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Ref-docs/manual gcc2.95.3/gcc_13.html
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Ref-docs/manual gcc2.95.3/gcc_13.html
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<HTML>
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<!-- Created on March, 17 2001 by texi2html 1.64 -->
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<!--
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Written by: Lionel Cons <Lionel.Cons@cern.ch> (original author)
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Karl Berry <karl@freefriends.org>
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Olaf Bachmann <obachman@mathematik.uni-kl.de>
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and many others.
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Maintained by: Olaf Bachmann <obachman@mathematik.uni-kl.de>
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Send bugs and suggestions to <texi2html@mathematik.uni-kl.de>
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-->
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<HEAD>
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<TITLE>Using and Porting the GNU Compiler Collection (GCC): Interface</TITLE>
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<META NAME="description" CONTENT="Using and Porting the GNU Compiler Collection (GCC): Interface">
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<META NAME="keywords" CONTENT="Using and Porting the GNU Compiler Collection (GCC): Interface">
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<META NAME="resource-type" CONTENT="document">
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<META NAME="distribution" CONTENT="global">
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<META NAME="Generator" CONTENT="texi2html 1.64">
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</HEAD>
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<BODY LANG="" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#800080" ALINK="#FF0000">
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<A NAME="SEC147"></A>
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<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
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<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gcc_12.html#SEC146" tppabs="http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_12.html#SEC146"> < </A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gcc_14.html#SEC148" tppabs="http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_14.html#SEC148"> > </A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gcc_2.html#SEC2" tppabs="http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_2.html#SEC2"> << </A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gcc.html#SEC_Top" tppabs="http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc.html#SEC_Top"> Up </A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gcc_14.html#SEC148" tppabs="http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_14.html#SEC148"> >> </A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gcc.html#SEC_Top" tppabs="http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc.html#SEC_Top">Top</A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gcc_toc.html#SEC_Contents" tppabs="http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_toc.html#SEC_Contents">Contents</A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gcc_24.html#SEC261" tppabs="http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_24.html#SEC261">Index</A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gcc_abt.html#SEC_About" tppabs="http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_abt.html#SEC_About"> ? </A>]</TD>
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</TR></TABLE>
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<H1> 13. Interfacing to GCC Output </H1>
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<!--docid::SEC147::-->
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<P>
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GCC is normally configured to use the same function calling convention
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normally in use on the target system. This is done with the
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machine-description macros described (see section <A HREF="gcc_17.html#SEC199" tppabs="http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_17.html#SEC199">17. Target Description Macros</A>).
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</P><P>
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<A NAME="IDX433"></A>
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<A NAME="IDX434"></A>
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<A NAME="IDX435"></A>
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However, returning of structure and union values is done differently on
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some target machines. As a result, functions compiled with PCC
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returning such types cannot be called from code compiled with GCC,
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and vice versa. This does not cause trouble often because few Unix
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library routines return structures or unions.
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</P><P>
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GCC code returns structures and unions that are 1, 2, 4 or 8 bytes
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long in the same registers used for <CODE>int</CODE> or <CODE>double</CODE> return
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values. (GCC typically allocates variables of such types in
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registers also.) Structures and unions of other sizes are returned by
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storing them into an address passed by the caller (usually in a
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register). The machine-description macros <CODE>STRUCT_VALUE</CODE> and
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<CODE>STRUCT_INCOMING_VALUE</CODE> tell GCC where to pass this address.
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</P><P>
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By contrast, PCC on most target machines returns structures and unions
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of any size by copying the data into an area of static storage, and then
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returning the address of that storage as if it were a pointer value.
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The caller must copy the data from that memory area to the place where
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the value is wanted. This is slower than the method used by GCC, and
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fails to be reentrant.
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</P><P>
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On some target machines, such as RISC machines and the 80386, the
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standard system convention is to pass to the subroutine the address of
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where to return the value. On these machines, GCC has been
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configured to be compatible with the standard compiler, when this method
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is used. It may not be compatible for structures of 1, 2, 4 or 8 bytes.
