Module java.base

Interface VaList

All Superinterfaces:
AddressablePREVIEW

public sealed interface VaList extends AddressablePREVIEW
VaList is a preview API of the Java platform.
Programs can only use VaList when preview features are enabled.
Preview features may be removed in a future release, or upgraded to permanent features of the Java platform.
A variable argument list, similar in functionality to a C va_list.

A variable argument list is a stateful cursor used to iterate over a set of arguments. A variable argument list can be passed by reference e.g. to a downcall method handlePREVIEW.

Per the C specification (see C99 standard 6.5.2.2 Function calls - item 6), arguments to variadic calls are erased by way of 'default argument promotions', which erases integral types by way of integer promotion (see C99 standard 6.3.1.1 - item 2), and which erases all float arguments to double.

As such, this interface only supports reading int, double, and any other type that fits into a long.

Safety considerations

It is possible for clients to access elements outside the spatial bounds of a variable argument list. Variable argument list implementations will try to detect out-of-bounds reads on a best-effort basis.

Whether this detection succeeds depends on the factory method used to create the variable argument list:

This class is not thread safe, and all accesses should occur within a single thread (regardless of the memory session associated with the variable arity list).

Since:
19
  • Method Details

    • session

      Returns the memory session associated with this variable argument list.
      Returns:
      the memory session associated with this variable argument list
    • nextVarg

      int nextVarg(ValueLayout.OfIntPREVIEW layout)
      Reads the next value as an int and advances this variable argument list's position. The behavior of this method is equivalent to the C va_arg function.
      Parameters:
      layout - the layout of the value to be read.
      Returns:
      the int value read from this variable argument list.
      Throws:
      IllegalStateException - if the session associated with this variable argument list is not alivePREVIEW.
      WrongThreadException - if this method is called from a thread other than the thread owning the session associated with this variable argument list.
      NoSuchElementException - if an out-of-bounds read is detected.
    • nextVarg

      long nextVarg(ValueLayout.OfLongPREVIEW layout)
      Reads the next value as a long and advances this variable argument list's position. The behavior of this method is equivalent to the C va_arg function.
      Parameters:
      layout - the layout of the value to be read.
      Returns:
      the long value read from this variable argument list.
      Throws:
      IllegalStateException - if the session associated with this variable argument list is not alivePREVIEW.
      WrongThreadException - if this method is called from a thread other than the thread owning the session associated with this variable argument list.
      NoSuchElementException - if an out-of-bounds read is detected.
    • nextVarg

      double nextVarg(ValueLayout.OfDoublePREVIEW layout)
      Reads the next value as a double and advances this variable argument list's position. The behavior of this method is equivalent to the C va_arg function.
      Parameters:
      layout - the layout of the value
      Returns:
      the double value read from this variable argument list.
      Throws:
      IllegalStateException - if the session associated with this variable argument list is not alivePREVIEW.
      WrongThreadException - if this method is called from a thread other than the thread owning the session associated with this variable argument list.
      NoSuchElementException - if an out-of-bounds read is detected.
    • nextVarg

      Reads the next value as a MemoryAddress and advances this variable argument list's position. The behavior of this method is equivalent to the C va_arg function.
      Parameters:
      layout - the layout of the value to be read.
      Returns:
      the MemoryAddress value read from this variable argument list.
      Throws:
      IllegalStateException - if the session associated with this variable argument list is not alivePREVIEW.
      WrongThreadException - if this method is called from a thread other than the thread owning the session associated with this variable argument list.
      NoSuchElementException - if an out-of-bounds read is detected.
    • nextVarg

      Reads the next value as a MemorySegment, and advances this variable argument list's position. The behavior of this method is equivalent to the C va_arg function. The provided group layout must correspond to a C struct or union type.

      How the value is read in the returned segment is ABI-dependent: calling this method on a group layout with member layouts L_1, L_2, ... L_n is not guaranteed to be semantically equivalent to perform distinct calls to nextVarg for each of the layouts in L_1, L_2, ... L_n.

      The memory segment returned by this method will be allocated using the given SegmentAllocatorPREVIEW.

