Re: documentation patch, oddities, and proposals

[Tom Tromey <tom@tromey.com>]

rth> Indeed.  It would be very nice to require a separate layout call,
rth> rather than imply it from ffi_prep_cif.  This would also nicely fix
rth> the problem that libgo encountered wrt zero-sized structures.

What was that problem btw?  If it's something users might encounter I
could update the docs to account for it.

rth> I'll have a look at the actual doc changes later.

Here's a newer version of the patch.  I've expanded the section on
ffi_arg use a bit.  I also updated the closure docs to note that this
behavior is not needed there.

I did a test to see what return pointer is passed to a closure declared
as returning void, since the docs had:

 @c FIXME: is this NULL for void-returning functions?

Experimentally it is not null, so I imagine it is just garbage... ?
I stuck another comment in there, maybe this is worth a bug report.

I'm not sure the thread safety section is correct.  It depends on what
parts of a CIF are referred to after it has bee prepped.  For example if
ffi_call refers to the alignment, then thread safety is hard to achieve
when also using multiple ABIs -- I think you'd have to re-prep the CIF
before each call and also only allow one call at a time (or, really,
just one ABI to be in use at a time).

Tom

diff --git a/doc/libffi.texi b/doc/libffi.texi
index ff72e58..50fc103 100644
--- a/doc/libffi.texi
+++ b/doc/libffi.texi
@@ -107,6 +107,7 @@ values passed between the two languages.
 * Multiple ABIs::               Different passing styles on one platform.
 * The Closure API::             Writing a generic function.
 * Closure Example::             A closure example.
+* Thread Safety::               Thread safety.
 @end menu
 
 
@@ -190,12 +191,29 @@ to ensure this.  If @var{cif} declares that the function returns
 @code{void} (using @code{ffi_type_void}), then @var{rvalue} is
 ignored.
 
+In most situations, @samp{libffi} will handle promotion according to
+the ABI.  However, for historical reasons, there is a special case
+with return values that must be handled by your code.  In particular,
+for integral (not @code{struct}) types that are narrower than the
+system register size, the return value will be widened by
+@samp{libffi}.  @samp{libffi} provides a type, @code{ffi_arg}, that
+can be used as the return type.  For example, if the CIF was defined
+with a return type of @code{char}, @samp{libffi} will try to store a
+full @code{ffi_arg} into the return value.
+
 @var{avalues} is a vector of @code{void *} pointers that point to the
 memory locations holding the argument values for a call.  If @var{cif}
 declares that the function has no arguments (i.e., @var{nargs} was 0),
 then @var{avalues} is ignored.  Note that argument values may be
 modified by the callee (for instance, structs passed by value); the
 burden of copying pass-by-value arguments is placed on the caller.
+
+Note that while the return value must be register-sized, arguments
+should exactly match their declared type.  For example, if an argument
+is a @code{short}, then the entry is @var{avalues} should point to an
+object declared as @code{short}; but if the return type is
+@code{short}, then @var{rvalue} should point to an object declared as
+a larger type -- usually @code{ffi_arg}.
 @end defun
 
 
@@ -246,6 +264,8 @@ int main()
 @menu
 * Primitive Types::             Built-in types.
 * Structures::                  Structure types.
+* Size and Alignment::          Size and alignment of types.
+* Arrays and Unions::           Arrays and unions.
 * Type Example::                Structure type example.
 * Complex::                     Complex types.
 * Complex Type Example::        Complex type example.
@@ -370,8 +390,7 @@ when passing to @code{ffi_prep_cif}.
 @node Structures
 @subsection Structures
 
-Although @samp{libffi} has no special support for unions or
-bit-fields, it is perfectly happy passing structures back and forth.
+@samp{libffi} is perfectly happy passing structures back and forth.
 You must first describe the structure to @samp{libffi} by creating a
 new @code{ffi_type} object for it.
 
@@ -391,9 +410,132 @@ For a structure, this should be set to @code{FFI_TYPE_STRUCT}.
 @item ffi_type **elements
 This is a @samp{NULL}-terminated array of pointers to @code{ffi_type}
 objects.  There is one element per field of the struct.
+
+Note that @samp{libffi} has no special support for bit-fields.  You
+must manage these manually.
 @end table
 @end deftp
 