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</P><P>
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<A NAME="IDX436"></A>
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<A NAME="IDX437"></A>
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GCC uses the system's standard convention for passing arguments. On
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some machines, the first few arguments are passed in registers; in
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others, all are passed on the stack. It would be possible to use
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registers for argument passing on any machine, and this would probably
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result in a significant speedup. But the result would be complete
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incompatibility with code that follows the standard convention. So this
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change is practical only if you are switching to GCC as the sole C
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compiler for the system. We may implement register argument passing on
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certain machines once we have a complete GNU system so that we can
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compile the libraries with GCC.
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</P><P>
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On some machines (particularly the Sparc), certain types of arguments
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are passed "by invisible reference". This means that the value is
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stored in memory, and the address of the memory location is passed to
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the subroutine.
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</P><P>
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<A NAME="IDX438"></A>
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If you use <CODE>longjmp</CODE>, beware of automatic variables. ANSI C says that
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automatic variables that are not declared <CODE>volatile</CODE> have undefined
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values after a <CODE>longjmp</CODE>. And this is all GCC promises to do,
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because it is very difficult to restore register variables correctly, and
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one of GCC's features is that it can put variables in registers without
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your asking it to.
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</P><P>
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If you want a variable to be unaltered by <CODE>longjmp</CODE>, and you don't
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want to write <CODE>volatile</CODE> because old C compilers don't accept it,
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just take the address of the variable. If a variable's address is ever
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taken, even if just to compute it and ignore it, then the variable cannot
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go in a register:
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</P><P>
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<TABLE><tr><td> </td><td class=example><pre>{
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int careful;
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&careful;
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<small>...</small>
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}
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</pre></td></tr></table></P><P>
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<A NAME="IDX439"></A>
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<A NAME="IDX440"></A>
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Code compiled with GCC may call certain library routines. Most of
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them handle arithmetic for which there are no instructions. This
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includes multiply and divide on some machines, and floating point
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operations on any machine for which floating point support is disabled
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with <SAMP>`-msoft-float'</SAMP>. Some standard parts of the C library, such as
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<CODE>bcopy</CODE> or <CODE>memcpy</CODE>, are also called automatically. The usual
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function call interface is used for calling the library routines.
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</P><P>
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These library routines should be defined in the library <TT>`libgcc.a'</TT>,
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which GCC automatically searches whenever it links a program. On
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machines that have multiply and divide instructions, if hardware
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floating point is in use, normally <TT>`libgcc.a'</TT> is not needed, but it
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is searched just in case.
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</P><P>
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Each arithmetic function is defined in <TT>`libgcc1.c'</TT> to use the
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corresponding C arithmetic operator. As long as the file is compiled
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with another C compiler, which supports all the C arithmetic operators,
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this file will work portably. However, <TT>`libgcc1.c'</TT> does not work if
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compiled with GCC, because each arithmetic function would compile
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into a call to itself!
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</P><P>
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<A NAME="Passes"></A>
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<HR SIZE="6">
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<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
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<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gcc_2.html#SEC2" tppabs="http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_2.html#SEC2"> << </A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gcc_14.html#SEC148" tppabs="http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_14.html#SEC148"> >> </A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT"> <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gcc.html#SEC_Top" tppabs="http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc.html#SEC_Top">Top</A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gcc_toc.html#SEC_Contents" tppabs="http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_toc.html#SEC_Contents">Contents</A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gcc_24.html#SEC261" tppabs="http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_24.html#SEC261">Index</A>]</TD>
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<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="gcc_abt.html#SEC_About" tppabs="http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_abt.html#SEC_About"> ? </A>]</TD>
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</TR></TABLE>
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<BR>
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<FONT SIZE="-1">
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This document was generated
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by <I>GCC Administrator</I> on <I>March, 17 2001</I>
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using <A HREF="tppmsgs/msgs0.htm#1" tppabs="http://www.mathematik.uni-kl.de/~obachman/Texi2html"><I>texi2html</I></A>
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