ld.1

早期freebsd实现

and the abbreviated library-names specified by
.RB ` \-l '.
These may appear
anywhere, and their influence is dependent upon their location.  
Some options may be obtained from
environment variables, such options are interpreted before any
on the command line (see
.SM ENVIRONMENT\s0,
below).
.SS Object File Processing
.LP
The files specified on the command line are processed in
the order listed.  Information is extracted from each file,
and concatenated to form the output.  The specific
processing performed on a given file depends upon whether it is
a simple object file, a library archive, or a shared library.
.LP
Simple object
.RB ( .o )
files are concatenated to the output as they are encountered.
.LP
Library archive
.RB  ( .a )
files are searched exactly once each, as each is encountered;
only those archive entries matching an unresolved external reference
are extracted and concatenated to the output.  If a member of an
archive references a symbol defined by another member of that same
archive, the member making the reference must appear before the
member containing the definition.
.LP
On Sun386i, a library contains a dictionary of symbols,  On
other Sun systems, processing library archives through
.BR ranlib (1)
provides this dictionary.  In addition, you can use 
.BR lorder (1),
in combination with
.BR tsort (1)
to place library members in calling order (see 
.BR lorder (1)
for details), or both (for fastest symbol lookup).
The first member of an archived processed by
.B ranlib 
has the reserved name of
.BR _\^_.\s-1SYMDEF\s0 ,
which
.B ld
takes to be the dictionary of all symbols defined by
members of the archive.
.LP
Sharable objects
.RB ( .so )
are scanned for symbol definitions and references, 
but are not normally included in the output from
.BR ld ,
except in cases where a shared library exports initialized data
structures and the
.B \-dc
option is in effect.  However, the
occurrence of each sharable object file in the
.BR ld
command line is noted in the resulting executable file; this
notation is utilized by an execution-time variant of
.BR ld ,
.BR ld.so ,
for
.I deferred
and
.I dynamic
loading and binding during execution.
See
.BR "Execution-Time Loading" ,
below, for details.
.LP
The
.B \-l
option specifies a short name for an object
file or archive used as a library.
The full name of the object file is
derived by adding the prefix 
.B lib
and a suffix of either
.B .a
or
.BR .so [ .\c
.IR v ]
to indicate an 
.BR ar (1V)
archive or a shared library, respectively.
The specific suffix used is determined through rules discussed in
.BR "Binding and Relocation Semantics" ,
below.
.LP
.B ld
searches for the desired object file through a list of
directories specified by
.B \-L
options, the environment variable
.BR \s-1LD_LIBRARY_PATH\s0 ,
and finally, the built-in list of standard library directories:
.BR /lib ,
.BR /usr/lib ,
and
.BR /usr/local/lib .
.
.SS Binding and Relocation Semantics
.LP
The manner in which
.B ld
processes a given object file is
dependent in part upon the \*(lqbinding mode\*(rq in which it is operating
at the time the file is encountered.
This binding mode is specified by the
.B \-B
flag, which takes the keyword arguments:
.RS
.TP 10
.B dynamic
Allow dynamic binding, do not resolve symbolic references, and allow
creation of execution-time symbol and relocation information.
This is the default setting.
.TP
.B static
Force static binding, implied by options that generate non-sharable
executable formats.
.RE
.LP
.B \-Bdynamic
and
.B \-Bstatic
may be specified several times, and may be used to toggle each other
on and off.
Like
.BR \-l ,
the influence of each depends upon its location within the
command line. 
When
.B \-Bdynamic
is in effect, 
.B \-l
searches may be satisfied by the first occurrence of either form of
library 
.RB ( .so
or
.BR .a ), 
but if both are encountered, the
.B .so
form is preferred.  When
.B \-Bstatic
is in effect,
.B ld
refuses to use any
.B .so
libraries it encounters; it continue searching
for the
.B .a
form.  Furthermore, an explicit request to load a
.B .so
file is treated as an error.  
.LP
After
.B ld
has processed all input files and command line options, the form of
the output it produces is based on the information provided in
both.  
.B ld
first tries to reduce all symbolic references to
relative numerical offsets within the executable it is building.  To
perform this \*(lqsymbolic reduction,\*(rq
.B ld
must be able to determine that:
.RS
.TP 3
\(bu
all information relating to the program has been provided, in
particular, no
.B .so
is to be added at execution time; and/or
.TP
\(bu
the program has an entry point, and symbolic reduction can
be performed for all symbols having definitions existing in the
material provided.
.RE
.LP
It should be noted that uninitialized \*(lqcommon\*(rq areas 
(for example, uninitialized C globals) are allocated by the
link editor
.I after
it has collected all references.
In particular, this allocation can not occur in a program that still
requires the addition of information contained in a 
.B \&.so
file, as the missing information may affect the allocation process.  
Initialized \*(lqcommons\*(rq however,  are allocated within the executable
in which their definition appears.
.LP
After
.B ld
has performed all the symbolic reductions it can, it
attempts to transform all relative references to absolute addresses.
.B ld
is able to perform this \*(lqrelative reduction\*(rq
only if it has been provided
.I some
absolute address, either implicitly through the specification of an
entry point, or explicitly through
.B ld
command-line options. 