      Parameters:
      layout - the layout of the value to be read.
      allocator - the allocator to be used to create a segment where the contents of the variable argument list will be copied.
      Returns:
      the MemorySegment value read from this variable argument list.
      Throws:
      IllegalStateException - if the session associated with this variable argument list is not alivePREVIEW.
      WrongThreadException - if this method is called from a thread other than the thread owning the session associated with this variable argument list.
      NoSuchElementException - if an out-of-bounds read is detected.
    • skip

      void skip(MemoryLayoutPREVIEW... layouts)
      Skips a number of elements with the given memory layouts, and advances this variable argument list's position.
      Parameters:
      layouts - the layouts of the values to be skipped.
      Throws:
      IllegalStateException - if the session associated with this variable argument list is not alivePREVIEW.
      WrongThreadException - if this method is called from a thread other than the thread owning the session associated with this variable argument list.
      NoSuchElementException - if an out-of-bounds read is detected.
    • copy

      Copies this variable argument list at its current position into a new variable argument list associated with the same memory session as this variable argument list. The behavior of this method is equivalent to the C va_copy function.

      Copying is useful to traverse the variable argument list elements, starting from the current position, without affecting the state of the original variable argument list, essentially allowing the elements to be traversed multiple times.

      Returns:
      a copy of this variable argument list.
      Throws:
      IllegalStateException - if the session associated with this variable argument list is not alivePREVIEW.
      WrongThreadException - if this method is called from a thread other than the thread owning the session associated with this variable argument list.
    • address

      Returns the memory addressPREVIEW associated with this variable argument list.
      Specified by:
      address in interface AddressablePREVIEW
      Returns:
      the memory addressPREVIEW associated with this variable argument list
      Throws:
      IllegalStateException - if the session associated with this variable argument list is not alivePREVIEW.
      WrongThreadException - if this method is called from a thread other than the thread owning the session associated with this variable argument list.
    • ofAddress

      static VaListPREVIEW ofAddress(MemoryAddressPREVIEW address, MemorySessionPREVIEW session)
      Creates a variable argument list from a memory address pointing to an existing variable argument list, with the given memory session.

      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.

      Implementation Note:
      variable argument lists created using this method can not detect out-of-bounds reads.
      Parameters:
      address - a memory address pointing to an existing variable argument list.
      session - the memory session to be associated with the returned variable argument list.
      Returns:
      a new variable argument list backed by the memory region at address.
      Throws:
      IllegalStateException - if session is not alivePREVIEW.
      WrongThreadException - if this method is called from a thread other than the thread owningPREVIEW session.
      UnsupportedOperationException - if the underlying native platform is not supported.
      IllegalCallerException - if access to this method occurs from a module M and the command line option --enable-native-access is specified, but does not mention the module name M, or ALL-UNNAMED in case M is an unnamed module.
    • make

      Creates a variable argument list using a builder (see VaList.BuilderPREVIEW), with the given memory session.

      If this method needs to allocate memory, such memory will be managed by the given memory session, and will be released when the memory session is closedPREVIEW.

      Note that when there are no elements added to the created va list, this method will return the same as empty().

      Implementation Note:
      variable argument lists created using this method can detect out-of-bounds reads.
      Parameters:
      actions - a consumer for a builder (see VaList.BuilderPREVIEW) which can be used to specify the elements of the underlying variable argument list.
      session - the memory session to be associated with the new variable arity list.
      Returns:
      a new variable argument list.
      Throws:
      UnsupportedOperationException - if the underlying native platform is not supported.
      IllegalStateException - if session is not alivePREVIEW.
      WrongThreadException - if this method is called from a thread other than the thread owningPREVIEW session.
    • empty

      static VaListPREVIEW empty()
      Returns an empty variable argument list, associated with the globalPREVIEW memory session. The resulting variable argument list does not contain any argument, and throws UnsupportedOperationException on all operations, except for address(), copy() and session().
      Returns:
      an empty variable argument list.
      Throws:
      UnsupportedOperationException - if the underlying native platform is not supported.