = LexAnalyse(IntrinsicName, StructDefinition, strlen(StructDefinition), NULL);
    LexInitParser(&Parser, StructDefinition, Tokens, IntrinsicName, true, false);
    TypeParse(&Parser, &ParsedType, &Identifier, &IsStatic);
    HeapFree(Tokens);
}

This code takes the structure definition in StructDefinition and runs the lexical analyser over it. This returns some lexical tokens. Then we initialize the parser and have it parse the type of the structure definition from the tokens we made. That’s enough to define the structure in the system. Finally we free the tokens.

Now let’s say we’re going to define a function to display a complex number. Our prototype will look like:

{ShowComplex,   "void ShowComplex(struct complex *)"},

And finally we can define the library function:

struct complex {int i; int j;};  /* make this C declaration match the picoc one */

void ShowComplex(struct ParseState *Parser,
				 struct Value *ReturnValue, struct Value **Param, int NumArgs)
{
    struct complex *ComplexVal = Param[0]->Val->NativePointer;  /* casts the pointer */

    /* print the result */
    PrintInt(ComplexVal->i, PlatformPutc);
    PlatformPutc(',');
    PrintInt(ComplexVal->j, PlatformPutc);
}

Unions work exactly the same way as structures. Define the prototype as “union” rather than “struct” and you’re away.

Returning values

Returning values from library functions is very much like accessing parameters. The type of return values is already set before your native C function is called so all you have to do is fill in the value.

Just as with parameters, ints, chars and enums are stored in ReturnValue->Val->Integer and floating point values are returned in ReturnValue->Val->FP.

Returning pointers

Returning a pointer to a static string or some other allocated data is easy. Your return code will look something like:

ReturnValue->Val->NativePointer = "hello";

Variable numbers of parameters

You can define your own stdarg-style library functions like printf(). Your function prototype should use “…” in the parameter list to indicate the potential extra parameters just like the standard stdarg system. Here’s an example from clibrary.c:

{LibPrintf, "void printf(char *, ...)"},

The NumArgs parameter to the native C function lets you know how many parameters were passed in. You access the variable parameters just like normal parameters using the Param[] array.

Take a look at clibrary.c for the full definition of LibPrintf() if you need a more complete example.

Sharing native values with PicoC

Sometimes you have native variables you’d like to share with picoc. We can define a picoc value which shares memory with a native variable. Then we store this variable in the picoc symbol table so your programs can find it by name. There’s an easy way to do this:

int RobotIsExploding = 0;

void PlatformLibraryInit()
{
    VariableDefinePlatformVar(NULL,
    						  "RobotIsExploding",
    						  &IntType,
    						  (union AnyValue*)&RobotIsExploding,
    						  false);
}

The variable RobotIsExploding can be written by your native C program and read by PicoC just like any other PicoC variable. In this case it’s protected from being written by the last parameter “IsWritable” being set to FALSE. Set it to TRUE and PicoC will be able to write it too.

How PicoC differs from C90

PicoC is a tiny C language, not a complete implementation of C90. It doesn’t aim to implement every single feature of C90 but it does aim to be close enough that most programs will run without modification.

PicoC also has scripting abilities which enhance it beyond what C90 offers.

C preprocessor

There is no true preprocessor in PicoC. The most popular preprocessor features are implemented in a slightly limited way.

#define

Macros are implemented but have some limitations. They can only be used as part of expressions and operate a bit like functions. Since they’re used in expressions they must result in a value.

#if/#ifdef/#else/#endif

The conditional compilation operators are implemented, but have some limitations. The operator “defined()” is not implemented. These operators can only be used at statement boundaries.

#include

Includes are supported however the level of support depends on the specific port of PicoC on your platform. Linux/UNIX and Windows support #include fully.

Function declarations

These styles of function declarations are supported:

int my_function(char param1, int param2, char *param3)
{
   ...
}

int my_function(char param1, int param2, char *param3) {
   ...
}

The old “K&R” form of function declaration is not supported.

Predefined macros

A few macros are pre-defined:

  • PICOC_VERSION - gives the picoc version as a string eg. “v2.1 beta r524”

Function pointers

Pointers to functions are supported. - typedefs on function pointer types are currently not supported - Pointers to pointers to functions are currently not supported - Arrays of function pointers are currently not supported

Storage classes

Many of the storage classes in C90 really only have meaning in a compiler so they’re not implemented in picoc. This includes: static, extern, volatile, register and auto. They’re recognised but currently ignored.

struct and unions

Structs and unions can only be defined globally. It’s not possible to define them within the scope of a function.

Bitfields in structs are not supported.

Linking with libraries

Because picoc is an interpreter (and not a compiler) libraries must be linked with picoc itself. Also a glue module must be written to interface to picoc. This is the same as other interpreters like python.

If you’re looking for an example check the interface to the C standard library time functions in cstdlib/time.c.

goto

The goto statement is implemented but only supports forward gotos, not backward. The rationale for this is that backward gotos are not necessary for any “legitimate” use of goto.

Some discussion on this topic: