Linker
is a preview API of the Java platform.
Foreign functions typically reside in libraries that can be loaded on-demand. Each library conforms to a specific ABI (Application Binary Interface). An ABI is a set of calling conventions and data types associated with the compiler, OS, and processor where the library was built. For example, a C compiler on Linux/x64 usually builds libraries that conform to the SystemV ABI.
A linker has detailed knowledge of the calling conventions and data types used by a specific ABI. For any library which conforms to that ABI, the linker can mediate between Java code running in the JVM and foreign functions in the library. In particular:
- A linker allows Java code to link against foreign functions, via downcall method handles; and
- A linker allows foreign functions to call Java method handles, via the generation of upcall stubs.
libc
and libm
. The functions in these
libraries are exposed via a symbol lookup.
Calling native functions
The native linker can be used to link against functions defined in C libraries (native functions). Suppose we wish to downcall from Java to thestrlen
function
defined in the standard C library:
size_t strlen(const char *s);
strlen
is obtained, using the native linker, as follows:
Linker linker = Linker.nativeLinker();
MethodHandle strlen = linker.downcallHandle(
linker.defaultLookup().find("strlen").orElseThrow(),
FunctionDescriptor.of(JAVA_LONG, ADDRESS)
);
strlen
native function. That address is then passed, along with
a platform-dependent description of the signature of the function expressed as a
FunctionDescriptor
PREVIEW (more on that below) to the native linker's
downcallHandle(MemorySegment, FunctionDescriptor, Option...)
method.
The obtained downcall method handle is then invoked as follows:
try (Arena arena = Arena.ofConfined()) {
MemorySegment str = arena.allocateUtf8String("Hello");
long len = (long) strlen.invokeExact(str); // 5
}
Describing C signatures
When interacting with the native linker, clients must provide a platform-dependent description of the signature of the C function they wish to link against. This description, afunction descriptor
PREVIEW,
defines the layouts associated with the parameter types and return type (if any) of the C function.
Scalar C types such as bool
, int
are modelled as value layoutsPREVIEW
of a suitable carrier. The mapping between a scalar type and its corresponding layout is dependent on the ABI
implemented by the native linker. For instance, the C type long
maps to the layout constant
ValueLayout.JAVA_LONG
PREVIEW on Linux/x64, but maps to the layout constant ValueLayout.JAVA_INT
PREVIEW on
Windows/x64. Similarly, the C type size_t
maps to the layout constant ValueLayout.JAVA_LONG
PREVIEW
on 64-bit platforms, but maps to the layout constant ValueLayout.JAVA_INT
PREVIEW on 32-bit platforms.
Composite types are modelled as group layoutsPREVIEW. More specifically, a C struct
type
maps to a struct layoutPREVIEW, whereas a C union
type maps to a union
layout
PREVIEW. When defining a struct or union layout, clients must pay attention to the size and alignment constraint
of the corresponding composite type definition in C. For instance, padding between two struct fields
must be modelled explicitly, by adding an adequately sized padding layoutPREVIEW member
to the resulting struct layout.
Finally, pointer types such as int**
and int(*)(size_t*, size_t*)
are modelled as
address layoutsPREVIEW. When the spatial bounds of the pointer type are known statically,
the address layout can be associated with a target layoutPREVIEW. For instance,
a pointer that is known to point to a C int[2]
array can be modelled as an address layout whose
target layout is a sequence layout whose element count is 2, and whose element type is ValueLayout.JAVA_INT
PREVIEW.
The following table shows some examples of how C types are modelled in Linux/x64:
C type Layout Java type bool
ValueLayout.JAVA_BOOLEAN
PREVIEWboolean
char
ValueLayout.JAVA_BYTE
PREVIEWbyte
short
ValueLayout.JAVA_SHORT
PREVIEWshort
int
ValueLayout.JAVA_INT
PREVIEWint
long
ValueLayout.JAVA_LONG
PREVIEWlong
long long
ValueLayout.JAVA_LONG
PREVIEWlong
float
ValueLayout.JAVA_FLOAT
PREVIEWfloat
double
ValueLayout.JAVA_DOUBLE
PREVIEWdouble
size_t
ValueLayout.JAVA_LONG
PREVIEWlong
char*
,int**
,struct Point*
ValueLayout.ADDRESS
PREVIEWMemorySegment
PREVIEWint (*ptr)[10]
ValueLayout.ADDRESS.withTargetLayout( MemoryLayout.sequenceLayout(10, ValueLayout.JAVA_INT) );MemorySegment
PREVIEWstruct Point { int x; long y; };
MemoryLayout.structLayout( ValueLayout.JAVA_INT.withName("x"), MemoryLayout.paddingLayout(32), ValueLayout.JAVA_LONG.withName("y") );MemorySegment
PREVIEWunion Choice { float a; int b; }
MemoryLayout.unionLayout( ValueLayout.JAVA_FLOAT.withName("a"), ValueLayout.JAVA_INT.withName("b") );MemorySegment
PREVIEW
All native linker implementations operate on a subset of memory layouts. More formally, a layout L
is supported by a native linker NL
if:
L
is a value layoutV
andV.withoutName()
is equalPREVIEW to one of the following layout constants:L
is an address layoutA
andA.withoutTargetLayout().withoutName()
is equalPREVIEW toValueLayout.ADDRESS
PREVIEWL
is a sequence layoutS
and all the following conditions hold:- the alignment constraint of
S
is set to its natural alignment, and S.elementLayout()
is a layout supported byNL
.
- the alignment constraint of
L
is a group layoutG
and all the following conditions hold:- the alignment constraint of
G
is set to its natural alignment; - the size of
G
is a multiple of its alignment constraint; - each member layout in
G.memberLayouts()
is either a padding layout or a layout supported byNL
, and G
does not contain padding other than what is strictly required to align its non-padding layout elements, or to satisfy (2).
- the alignment constraint of
Function pointers
Sometimes, it is useful to pass Java code as a function pointer to some native function; this is achieved by using an upcall stub. To demonstrate this, let's consider the following function from the C standard library:void qsort(void *base, size_t nmemb, size_t size,
int (*compar)(const void *, const void *));
qsort
function can be used to sort the contents of an array, using a custom comparator function which is
passed as a function pointer (the compar
parameter). To be able to call the qsort
function from Java,
we must first create a downcall method handle for it, as follows:
Linker linker = Linker.nativeLinker();
MethodHandle qsort = linker.downcallHandle(
linker.defaultLookup().find("qsort").orElseThrow(),
FunctionDescriptor.ofVoid(ADDRESS, JAVA_LONG, JAVA_LONG, ADDRESS)
);
ValueLayout.JAVA_LONG
PREVIEW to map the C type size_t
type, and ValueLayout.ADDRESS
PREVIEW
for both the first pointer parameter (the array pointer) and the last parameter (the function pointer).
To invoke the qsort
downcall handle obtained above, we need a function pointer to be passed as the last
parameter. That is, we need to create a function pointer out of an existing method handle. First, let's write a
Java method that can compare two int elements passed as pointers (i.e. as memory segmentsPREVIEW):
class Qsort {
static int qsortCompare(MemorySegment elem1, MemorySegment elem2) {
return Integer.compare(elem1.get(JAVA_INT, 0), elem2.get(JAVA_INT, 0));
}
}
FunctionDescriptor comparDesc = FunctionDescriptor.of(JAVA_INT,
ADDRESS.withTargetLayout(JAVA_INT),
ADDRESS.withTargetLayout(JAVA_INT));
MethodHandle comparHandle = MethodHandles.lookup()
.findStatic(Qsort.class, "qsortCompare",
comparDesc.toMethodType());
int[]
array, we can specify ValueLayout.JAVA_INT
PREVIEW
as the target layout for the address layouts of both parameters. This will allow the comparator method to access
the contents of the array elements to be compared. We then turnPREVIEW
that function descriptor into a suitable method type which we then use to look up
the comparator method handle. We can now create an upcall stub which points to that method, and pass it, as a function
pointer, to the qsort
downcall handle, as follows:
try (Arena arena = Arena.ofConfined()) {
MemorySegment comparFunc = linker.upcallStub(comparHandle, comparDesc, arena);
MemorySegment array = arena.allocateArray(JAVA_INT, 0, 9, 3, 4, 6, 5, 1, 8, 2, 7);
qsort.invokeExact(array, 10L, 4L, comparFunc);
int[] sorted = array.toArray(JAVA_INT); // [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 ]
}
qsort
method handle along with the comparator function we obtained from the native linker. After the invocation, the contents
of the off-heap array will be sorted according to our comparator function, written in Java. We then extract a
new Java array from the segment, which contains the sorted elements.
Functions returning pointers
When interacting with native functions, it is common for those functions to allocate a region of memory and return a pointer to that region. Let's consider the following function from the C standard library:void *malloc(size_t size);
malloc
function allocates a region of memory of given size,
and returns a pointer to that region of memory, which is later deallocated using another function from
the C standard library:
void free(void *ptr);
free
function takes a pointer to a region of memory and deallocates that region. In this section we
will show how to interact with these native functions, with the aim of providing a safe allocation
API (the approach outlined below can of course be generalized to allocation functions other than malloc
and free
).
First, we need to create the downcall method handles for malloc
and free
, as follows:
Linker linker = Linker.nativeLinker();
MethodHandle malloc = linker.downcallHandle(
linker.defaultLookup().find("malloc").orElseThrow(),
FunctionDescriptor.of(ADDRESS, JAVA_LONG)
);
MethodHandle free = linker.downcallHandle(
linker.defaultLookup().find("free").orElseThrow(),
FunctionDescriptor.ofVoid(ADDRESS)
);
malloc
) is invoked using a downcall method handle,
the Java runtime has no insight into the size or the lifetime of the returned pointer. Consider the following code:
MemorySegment segment = (MemorySegment)malloc.invokeExact(100);
malloc
downcall method handle is
zero. Moreover, the scope of the
returned segment is a fresh scope that is always alive. To provide safe access to the segment, we must,
unsafely, resize the segment to the desired size (100, in this case). It might also be desirable to
attach the segment to some existing arenaPREVIEW, so that the lifetime of the region of memory
backing the segment can be managed automatically, as for any other native segment created directly from Java code.
Both of these operations are accomplished using the restricted method MemorySegment.reinterpret(long, Arena, Consumer)
PREVIEW,
as follows:
MemorySegment allocateMemory(long byteSize, Arena arena) throws Throwable {
MemorySegment segment = (MemorySegment) malloc.invokeExact(byteSize); // size = 0, scope = always alive
return segment.reinterpret(byteSize, arena, s -> {
try {
free.invokeExact(s);
} catch (Throwable e) {
throw new RuntimeException(e);
}
}); // size = byteSize, scope = arena.scope()
}
allocateMemory
method defined above accepts two parameters: a size and an arena. The method calls the
malloc
downcall method handle, and unsafely reinterprets the returned segment, by giving it a new size
(the size passed to the allocateMemory
method) and a new scope (the scope of the provided arena).
The method also specifies a cleanup action to be executed when the provided arena is closed. Unsurprisingly,
the cleanup action passes the segment to the free
downcall method handle, to deallocate the underlying
region of memory. We can use the allocateMemory
method as follows:
try (Arena arena = Arena.ofConfined()) {
MemorySegment segment = allocateMemory(100, arena);
} // 'free' called here
allocateMemory
acts as any other segment managed by the confined arena. More
specifically, the obtained segment has the desired size, can only be accessed by a single thread (the thread which created
the confined arena), and its lifetime is tied to the surrounding try-with-resources block.
Variadic functions
Variadic functions are C functions which can accept a variable number and type of arguments. They are declared:- With a trailing ellipsis (
...
) at the end of the formal parameter list, such as:void foo(int x, ...);
- With an empty formal parameter list, called a prototype-less function, such as:
void foo();
...
or empty formal parameter list with a list of variadic parameters
of a fixed number and type.
It should be noted that values passed as variadic arguments undergo default argument promotion in C. For instance, the following argument promotions are applied:
_Bool
->unsigned int
[signed] char
->[signed] int
[signed] short
->[signed] int
float
->double
The native linker only supports linking the specialized form of a variadic function. A variadic function in its specialized
form can be linked using a function descriptor describing the specialized form. Additionally, the
Linker.Option.firstVariadicArg(int)
PREVIEW linker option must be provided to indicate the first variadic parameter in
the parameter list. The corresponding argument layout (if any), and all following argument layouts in the specialized
function descriptor, are called variadic argument layouts. For a prototype-less function, the index passed to
Linker.Option.firstVariadicArg(int)
PREVIEW should always be 0
.
The native linker will reject an attempt to link a specialized function descriptor with any variadic argument layouts
corresponding to a C type that would be subject to default argument promotion (as described above). Exactly which layouts
will be rejected is platform specific, but as an example: on Linux/x64 the layouts ValueLayout.JAVA_BOOLEAN
PREVIEW,
ValueLayout.JAVA_BYTE
PREVIEW, ValueLayout.JAVA_CHAR
PREVIEW, ValueLayout.JAVA_SHORT
PREVIEW, and
ValueLayout.JAVA_FLOAT
PREVIEW will be rejected.
A well-known variadic function is the printf
function, defined in the C standard library:
int printf(const char *format, ...);
printf("%d plus %d equals %d", 2, 2, 4);
(char*, int, int, int)
as the format string accepts three integer parameters. We then need to use
a linker optionPREVIEW to specify the position of the first variadic layout
in the provided function descriptor (starting from 0). In this case, since the first parameter is the format string
(a non-variadic argument), the first variadic index needs to be set to 1, as follows:
Linker linker = Linker.nativeLinker();
MethodHandle printf = linker.downcallHandle(
linker.defaultLookup().find("printf").orElseThrow(),
FunctionDescriptor.of(JAVA_INT, ADDRESS, JAVA_INT, JAVA_INT, JAVA_INT),
Linker.Option.firstVariadicArg(1) // first int is variadic
);
try (Arena arena = Arena.ofConfined()) {
int res = (int)printf.invokeExact(arena.allocateUtf8String("%d plus %d equals %d"), 2, 2, 4); //prints "2 plus 2 equals 4"
}
Safety considerations
Creating a downcall method handle is intrinsically unsafe. A symbol in a foreign library does not, in general, contain enough signature information (e.g. arity and types of foreign function parameters). As a consequence, the linker runtime cannot validate linkage requests. When a client interacts with a downcall method handle obtained through an invalid linkage request (e.g. by specifying a function descriptor featuring too many argument layouts), the result of such interaction is unspecified and can lead to JVM crashes.When an upcall stub is passed to a foreign function, a JVM crash might occur, if the foreign code casts the function pointer associated with the upcall stub to a type that is incompatible with the type of the upcall stub, and then attempts to invoke the function through the resulting function pointer. Moreover, if the method handle associated with an upcall stub returns a memory segmentPREVIEW, clients must ensure that this address cannot become invalid after the upcall completes. This can lead to unspecified behavior, and even JVM crashes, since an upcall is typically executed in the context of a downcall method handle invocation.
- Implementation Requirements:
- Implementations of this interface are immutable, thread-safe and value-based.
