1@c Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,
2@c 1999, 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc.
3@c This is part of the GCC manual.
4@c For copying conditions, see the file gcc.texi.
5
6@node Interface
7@chapter Interfacing to GCC Output
8@cindex interfacing to GCC output
9@cindex run-time conventions
10@cindex function call conventions
11@cindex conventions, run-time
12
13GCC is normally configured to use the same function calling convention
14normally in use on the target system.  This is done with the
15machine-description macros described (@pxref{Target Macros}).
16
17@cindex unions, returning
18@cindex structures, returning
19@cindex returning structures and unions
20However, returning of structure and union values is done differently on
21some target machines.  As a result, functions compiled with PCC
22returning such types cannot be called from code compiled with GCC,
23and vice versa.  This does not cause trouble often because few Unix
24library routines return structures or unions.
25
26GCC code returns structures and unions that are 1, 2, 4 or 8 bytes
27long in the same registers used for @code{int} or @code{double} return
28values.  (GCC typically allocates variables of such types in
29registers also.)  Structures and unions of other sizes are returned by
30storing them into an address passed by the caller (usually in a
31register).  The target hook @code{TARGET_STRUCT_VALUE_RTX}
32tells GCC where to pass this address.
33
34By contrast, PCC on most target machines returns structures and unions
35of any size by copying the data into an area of static storage, and then
36returning the address of that storage as if it were a pointer value.
37The caller must copy the data from that memory area to the place where
38the value is wanted.  This is slower than the method used by GCC, and
39fails to be reentrant.
40
41On some target machines, such as RISC machines and the 80386, the
42standard system convention is to pass to the subroutine the address of
43where to return the value.  On these machines, GCC has been
44configured to be compatible with the standard compiler, when this method
45is used.  It may not be compatible for structures of 1, 2, 4 or 8 bytes.
46
47@cindex argument passing
48@cindex passing arguments
49GCC uses the system's standard convention for passing arguments.  On
50some machines, the first few arguments are passed in registers; in
51others, all are passed on the stack.  It would be possible to use
52registers for argument passing on any machine, and this would probably
53result in a significant speedup.  But the result would be complete
54incompatibility with code that follows the standard convention.  So this
55change is practical only if you are switching to GCC as the sole C
56compiler for the system.  We may implement register argument passing on
57certain machines once we have a complete GNU system so that we can
58compile the libraries with GCC@.
59
60On some machines (particularly the SPARC), certain types of arguments
61are passed ``by invisible reference''.  This means that the value is
62stored in memory, and the address of the memory location is passed to
63the subroutine.
64
65@cindex @code{longjmp} and automatic variables
66If you use @code{longjmp}, beware of automatic variables.  ISO C says that
67automatic variables that are not declared @code{volatile} have undefined
68values after a @code{longjmp}.  And this is all GCC promises to do,
69because it is very difficult to restore register variables correctly, and
70one of GCC's features is that it can put variables in registers without
71your asking it to.
72