Natives (SourceMod Development)

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Natives are functions which are available to plugins via Core itself, or a C++ extension. They are called "natives" because they must be translated via a native interface. This article explains the various parameter passing conventions in SourcePawn, as well as how to use them in your own natives.

To understand the contents of this article, you will need to read Creating Natives, the Native section in the introductory article on creating extensions.

In this article, float refers to the C/C++ data type, and Float (note capital 'F') refers to the SourcePawn tag.

By Value versus By Reference

There are two ways to pass integers/Floats to native implementations. They are by value (ByVal) and by reference (ByRef). ByVal means that a copy of the value is passed to the native. This is the default behaviour. ByRef means a reference is passed to the native, and this reference points to the value. Both will be explained below.

Note that arrays and strings, as will be explained later, are always passed by reference. This is because they are usually larger structures, and passing them ByVal would require copying their data, wasting CPU cycles.

By Value

Passing by value is the default behaviour for primitive data types (integers, Floats, or any tag that's cell-based). Data is treated normally as raw, copied input. Observe the following native:

/**
 * Returns the number decremented by one.
 *
 * @param num		Number to decrement.
 * @return		Decremented number.
 */
native Decrement(num);

How would we use this native? Since num is passed as a value, it cannot change in the native code. This means we have to use the return value as such:

//Decrements a number 5 times
Example(num)
{
	num = Decrement(num);
	num = Decrement(num);
	num = Decrement(num);
	num = Decrement(num);
	num = Decrement(num);
	return num;
}

By Reference

Let's reuse the above example to use reference passing. Observe the new native below:

/**
 * Subtracts one from the given number, by reference.
 *
 * @param num		Number to subtract, by reference.
 * @noreturn
 */
native Decrement(&num);

Note the ampersand ('&') before the parameter name -- this specifies that it is passed by reference. Now, let's see how this would look in our script:

Example(num)
{
	Decrement(num);
	Decrement(num);
	Decrement(num);
	Decrement(num);
	Decrement(num);
	return num;
}

In this example, Decrement is acting on the num variable itself, not a copy of it. Thus, num will decrease 5 times. In fact, scripts can even do this internally. We can shorten the example even more:

Example(&num)
{
	Decrement(num);
	Decrement(num);
	Decrement(num);
	Decrement(num);
	Decrement(num);
}

When to use By Ref

There is a misconception that passing by reference is always better then by value. After all, copying data sounds excessive. However, by value in SourcePawn only works on 32bit values, and thus copying the value is inherent to the processor, and trivial.

On the other hand, passing by reference is slightly more expensive. First, the compiler has to generate a little extra code to compute the local address of the variable. Second, the native code itself has to translate the local address to a real virtual address (native memory).

It is a good idea to only use by reference when you need it. A common usage is when you need to return more than one piece of data, and you can't fit it into the return value of your native or function. In this case, by reference is ideal.

Integers/Floats

By Value

Natives based purely on by-value floating point or integer input are generally the easiest to make. Let's take a simple function which takes in a Float and an integer, and returns a Float:

/**
 * Raises a number to an integer power.
 *
 * @param fNum		Base number (Float).
 * @param exp		Exponent (integer).
 * @return		Computed exponent result as a Float.
 */
native Float:FloatIntPower(Float:fNum, exp);

An implementation of this native might look like:

#include <stdlib.h>
 
static cell_t sm_FloatIntPower(IPluginContext *pContext, const cell_t *params)
{
	float f1 = sp_ctof(params[1]);
	int num = params[2];
 
	float result = (float)pow(f1, (double)f2);
	return sp_ftoc(result);
}

Integers, as we saw in the introduction, are accessed from the parameter stack normally. Floats, however, must be reinterpret casted from a cell_t to float. Two inline functions are provided for this:

  • sp_ctof: Convert a cell_t to a float.
  • sp_ftoc: Convert a float to a cell_t.


By Reference

By reference is a little more tricky. Let's first implement our Decrement native from earlier. A refresher:

native Decrement(&num);

If we try to use our above code, params[1] will no longer hold a value. Instead, it holds a local address in the plugin. We must use the LocalToPhysAddr function to translate this. It takes in a local address and returns back a physical pointer, which we can then modify. This will modify the value in the script.

static cell_t sm_Decrement(IPluginContext *pContext, const cell_t *params)
{
	cell_t *addr;
 
	/* Translate the address. */
	pContext->LocalToPhysAddr(params[1], &addr);
 
	/* Decrement the number */
	*addr -= 1;
 
	/* We have to return something, even if the plugin doesn't use the value */
	return 1;
}

Note: LocalToPhysAddr can return an error code. For all practical purposes, this will never error, unless there is some memory or corruption issue, so checking it isn't necessary.