- Since:
- 19
-
Nested Class Summary
Modifier and TypeInterfaceDescriptionstatic interface
Preview.A linker option is used to provide additional parameters to a linkage request. -
Method Summary
Modifier and TypeMethodDescriptionReturns a symbol lookup for symbols in a set of commonly used libraries.downcallHandle
(FunctionDescriptorPREVIEW function, Linker.OptionPREVIEW... options) Creates a method handle which is used to call a foreign function with the given signature.downcallHandle
(MemorySegmentPREVIEW address, FunctionDescriptorPREVIEW function, Linker.OptionPREVIEW... options) Creates a method handle which is used to call a foreign function with the given signature and address.Returns a linker for the ABI associated with the underlying native platform.upcallStub
(MethodHandle target, FunctionDescriptorPREVIEW function, ArenaPREVIEW arena, Linker.OptionPREVIEW... options) Creates an upcall stub which can be passed to other foreign functions as a function pointer, associated with the given arena.
-
Method Details
-
nativeLinker
Returns a linker for the ABI associated with the underlying native platform. The underlying native platform is the combination of OS and processor where the Java runtime is currently executing.- API Note:
- It is not currently possible to obtain a linker for a different combination of OS and processor.
- Implementation Note:
- The libraries exposed by the default lookup associated with the returned
linker are the native libraries loaded in the process where the Java runtime is currently executing. For example,
on Linux, these libraries typically include
libc
,libm
andlibdl
. - Returns:
- a linker for the ABI associated with the underlying native platform
- Throws:
UnsupportedOperationException
- if the underlying native platform is not supported.
-
downcallHandle
MethodHandle downcallHandle(MemorySegmentPREVIEW address, FunctionDescriptorPREVIEW function, Linker.OptionPREVIEW... options) Creates a method handle which is used to call a foreign function with the given signature and address.Calling this method is equivalent to the following code:
linker.downcallHandle(function).bindTo(symbol);
This method is restricted. Restricted methods are unsafe, and, if used incorrectly, their use might crash the JVM or, worse, silently result in memory corruption. Thus, clients should refrain from depending on restricted methods, and use safe and supported functionalities, where possible.
- Parameters:
address
- the native memory segment whose base addressPREVIEW is the address of the target foreign function.function
- the function descriptor of the target foreign function.options
- the linker options associated with this linkage request.- Returns:
- a downcall method handle.
- Throws:
IllegalArgumentException
- if the provided function descriptor is not supported by this linker.IllegalArgumentException
- if!address.isNative()
, or ifaddress.equals(MemorySegment.NULL)
.IllegalArgumentException
- if an invalid combination of linker options is given.IllegalCallerException
- If the caller is in a module that does not have native access enabled.- See Also:
-
downcallHandle
Creates a method handle which is used to call a foreign function with the given signature.The Java method type associated with the returned method handle is derivedPREVIEW from the argument and return layouts in the function descriptor, but features an additional leading parameter of type
MemorySegment
PREVIEW, from which the address of the target foreign function is derived. Moreover, if the function descriptor's return layout is a group layout, the resulting downcall method handle accepts an additional leading parameter of typeSegmentAllocator
PREVIEW, which is used by the linker runtime to allocate the memory region associated with the struct returned by the downcall method handle.Upon invoking a downcall method handle, the linker provides the following guarantees for any argument
A
of typeMemorySegment
PREVIEW whose corresponding layout is an address layoutPREVIEW:A.scope().isAlive() == true
. Otherwise, the invocation throwsIllegalStateException
;- The invocation occurs in a thread
T
such thatA.isAccessibleBy(T) == true
. Otherwise, the invocation throwsWrongThreadException
; and A
is kept alive during the invocation. For instance, ifA
has been obtained using a shared arenaPREVIEW, any attempt to closePREVIEW the arena while the downcall method handle is still executing will result in anIllegalStateException
.
Moreover, if the provided function descriptor's return layout is an address layoutPREVIEW, invoking the returned method handle will return a native segment associated with a fresh scope that is always alive. Under normal conditions, the size of the returned segment is
0
. However, if the function descriptor's return layout has a target layoutPREVIEWT
, then the size of the returned segment is set toT.byteSize()
.The returned method handle will throw an
IllegalArgumentException
if theMemorySegment
PREVIEW representing the target address of the foreign function is theMemorySegment.NULL
PREVIEW address. The returned method handle will additionally throwNullPointerException
if any argument passed to it isnull
.This method is restricted. Restricted methods are unsafe, and, if used incorrectly, their use might crash the JVM or, worse, silently result in memory corruption. Thus, clients should refrain from depending on restricted methods, and use safe and supported functionalities, where possible.
