link.5

早期freebsd实现

.\" This source code is a product of Sun Microsystems, Inc. and is provided
.\" for unrestricted use provided that this legend is included on all tape
.\" media and as a part of the software program in whole or part.  Users
.\" may copy or modify this source code without charge, but are not authorized
.\" to license or distribute it to anyone else except as part of a product or
.\" program developed by the user.
.\"
.\" THIS PROGRAM CONTAINS SOURCE CODE COPYRIGHTED BY SUN MICROSYSTEMS, INC.
.\" SUN MICROSYSTEMS, INC., MAKES NO REPRESENTATIONS ABOUT THE SUITABLITY
.\" OF SUCH SOURCE CODE FOR ANY PURPOSE.  IT IS PROVIDED "AS IS" WITHOUT
.\" EXPRESS OR IMPLIED WARRANTY OF ANY KIND.  SUN MICROSYSTEMS, INC. DISCLAIMS
.\" ALL WARRANTIES WITH REGARD TO SUCH SOURCE CODE, INCLUDING ALL IMPLIED
.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  IN
.\" NO EVENT SHALL SUN MICROSYSTEMS, INC. BE LIABLE FOR ANY SPECIAL, INDIRECT,
.\" INCIDENTAL, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING
.\" FROM USE OF SUCH SOURCE CODE, REGARDLESS OF THE THEORY OF LIABILITY.
.\" 
.\" This source code is provided with no support and without any obligation on
.\" the part of Sun Microsystems, Inc. to assist in its use, correction, 
.\" modification or enhancement.
.\"
.\" SUN MICROSYSTEMS, INC. SHALL HAVE NO LIABILITY WITH RESPECT TO THE
.\" INFRINGEMENT OF COPYRIGHTS, TRADE SECRETS OR ANY PATENTS BY THIS
.\" SOURCE CODE OR ANY PART THEREOF.
.\"
.\" Sun Microsystems, Inc.
.\" 2550 Garcia Avenue
.\" Mountain View, California 94043
.\"
.\" Copyright (c) 1991 Sun Microsystems, Inc.
.\"
.\" @(#)link.5 1.6 90/01/04 SMI
.TH LINK 5 "17 February 1988"
.SH NAME
link \- link editor interfaces
.SH SYNOPSIS
.B #include <link.h>
.SH DESCRIPTION
.IX "link editor" "" "link editor data structures"
.LP
Dynamically linked executables created by
.BR ld (1)
contain data structures used by the dynamic link editor
to finish link-editing the program during program execution.
These data
structures are described with a 
.B link_dynamic 
structure, as defined in the
.B link.h
file.  
.B ld
always identifies the location of this structure in the executable file with
the symbol
.BR _\^_DYNAMIC .
This symbol is 
.BR ld -defined
and if referenced in an executable that does not require dynamic linking 
will have the value zero.
.LP
The program stub linked with ``main'' programs by compiler drivers such as
.BR cc (1V)
(called 
.BR crt0 )
tests the definition of
.B _\^_DYNAMIC
to determine whether or not the dynamic link editor should be invoked.
Programs supplying a substitute for 
.B crt0
must either duplicate this functionality or else require that the programs with
which they are linked be linked 
.IR statically .
Otherwise, such replacement
.BR crt0 's
must open and map in the executable
.B /usr/lib/ld.so
using 
.BR mmap (2).
Care should be taken to ensure that the expected mapping relationship between
the ``text'' and ``data'' segments of the executable is maintained in the
same manner that the 
.BR execve (2V)
system call does.
The first location following the
.B a.out
header of this executable is the entry point to a function that begins the
dynamic link-editing process.  This function must be called and supplied
with two
arguments.  The first argument is an integer representing the revision level
of the argument list, and should have the value ``1''.  The second should be a
pointer to an argument list structure of the form:
.LP
.RS
.nf
.ta .75i 1.25i 2.75i
.ft B
struct {
	int	crt_ba;			/* base address of ld.so */
	int	crt_dzfd;		/* open fd to /dev/zero */
	int	crt_ldfd;		/* open fd to ld.so */
	struct	link_dynamic *crt_dp;	/* pointer to program's _\^_DYNAMIC */
	char	**crt_ep;		/* environment strings */
	caddr_t	crt_bp;			/* debugger hook */
}
.ft R
.fi
.DT
.RE
.LP
The members of the structure are:
.TP 15
.B crt_ba
The address at which 
.B /usr/lib/ld.so
has been mapped.
.TP 
.B crt_dzfd
An open file descriptor for 
.BR /dev/zero .
.B ld.so
will close this file descriptor before returning.
.TP 
.B crt_ldfd
The file descriptor used to map 
.BR /usr/lib/ld.so .
.B ld.so
will close this file descriptor before returning.
.TP 
.B crt_dp
A pointer to the label
.B _\^_DYNAMIC
in the executable which is calling 
.BR ld.so .
.TP 
.B crt_ep
A pointer to the environment strings provided to the program.
.TP 
.B crt_bp
A location in the executable which contains an instruction that will
be executed after the call to
.B ld.so
returns.  This location is used as a breakpoint in programs that are
being executed under the control of a debugger such as
.BR adb (1).
.SH "SEE ALSO"
.BR ld (1),
.BR mmap (2),
.BR a.out (5)
.SH "BUGS"
These interfaces are under development and are subject to rapid change.