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diff --git a/doc/libffi.texi b/doc/libffi.texi new file mode 100644 index 0000000..90c676a --- /dev/null +++ b/doc/libffi.texi @@ -0,0 +1,541 @@ +\input texinfo @c -*-texinfo-*- +@c %**start of header +@setfilename libffi.info +@settitle libffi +@setchapternewpage off +@c %**end of header + +@c Merge the standard indexes into a single one. +@syncodeindex fn cp +@syncodeindex vr cp +@syncodeindex ky cp +@syncodeindex pg cp +@syncodeindex tp cp + +@include version.texi + +@copying + +This manual is for Libffi, a portable foreign-function interface +library. + +Copyright @copyright{} 2008 Red Hat, Inc. + +@quotation +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU General Public License as published by the +Free Software Foundation; either version 2, or (at your option) any +later version. A copy of the license is included in the +section entitled ``GNU General Public License''. + +@end quotation +@end copying + +@dircategory +@direntry +* libffi: (libffi). Portable foreign-function interface library. +@end direntry + +@titlepage +@title Libffi +@page +@vskip 0pt plus 1filll +@insertcopying +@end titlepage + + +@ifnottex +@node Top +@top libffi + +@insertcopying + +@menu +* Introduction:: What is libffi? +* Using libffi:: How to use libffi. +* Missing Features:: Things libffi can't do. +* Index:: Index. +@end menu + +@end ifnottex + + +@node Introduction +@chapter What is libffi? + +Compilers for high level languages generate code that follow certain +conventions. These conventions are necessary, in part, for separate +compilation to work. One such convention is the @dfn{calling +convention}. The calling convention is a set of assumptions made by +the compiler about where function arguments will be found on entry to +a function. A calling convention also specifies where the return +value for a function is found. The calling convention is also +sometimes called the @dfn{ABI} or @dfn{Application Binary Interface}. +@cindex calling convention +@cindex ABI +@cindex Application Binary Interface + +Some programs may not know at the time of compilation what arguments +are to be passed to a function. For instance, an interpreter may be +told at run-time about the number and types of arguments used to call +a given function. @samp{Libffi} can be used in such programs to +provide a bridge from the interpreter program to compiled code. + +The @samp{libffi} library provides a portable, high level programming +interface to various calling conventions. This allows a programmer to +call any function specified by a call interface description at run +time. + +@acronym{FFI} stands for Foreign Function Interface. A foreign +function interface is the popular name for the interface that allows +code written in one language to call code written in another language. +The @samp{libffi} library really only provides the lowest, machine +dependent layer of a fully featured foreign function interface. A +layer must exist above @samp{libffi} that handles type conversions for +values passed between the two languages. +@cindex FFI +@cindex Foreign Function Interface + + +@node Using libffi +@chapter Using libffi + +@menu +* The Basics:: The basic libffi API. +* Simple Example:: A simple example. +* Types:: libffi type descriptions. +* Multiple ABIs:: Different passing styles on one platform. +* The Closure API:: Writing a generic function. +@end menu + + +@node The Basics +@section The Basics + +@samp{Libffi} assumes that you have a pointer to the function you wish +to call and that you know the number and types of arguments to pass +it, as well as the return type of the function. + +The first thing you must do is create an @code{ffi_cif} object that +matches the signature of the function you wish to call. This is a +separate step because it is common to make multiple calls using a +single @code{ffi_cif}. The @dfn{cif} in @code{ffi_cif} stands for +Call InterFace. To prepare a call interface object, use the function +@code{ffi_prep_cif}. +@cindex cif + +@findex ffi_prep_cif +@defun ffi_status ffi_prep_cif (ffi_cif *@var{cif}, ffi_abi @var{abi}, unsigned int @var{nargs}, ffi_type *@var{rtype}, ffi_type **@var{argtypes}) +This initializes @var{cif} according to the given parameters. + +@var{abi} is the ABI to use; normally @code{FFI_DEFAULT_ABI} is what +you want. @ref{Multiple ABIs} for more information. + +@var{nargs} is the number of arguments that this function accepts. +@samp{libffi} does not yet handle varargs functions; see @ref{Missing +Features} for more information. + +@var{rtype} is a pointer to an @code{ffi_type} structure that +describes the return type of the function. @xref{Types}. + +@var{argtypes} is a vector of @code{ffi_type} pointers. +@var{argtypes} must have @var{nargs} elements. If @var{nargs} is 0, +this argument is ignored. + +@code{ffi_prep_cif} returns a @code{libffi} status code, of type +@code{ffi_status}. This will be either @code{FFI_OK} if everything +worked properly; @code{FFI_BAD_TYPEDEF} if one of the @code{ffi_type} +objects is incorrect; or @code{FFI_BAD_ABI} if the @var{abi} parameter +is invalid. +@end defun + + +To call a function using an initialized @code{ffi_cif}, use the +@code{ffi_call} function: + +@findex ffi_call +@defun void ffi_call (ffi_cif *@var{cif}, void *@var{fn}, void *@var{rvalue}, void **@var{avalues}) +This calls the function @var{fn} according to the description given in +@var{cif}. @var{cif} must have already been prepared using +@code{ffi_prep_cif}. + +@var{rvalue} is a pointer to a chunk of memory that will hold the +result of the function call. This must be large enough to hold the +result and must be suitably aligned; it is the caller's responsibility +to ensure this. If @var{cif} declares that the function returns +@code{void} (using @code{ffi_type_void}), then @var{rvalue} is +ignored. If @var{rvalue} is @samp{NULL}, then the return value is +discarded. + +@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. +@end defun + + +@node Simple Example +@section Simple Example + +Here is a trivial example that calls @code{puts} a few times. + +@example +#include <stdio.h> +#include <ffi.h> + +int main() +@{ + ffi_cif cif; + ffi_type *args[1]; + void *values[1]; + char *s; + int rc; + + /* Initialize the argument info vectors */ + args[0] = &ffi_type_pointer; + values[0] = &s; + + /* Initialize the cif */ + if (ffi_prep_cif(&cif, FFI_DEFAULT_ABI, 1, + &ffi_type_uint, args) == FFI_OK) + @{ + s = "Hello World!"; + ffi_call(&cif, puts, &rc, values); + /* rc now holds the result of the call to puts */ + + /* values holds a pointer to the function's arg, so to + call puts() again all we need to do is change the + value of s */ + s = "This is cool!"; + ffi_call(&cif, puts, &rc, values); + @} + + return 0; +@} +@end example + + +@node Types +@section Types + +@menu +* Primitive Types:: Built-in types. +* Structures:: Structure types. +* Type Example:: Structure type example. +@end menu + +@node Primitive Types +@subsection Primitive Types + +@code{Libffi} provides a number of built-in type descriptors that can +be used to describe argument and return types: + +@table @code +@item ffi_type_void +@tindex ffi_type_void +The type @code{void}. This cannot be used for argument types, only +for return values. + +@item ffi_type_uint8 +@tindex ffi_type_uint8 +An unsigned, 8-bit integer type. + +@item ffi_type_sint8 +@tindex ffi_type_sint8 +A signed, 8-bit integer type. + +@item ffi_type_uint16 +@tindex ffi_type_uint16 +An unsigned, 16-bit integer type. + +@item ffi_type_sint16 +@tindex ffi_type_sint16 +A signed, 16-bit integer type. + +@item ffi_type_uint32 +@tindex ffi_type_uint32 +An unsigned, 32-bit integer type. + +@item ffi_type_sint32 +@tindex ffi_type_sint32 +A signed, 32-bit integer type. + +@item ffi_type_uint64 +@tindex ffi_type_uint64 +An unsigned, 64-bit integer type. + +@item ffi_type_sint64 +@tindex ffi_type_sint64 +A signed, 64-bit integer type. + +@item ffi_type_float +@tindex ffi_type_float +The C @code{float} type. + +@item ffi_type_double +@tindex ffi_type_double +The C @code{double} type. + +@item ffi_type_uchar +@tindex ffi_type_uchar +The C @code{unsigned char} type. + +@item ffi_type_schar +@tindex ffi_type_schar +The C @code{signed char} type. (Note that there is not an exact +equivalent to the C @code{char} type in @code{libffi}; ordinarily you +should either use @code{ffi_type_schar} or @code{ffi_type_uchar} +depending on whether @code{char} is signed.) + +@item ffi_type_ushort +@tindex ffi_type_ushort +The C @code{unsigned short} type. + +@item ffi_type_sshort +@tindex ffi_type_sshort +The C @code{short} type. + +@item ffi_type_uint +@tindex ffi_type_uint +The C @code{unsigned int} type. + +@item ffi_type_sint +@tindex ffi_type_sint +The C @code{int} type. + +@item ffi_type_ulong +@tindex ffi_type_ulong +The C @code{unsigned long} type. + +@item ffi_type_slong +@tindex ffi_type_slong +The C @code{long} type. + +@item ffi_type_longdouble +@tindex ffi_type_longdouble +On platforms that have a C @code{long double} type, this is defined. +On other platforms, it is not. + +@item ffi_type_pointer +@tindex ffi_type_pointer +A generic @code{void *} pointer. You should use this for all +pointers, regardless of their real type. +@end table + +Each of these is of type @code{ffi_type}, so you must take the address +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. +You must first describe the structure to @samp{libffi} by creating a +new @code{ffi_type} object for it. + +@tindex ffi_type +@deftp ffi_type +The @code{ffi_type} has the following members: +@table @code +@item size_t size +This is set by @code{libffi}; you should initialize it to zero. + +@item unsigned short alignment +This is set by @code{libffi}; you should initialize it to zero. + +@item unsigned short type +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. +@end table +@end deftp + + +@node Type Example +@subsection Type Example + +The following example initializes a @code{ffi_type} object +representing the @code{tm} struct from Linux's @file{time.h}. + +Here is how the struct is defined: + +@example +struct tm @{ + int tm_sec; + int tm_min; + int tm_hour; + int tm_mday; + int tm_mon; + int tm_year; + int tm_wday; + int tm_yday; + int tm_isdst; + /* Those are for future use. */ + long int __tm_gmtoff__; + __const char *__tm_zone__; +@}; +@end example + +Here is the corresponding code to describe this struct to +@code{libffi}: + +@example + @{ + ffi_type tm_type; + ffi_type *tm_type_elements[12]; + int i; + + tm_type.size = tm_type.alignment = 0; + tm_type.elements = &tm_type_elements; + + for (i = 0; i < 9; i++) + tm_type_elements[i] = &ffi_type_sint; + + tm_type_elements[9] = &ffi_type_slong; + tm_type_elements[10] = &ffi_type_pointer; + tm_type_elements[11] = NULL; + + /* tm_type can now be used to represent tm argument types and + return types for ffi_prep_cif() */ + @} +@end example + + +@node Multiple ABIs +@section Multiple ABIs + +A given platform may provide multiple different ABIs at once. For +instance, the x86 platform has both @samp{stdcall} and @samp{fastcall} +functions. + +@code{libffi} provides some support for this. However, this is +necessarily platform-specific. + +@c FIXME: document the platforms + +@node The Closure API +@section The Closure API + +@code{libffi} also provides a way to write a generic function -- a +function that can accept and decode any combination of arguments. +This can be useful when writing an interpreter, or to provide wrappers +for arbitrary functions. + +This facility is called the @dfn{closure API}. Closures are not +supported on all platforms; you can check the @code{FFI_CLOSURES} +define to determine whether they are supported on the current +platform. +@cindex closures +@cindex closure API +@findex FFI_CLOSURES + +Because closures work by assembling a tiny function at runtime, they +require special allocation on platforms that have a non-executable +heap. Memory management for closures is handled by a pair of +functions: + +@findex ffi_closure_alloca +@defun void *ffi_closure_alloc (size_t @var{size}, void **@var{code}) +Allocate a chunk of memory holding @var{size} bytes. This returns a +pointer to the writable address, and sets *@var{code} to the +corresponding executable address. + +@var{size} should be sufficient to hold a @code{ffi_closure} object. +@end defun + +@findex ffi_closure_free +@defun void ffi_closure_free (void *@var{writable}) +Free memory allocated using @code{ffi_closure_alloc}. The argument is +the writable address that was returned. +@end defun + + +Once you have allocated the memory for a closure, you must construct a +@code{ffi_cif} describing the function call. Finally you can prepare +the closure function: + +@findex ffi_prep_closure_loc +@defun ffi_status ffi_prep_closure_loc (ffi_closure *@var{closure}, ffi_cif *@var{cif}, void (*@var{fun}) (ffi_cif *@var{cif}, void *@var{ret}, void **@var{args}, void *@var{user_data}), void *@var{user_data}, void *@var{codeloc}) +Prepare a closure function. + +@var{closure} is the address of a @code{ffi_closure} object; this is +the writable address returned by @code{ffi_closure_alloc}. + +@var{cif} is the @code{ffi_cif} describing the function parameters. + +@var{user_data} is an arbitrary datum that is passed, uninterpreted, +to your closure function. + +@var{codeloc} is the executable address returned by +@code{ffi_closure_alloc}. + +@var{fun} is the function which will be called when the closure is +invoked. It is called with the arguments: +@table @var +@item cif +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}. +@c FIXME: is this NULL for void-returning functions? + +@item args +A vector of pointers to memory holding the arguments to the function. + +@item user_data +The same @var{user_data} that was passed to +@code{ffi_prep_closure_loc}. +@end table + +@code{ffi_prep_closure_loc} will return @code{FFI_OK} if everything +went ok, and something else on error. +@c FIXME: what? + +After calling @code{ffi_prep_closure_loc}, you can cast @var{codeloc} +to the appropriate pointer-to-function type. +@end defun + +@c FIXME: example + +You may see old code referring to @code{ffi_prep_closure}. This +function is deprecated, as it cannot handle the need for separate +writable and executable addresses. + + +@node Missing Features +@chapter Missing Features + +@code{libffi} is missing a few features. We welcome patches to add +support for these. + +@itemize @bullet +@item +There is no support for calling varargs functions. This may work on +some platforms, depending on how the ABI is defined, but it is not +reliable. + +@item +There is no support for bit fields in structures. + +@item +The closure API is + +@item +The ``raw'' API is undocumented. +@c argument promotion? +@c unions? +@c anything else? +@end itemize + + +@node Index +@unnumbered Index + +@printindex cp + +@bye |