- Parameters:
function
- the function descriptor of the target foreign function.options
- the linker options associated with this linkage request.- Returns:
- a downcall method handle.
- Throws:
IllegalArgumentException
- if the provided function descriptor is not supported by this linker.IllegalArgumentException
- if an invalid combination of linker options is given.IllegalCallerException
- If the caller is in a module that does not have native access enabled.
-
upcallStub
MemorySegmentPREVIEW upcallStub(MethodHandle target, FunctionDescriptorPREVIEW function, ArenaPREVIEW arena, Linker.OptionPREVIEW... options) Creates an upcall stub which can be passed to other foreign functions as a function pointer, associated with the given arena. Calling such a function pointer from foreign code will result in the execution of the provided method handle.The returned memory segment's address points to the newly allocated upcall stub, and is associated with the provided arena. As such, the lifetime of the returned upcall stub segment is controlled by the provided arena. For instance, if the provided arena is a confined arena, the returned upcall stub segment will be deallocated when the provided confined arena is closedPREVIEW.
An upcall stub argument whose corresponding layout is an address layoutPREVIEW is a native segment associated with a fresh scope that is always alive. Under normal conditions, the size of this segment argument is
0
. However, if the address layout has a target layoutPREVIEWT
, then the size of the segment argument is set toT.byteSize()
.The target method handle should not throw any exceptions. If the target method handle does throw an exception, the JVM will terminate abruptly. To avoid this, clients should wrap the code in the target method handle in a try/catch block to catch any unexpected exceptions. This can be done using the
MethodHandles.catchException(MethodHandle, Class, MethodHandle)
method handle combinator, and handle exceptions as desired in the corresponding catch block.This method is restricted. Restricted methods are unsafe, and, if used incorrectly, their use might crash the JVM or, worse, silently result in memory corruption. Thus, clients should refrain from depending on restricted methods, and use safe and supported functionalities, where possible.
- Parameters:
target
- the target method handle.function
- the upcall stub function descriptor.arena
- the arena associated with the returned upcall stub segment.options
- the linker options associated with this linkage request.- Returns:
- a zero-length segment whose address is the address of the upcall stub.
- Throws:
IllegalArgumentException
- if the provided function descriptor is not supported by this linker.IllegalArgumentException
- if the type oftarget
is incompatible with the type derivedPREVIEW fromfunction
.IllegalArgumentException
- if it is determined that the target method handle can throw an exception.IllegalStateException
- ifarena.scope().isAlive() == false
WrongThreadException
- ifarena
is a confined arena, and this method is called from a threadT
, other than the arena's owner thread.IllegalCallerException
- If the caller is in a module that does not have native access enabled.
-
defaultLookup
SymbolLookupPREVIEW defaultLookup()Returns a symbol lookup for symbols in a set of commonly used libraries.Each
Linker
PREVIEW is responsible for choosing libraries that are widely recognized as useful on the OS and processor combination supported by theLinker
PREVIEW. Accordingly, the precise set of symbols exposed by the symbol lookup is unspecified; it varies from oneLinker
PREVIEW to another.- Implementation Note:
- It is strongly recommended that the result of
defaultLookup()
exposes a set of symbols that is stable over time. Clients ofdefaultLookup()
are likely to fail if a symbol that was previously exposed by the symbol lookup is no longer exposed.If an implementer provides
Linker
PREVIEW implementations for multiple OS and processor combinations, then it is strongly recommended that the result ofdefaultLookup()
exposes, as much as possible, a consistent set of symbols across all the OS and processor combinations. - Returns:
- a symbol lookup for symbols in a set of commonly used libraries.
-
Linker
when preview features are enabled.