This works the same way for floats. Let's say we change our native to this:

/**
 * Decrements a Float by an integer, and stores the result by reference.
 *
 * @param fNum		Float number to decrement (by ref).
 * @param decamt	Amount to decrement by.
 * @noreturn
 */
native Decrement(&Float:fNum, decamt);

This example is fairly contrived -- our native simply performs a subtract operation. But let's look at the implementation:

static cell_t sm_Decrement(IPluginContext *pContext, const cell_t *params)
{
	cell_t *addr;
 
	/* Translate the address. */
	pContext->LocalToPhysAddr(params[1], &addr);
 
	/* Get the value */
	float val = sp_ctof(*addr);
	/* Decrement */
	val -= params[2];
	/* Store back */
	*addr = sp_ftoc(val);
 
	/* We have to return something, even if the plugin doesn't use the value */
	return 1;
}

Note that even though the first parameter was passed by reference, the second was not, and is accessed normally.

Arrays

Basic Arrays

Arrays are always passed by reference. The first example is an array which contains a list of numbers to sum. Observe the native:

/**
 * Averages an array of numbers.
 *
 * @param array		Array of numbers to average.
 * @param num		Number of slots in the array.
 * @return		Average number as a Float.
 */
native Float:Average(array[], num);

Usage might look like:

Example()
{
	new numbers[5] = {5, 6, 1, 3, 8};
	return Average(numbers, 5);
}

The implementation, like the by ref examples above, requires LocalToPhysAddr:

static cell_t sm_Average(IPluginContext *pContext, cell_t *params)
{
	cell_t *array;
	float sum = 0.0f, average;
 
	if (params[2] < 1)
	{
		/* 0 works without sp_ftoc() */
		return 0;
	}
 
	pContext->LocalToPhysAddr(params[1], &array);
	for (cell_t i=0; i<params[2]; i++)
	{
		sum += array[i];
	}
 
	average = sum / params[2];
 
	return sp_ftoc(average);
}

Float Arrays

This works similarly for Float arrays. Let's take Vectors an example, with the following native:

/**
 * Adds two vectors together.
 *
 * @param r	Array to store the result in.
 * @param v1	First vector to add.
 * @param v2	Second vector to add.
 * @noreturn
 */
native AddVectors(Float:r[3], const Float:v1[3], const Float:v2[3]);

The implementation is straightforward:

static cell_t sm_AddVectors(IPluginContext *pContext, cell_t *params)
{
	cell_t *v1, *v2;
	float result[3];
 
	pContext->LocalToPhysAddr(params[2], &v1);
	pContext->LocalToPhysAddr(params[3], &v2);
 
	result[0] = sp_ctof(v1[0]) + sp_ctof(v2[0]);
	result[1] = sp_ctof(v1[1]) + sp_ctof(v2[1]);
	result[2] = sp_ctof(v1[2]) + sp_ctof(v2[2]);
 
	cell_t *r;
	pContext->LocalToPhysAddr(params[1], &r);
	r[0] = sp_ftoc(result[0]);
	r[1] = sp_ftoc(result[1]);
	r[2] = sp_ftoc(result[2]);
 
	return 1;
}

Note that we only store the result after we have computed the input, rather than store directly. This is a bit more work, but is good practice in case users re-use data inputs, and risk overwriting inputs as they are written. For example:

new Float:origin[3];
AddVectors(origin, origin, origin);


Strings

Strings are just like arrays, in that they are passed by reference. The only difference is that each character is stored in a byte, not a 32bit cell_t. However, to make coding a bit easier for you, the coder, there are separate functions.

  • LocalToString: Converts a local address to a physical string address.
  • StringToLocal: Copies a physical string into a local address buffer.
  • StringToLocalUTF8: Same as StringToLocal, but only copies the maximum amount possible without cutting off any multi byte characters.

First, let's take an easy example: the infamous strlen.

/**
 * Returns the length of a string.
 * 
 * @param string	String to calculate.
 * @return		Number of bytes in the string.
 */
native strlen(const String:string[]);

Implementation:

static cell_t sm_StrLen(IPluginContext *pContext, cell_t *params)
{
	char *str;
	pContext->LocalToString(params[1], &str);
	return strlen(str);
}

Now, let's say we want to modify the string. There is an important note here: since the string is passed by reference, when you modify the string, it is also modified in the plugin. This is a huge difference from AMX Mod X, where strings were essentially by value in most cases, and you had to copy results back.

