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LINK(5)								       LINK(5)

NAME
       link - link editor interfaces

SYNOPSIS
       #include <link.h>

DESCRIPTION
       Dynamically linked executables created by ld(1) contain data structures
       used by the dynamic link editor to finish link-editing the program dur‐
       ing  program  execution.	  These	 data  structures are described with a
       link_dynamic structure, as defined in the link.h file.  ld always iden‐
       tifies  the  location of this structure in the executable file with the
       symbol __DYNAMIC.  This symbol is ld-defined and if  referenced	in  an
       executable  that	 does  not require dynamic linking will have the value
       zero.

       The program stub linked with ``main'' programs by compiler drivers such
       as  cc(1V) (called crt0) tests the definition of __DYNAMIC to determine
       whether or not the dynamic link editor  should  be  invoked.   Programs
       supplying  a substitute for crt0 must either duplicate this functional‐
       ity or else require that the programs with which	 they  are  linked  be
       linked  statically.   Otherwise,	 such replacement crt0's must open and
       map in the executable /usr/lib/ld.so using  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 execve(2V) system call does.  The first location
       following the 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:

	      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 */
	      }

       The members of the structure are:

       crt_ba	      The address at which /usr/lib/ld.so has been mapped.

       crt_dzfd	      An open file descriptor for /dev/zero.  ld.so will close
		      this file descriptor before returning.

       crt_ldfd	      The file descriptor used to map  /usr/lib/ld.so.	 ld.so
		      will close this file descriptor before returning.

       crt_dp	      A pointer to the label __DYNAMIC in the executable which
		      is calling ld.so.

       crt_ep	      A pointer to the environment  strings  provided  to  the
		      program.

       crt_bp	      A	 location in the executable which contains an instruc‐
		      tion that will be	 executed  after  the  call  to	 ld.so
		      returns.	 This location is used as a breakpoint in pro‐
		      grams that are being executed under  the	control	 of  a
		      debugger such as adb(1).

SEE ALSO
       ld(1), mmap(2), a.out(5)

BUGS
       These interfaces are under development and are subject to rapid change.

			       17 February 1988			       LINK(5)
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