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..
Copyright 1988-2022 Free Software Foundation, Inc.
This is part of the GCC manual.
For copying conditions, see the copyright.rst file.
.. index:: extern, external names
.. _function-names:
Function Names
**************
Go code can call C functions directly using the ``//extern`` or
``//go:linkname`` compiler directives. An ``//extern``
directive must be at the beginning of the line and must start with
``//extern``. This must be followed by a space and then the
external name of the function. The function declaration must be on
the line immediately after the comment. For example, here is how the
C function ``open`` can be declared in Go:
.. code-block:: c++
//extern open
func c_open(name *byte, mode int, perm int) int
You can do the same thing using the ``//go:linkname`` compiler
directive. The ``//go:linkname`` directive must be at the start of
the line. It is followed by whitespace, the name of the Go function,
more whitespace, and the external name of the function. Unlike
``//extern``, ``//go:linkname`` does not need to appear
immediately adjacent to the function definition or declaration.
.. code-block:: c++
//go:linkname c_open open
func c_open(name *byte, mode int, perm int) int
The C function naturally expects a nul terminated string, which in Go
is equivalent to a pointer to an array (not a slice!) of ``byte``
with a terminating zero byte. So a sample call from Go would look
like (after importing the ``os`` package):
.. code-block:: c++
var name = [4]byte{'f', 'o', 'o', 0};
i := c_open(&name[0], os.O_RDONLY, 0);
Note that this serves as an example only. To open a file in Go please
use Go's ``os.Open`` function instead.
The name of Go functions accessed from C is subject to change. At
present the name of a Go function that does not have a receiver is
``pkgpath.Functionname``. The :samp:`{pkgpath}` is set by the
:option:`-fgo-pkgpath` option used when the package is compiled; if the
option is not used, the default is ``go.packagename``. To
call the function from C you must set the name using the :command:`gcc`
``__asm__`` extension.
.. code-block:: c++
extern int go_function(int) __asm__ ("mypkgpath.Function");
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