Thus, if we wanted to copy a new result into str in the above implementation, it would be very easy. If we were implementing strcpy(), we would have to use memmove() to make sure there are no overlaps. Let's do this:

/**
 * Copies one string onto another.
 *
 * @param dest		Destination buffer to copy to.
 * @param length	Maximum length of the buffer.
 * @param source	Source to copy from.
 * @noreturn
 */
native StringCopy(String:dest[], length, const String:source);

Implementation using memmove for safety:

static cell_t sm_StringCopy(IPluginContext *pContext, cell_t *params)
{
	char *dest, *src;
	size_t len;
 
	pContext->LocalToString(params[1], &dest);
	pContext->LocalToString(params[3], &src);
 
	/* Perform bounds checking */
	len = strlen(src);
	if (len >= params[2])
	{
		len = params[2] - 1;
	} else {
		len = params[2];
	}
 
	/* Copy */
	memmove(dest, src, len);
 
	dest[len] = '\0';
 
	return 1;
}

We can also use StringToLocal, which would requires a temporary buffer for the original string:

static cell_t sm_StringCopy(IPluginContext *pContext, cell_t *params)
{
	char *src;
	char buffer[2048];
	size_t len;
 
	pContext->LocalToString(params[3], &src);
 
	snprintf(buffer, sizeof(buffer), "%s", src);
	pContext->StringToLocal(params[1], params[2], buffer);
 
	return 1;
}

Generally, you will be using StringToLocal for setting strings from outside sources, because it calculates addresses and overflows for you. However, you must make sure that you are not overwriting memory as you are reading it, or else programmers who are using certain types of array slicing may find themselves getting unexpected results.

Advanced

Default Arguments

Any type of parameter can have a default argument. For example, here is a native where every argument has a default parameter:

native RandomStuff(a=0, Float:b=1.0f, &c=0, const String:d[]="", e[3] = {0,1,2});

Note that even by reference parameters can have default arguments, as shown above. This does not change the code; it just means the address will contain the default value.

Scripts can force default values. Example:

native RandomStuff(a, b, c=0, d=0);
 
Example()
{
	RandomStuff(1, 2, _, d);
}

The underscore ('_') means "use the default value for this argument."

Variable Arguments

Variable arguments means any number of arguments can be passed. An example of this looks like:

native FormatText(const String:format[], {Float,String,_}:...);

The {Float,Handle_}: is a multi-tag specifier that means any extra parameters must be either a Float, String, or have no tag at all. The ... characters mean any number of parameters can follow.

There is one important note about variable arguments. They are always passed by reference. This means if you have a function which supports variable arguments, and an integer is passed, you will need to use LocalToPhysAddr as required with by reference parameters.

Argument Counts/Backwards Compatibility

In native handlers, the params array has a 0th index which stores the number of parameters it contains. This is useful for both Default Arguments and Variable Arguments.

For example, consider the following native:

native DoSomething(index);

Let's say that this native exists in plugins for six months. After that, you decide to add a new parameter. You want two conditions to be true after you release this update:

  • Old plugins in binary form should still work on newer installations.
  • Old plugins should still compile fine on the new API.

The second condition is solved by using default arguments. You should choose a value that will mimic the old functionality of the native.

native DoSomething(index, newparam = 0);

Next, when changing the implementation, you should detect whether params[0] contains one parameter or two parameters. If it only contains one, you know to use the old version. If it contains two, you know to use the later version and accept the second parameter.

Similarly, you can use the params[0] count to detect how many arguments were passed to a variable argument native. This is used primarily in Format Class Functions for parameter-count validation.


Throwing Errors/Invoking the Debugger

Often, you will write a native and realize that you need to tell the plugin that a serious error has occurred. A good example of this is if you are writing a function which requires a player index, but that index is beyond the reasonable bounds for the maximum players in the server.

In this case, it is a good idea to throw a runtime error. This is an error that halts the plugin and invokes a call trace from the debugger. The halt is not permanent, but it does break the execution flow of the current callback.

There are two ways to throw a native error: ThrowNativeErrorEx() or ThrowNativeError(). The former lets you specify a detailed error code from sp_vm_types.h. The latter is a helper function for generic errors, and it returns 0, allowing you to return with the function itself. For example:

static cell_t GetPlayerFrags(IPluginContext *pContext, const cell_t *params)
{
	if (params[1] < 0 || params[1] > maxplayers)
	{
		return pContext->ThrowNativeError("Invalid client index: %d", params[1]);
	}
	/* ... rest of code ... */
}

Don't worry about including extra information, such as your native name or the module name. The debugger knows all of this information and will decide what to print to the user.