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@c Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,

@c 1999, 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc.

@c This is part of the GCC manual.

@c For copying conditions, see the file gcc.texi.

@node Interface

@chapter Interfacing to GCC Output

@cindex interfacing to GCC output

@cindex run-time conventions

@cindex function call conventions

@cindex conventions, run-time

GCC is normally configured to use the same function calling convention

normally in use on the target system.  This is done with the

machine-description macros described (@pxref{Target Macros}).

@cindex unions, returning

@cindex structures, returning

@cindex returning structures and unions

However, returning of structure and union values is done differently on

some target machines.  As a result, functions compiled with PCC

returning such types cannot be called from code compiled with GCC,

and vice versa.  This does not cause trouble often because few Unix

library routines return structures or unions.

GCC code returns structures and unions that are 1, 2, 4 or 8 bytes

long in the same registers used for @code{int} or @code{double} return

values.  (GCC typically allocates variables of such types in

registers also.)  Structures and unions of other sizes are returned by

storing them into an address passed by the caller (usually in a

register).  The target hook @code{TARGET_STRUCT_VALUE_RTX}

tells GCC where to pass this address.

By contrast, PCC on most target machines returns structures and unions

of any size by copying the data into an area of static storage, and then

returning the address of that storage as if it were a pointer value.

The caller must copy the data from that memory area to the place where

the value is wanted.  This is slower than the method used by GCC, and

fails to be reentrant.

On some target machines, such as RISC machines and the 80386, the

standard system convention is to pass to the subroutine the address of

where to return the value.  On these machines, GCC has been

configured to be compatible with the standard compiler, when this method

is used.  It may not be compatible for structures of 1, 2, 4 or 8 bytes.

@cindex argument passing

@cindex passing arguments

GCC uses the system's standard convention for passing arguments.  On

some machines, the first few arguments are passed in registers; in

others, all are passed on the stack.  It would be possible to use

registers for argument passing on any machine, and this would probably

result in a significant speedup.  But the result would be complete

incompatibility with code that follows the standard convention.  So this

change is practical only if you are switching to GCC as the sole C

compiler for the system.  We may implement register argument passing on

certain machines once we have a complete GNU system so that we can

compile the libraries with GCC@.

On some machines (particularly the SPARC), certain types of arguments

are passed ``by invisible reference''.  This means that the value is

stored in memory, and the address of the memory location is passed to

the subroutine.

@cindex @code{longjmp} and automatic variables

If you use @code{longjmp}, beware of automatic variables.  ISO C says that

automatic variables that are not declared @code{volatile} have undefined

values after a @code{longjmp}.  And this is all GCC promises to do,

because it is very difficult to restore register variables correctly, and

one of GCC's features is that it can put variables in registers without

your asking it to.