+The @code{size} and @code{alignment} fields will be filled in by
+@code{ffi_prep_cif} or @code{ffi_prep_cif_var}, as needed.
+
+@node Size and Alignment
+@subsection Size and Alignment
+
+@code{libffi} will set the @code{size} and @code{alignment} fields of
+an @code{ffi_type} object for you.  It does so using its knowledge of
+the ABI.
+
+You might expect that you can simply read these fields for a type that
+has been laid out by @code{libffi}.  However, there are some caveats.
+
+@itemize @bullet
+@item
+The size or alignment of some of the built-in types may vary depending
+on the chosen ABI.
+
+@item
+The size and alignment of a new structure type will not be set by
+@code{libffi} until it has been passed to @code{ffi_prep_cif}.
+
+@item
+A structure type cannot be shared across ABIs.  Instead each ABI needs
+its own copy of the structure type.
+@end itemize
+
+So, before examining these fields, it is safest to pass the
+@code{ffi_type} object to @code{ffi_prep_cif} first.  This function
+will do all the needed setup.
+
+@example
+ffi_type *desired_type;
+ffi_abi desired_abi;
+@dots{}
+ffi_cif cif;
+if (ffi_prep_cif (&cif, desired_abi, 0, desired_type, NULL) == FFI_OK)
+  @{
+    size_t size = desired_type->size;
+    unsigned short alignment = desired_type->alignment;
+  @}
+@end example
+
+@node Arrays and Unions
+@subsection Arrays and Unions
+
+@subsubsection Arrays
+
+@samp{libffi} does not have direct support for arrays or unions.
+However, they can be emulated using structures.
+
+To emulate an array, simply create an @code{ffi_type} using
+@code{FFI_TYPE_STRUCT} with as many members as there are elements in
+the array.
+
+@example
+ffi_type array_type;
+ffi_type **elements
+int i;
+
+elements = malloc ((n + 1) * sizeof (ffi_type *));
+for (i = 0; i < n; ++i)
+  elements[i] = array_element_type;
+elements[n] = NULL;
+
+array_type.size = array_type.alignment = 0;
+array_type.type = FFI_TYPE_STRUCT;
+array_type.elements = elements;
+@end example
+
+Note that arrays cannot be passed or returned by value in C --
+structure types created like this should only be used to refer to
+members of real @code{FFI_TYPE_STRUCT} objects.
+
+However, a phony array type like this will not cause any errors from
+@samp{libffi} if you use it as an argument or return type.  This may
+be confusing.
+
+@subsubsection Unions
+
+A union can also be emulated using @code{FFI_TYPE_STRUCT}.  In this
+case, however, you must make sure that the size and alignment match
+the real requirements of the union.
+
+One simple way to do this is to ensue that each element type is laid
+out.  Then, give the new structure type a single element; the size of
+the largest element; and the largest alignment seen as well.
+
+This example uses the @code{ffi_prep_cif} trick to ensure that each
+element type is laid out.
+
+@example
+ffi_abi desired_abi;
+ffi_type union_type;
+ffi_type **union_elements;
+
+int i;
+ffi_type element_types[2];
+
+element_types[1] = NULL;
+
+union_type.size = union_type.alignment = 0;
+union_type.type = FFI_TYPE_STRUCT;
+union_type.elements = element_types;
+
+for (i = 0; union_elements[i]; ++i)
+  @{
+    ffi_cif cif;
+    if (ffi_prep_cif (&cif, desired_abi, 0, union_elements[i], NULL) == FFI_OK)
+      @{
+        if (union_elements[i]->size > union_type.size)
+          @{
+            union_type.size = union_elements[i];
+            size = union_elements[i]->size;
+          @}
+        if (union_elements[i]->alignment > union_type.alignment)
+          union_type.alignment = union_elements[i]->alignment;
+      @}
+  @}
+@end example
 
 @node Type Example
 @subsection Type Example
@@ -636,6 +778,8 @@ Prepare a closure function.
 the writable address returned by @code{ffi_closure_alloc}.
 
 @var{cif} is the @code{ffi_cif} describing the function parameters.
+Note that this object, and the types to which it refers, must be kept
+alive until the closure itself is freed.
 
 @var{user_data} is an arbitrary datum that is passed, uninterpreted,
 to your closure function.
@@ -652,8 +796,12 @@ The @code{ffi_cif} passed to @code{ffi_prep_closure_loc}.
 @item ret
 A pointer to the memory used for the function's return value.
 @var{fun} must fill this, unless the function is declared as returning
-@code{void}.
+@code{void}.  Note that this points to memory that is exactly the size
+of the type given as the return type when initializing the CIF.  In
+particular, closures do not have the special promotion behavior of
+@code{ffi_call}.
 @c FIXME: is this NULL for void-returning functions?
+@c (experimentally it is not, but it seems like a good idea)
 
 @item args
 A vector of pointers to memory holding the arguments to the function.
@@ -664,8 +812,7 @@ The same @var{user_data} that was passed to
 @end table
 
 @code{ffi_prep_closure_loc} will return @code{FFI_OK} if everything
-went ok, and something else on error.
-@c FIXME: what?
+went ok, and one of the other @code{ffi_status} values on error.
 
 After calling @code{ffi_prep_closure_loc}, you can cast @var{codeloc}
 to the appropriate pointer-to-function type.
@@ -733,6 +880,28 @@ int main()
 
 @end example
 
+@node Thread Safety
+@section Thread Safety
+
+@code{libffi} is not completely thread-safe.  However, many parts are,
+and if you follow some simple rules, you can use it safely in a
+multi-threaded program.
+
+@itemize @bullet
+@item
+@code{ffi_prep_cif} may modify the @code{ffi_type} objects passed to
+it.  It is best to ensure that only a single thread prepares a given
+@code{ffi_cif} at a time.
+
+@item
+On some platforms, @code{ffi_prep_cif} may modify the size and
+alignment of some types, depending on the chosen ABI.  On these
+platforms, if you switch between ABIs, you must ensure that there is
+only one call to @code{ffi_prep_cif} at a time.
+
+Currently the only affected platform is PowerPC and the only affected
+type is @code{long double}.
+@end itemize
 
 @node Missing Features
 @chapter Missing Features
@@ -749,9 +918,10 @@ There is no support for bit fields in structures.
 
 @item
 The ``raw'' API is undocumented.
-@c argument promotion?
-@c unions?
 @c anything else?
+
+@item
+The Go API is undocumented.
 @end itemize
 
 Note that variadic support is very new and tested on a relatively