If, after performing all the reductions it can,
there are no further relocations or definitions to perform, then
.B ld
has produced a completely linked executable.
.SS Execution-Time Loading
.LP
In the event that one or more reductions can not be completed,
the executable will require further link editing at execution
time in order to be usable.  Such executables contain an
data structure identified with the symbol
.BR _\^_\s-1DYNAMIC\s0 .
An incompletely linked \*(lqmain\*(rq program should be
linked with a \*(lqbootstrap\*(rq routine that invokes
.BR ld.so ,
which uses the information
contained in the main program's 
.SB _\^_DYNAMIC
to assemble the rest
of the executables constituting the entire program.  A standard
Sun compilation driver (such as 
.BR cc (1V))
automatically includes such a module in each \*(lqmain\*(rq executable.
.LP
When
.B ld.so
is given control on program startup, it finds all
.B .so
files specified when the program was constructed (and
all
.BR .so 's
on which they depend), and loads
them into the address space.  The algorithm by which such files are found
mimics that used when
.B ld
is run, and like
.BR ld ,
can be influenced by the setting of
.SB LD_LIBRARY_PATH 
and any
.B \-L
options specified to
.B ld
when the program was built.
.B ld.so
then completes all
remaining relocations, with the exception of procedure call
relocations; failure to resolve a given non-procedural relocation
results in termination of the program with an appropriate diagnostic.
.LP
Procedure relocations are resolved when the referencing instruction
is first executed.  It should be noted that it is possible for
\*(lqundefined symbol\*(rq diagnostics to be produced during program
execution if a given target is not defined when referenced.
.LP
Although it is possible for binding errors to occur at execution-time,
such an occurrence generally indicates something wrong in the
maintenance of shared objects.
.BR ld 's
.B "\-assert  nodefinitions"
function (on by default) checks at
.BR ld -time
whether or not an execution-time binding error would occur.
.SS Version Handling for Shared Libraries
.LP
To allow the independent evolution of
.BR .so 's
used as libraries and the programs which use them,
.BR ld 's
handling of
.B .so
files found through
.B \-l
options involves the retention and
management of version control information.  The
.B .so
files used as such \*(lqshared libraries\*(rq are post-fixed with a
Dewey-decimal format string describing the version of the \*(lqlibrary\*(rq
contained in the file.
.LP
The first decimal component is called the library's
\*(lqmajor version\*(rq number,
and the second component its \*(lqminor version\*(rq number.  When
.B ld
records a 
.B .so
used as a library, it also records these two
numbers in the database used by
.B ld.so
at execution time.  In turn,
.B ld.so
uses these numbers to
decide which of multiple versions of a given library is \*(lqbest\*(rq or
whether
.I any
of the available versions are acceptable.  The
rules are:
.RS
.TP 3
\(bu
Major Versions Identical: the major version used at
execution time must exactly match the version found at
.BR ld -time.
Failure to find an instance of the library with a matching major
version causes a diagnostic to be issued and the program's
execution to be terminated.
.TP
\(bu
Highest Minor Version: in the presence of multiple instances of
libraries that match the desired major version, 
.B ld.so
uses the highest minor version it finds.  However, if the highest minor
version found at execution time is less than the version found at
.BR ld -time,
a warning diagnostic is issued; program execution continues.
.RE
.LP
The semantics of version numbers are such that major version numbers
should be changed whenever interfaces are changed.  Minor versions
should be changed to reflect compatible updates to libraries, and
programs will silently favor the highest compatible version they can
obtain.  
.SS Special Symbols
.LP
A number of symbols have special meanings to 
.B ld
and programs should not define these symbols.  The symbols described
below are those actually seen by
.BR ld .
Note: C and several
other languages prepend symbols they use with 
.RB ` _ '.
.TP
.BR _etext
The first location after the text of the program.
.TP
.BR _edata
The first location after initialized data.
.TP
.B _end
The first location after all data.
.br
.ne 5
.TP
.SB _\^_DYNAMIC
Identifies an
.BR ld -produced
data structure.  It is defined with a non-zero value in
executables which require execution-time link editing.
By convention, if defined, it is the
first symbol in the symbol table associated with an
.B a.out
file.
.br
.ne 8
.TP
.SB _\^_GLOBAL_OFFSET_TABLE_
A position-independent reference to an
.BR ld -constructed
table of addresses.  This table is constructed from
\*(lqposition-independent\*(rq data references occurring in objects
that have been assembled with the assembler's
.B \-k
flag (invoked on behalf of C compilations
performed with the
.B \-pic
flag).  A related table (for which no
symbol is currently defined) contains a series of transfer
instructions and is created from \*(lqposition-independent\*(rq procedure
calls or, if
.B \-dp
is specified to
.BR ld ,
a list of undefined symbols.
.LP
Symbols in object files beginning with the letter
.B L
are taken to be
local symbols and unless otherwise specified are purged from
.B ld
output files.
.br
.ne 6
.SH ENVIRONMENT
.TP
.SB LD_LIBRARY_PATH
A colon-separated list of directories in which to search for
libraries specified with the
.B \-l
option.  Similar to the 
.SB PATH
environment variable.
.SB LD_LIBRARY_PATH
also affects library searching during execution-time loading, causing
the search to use first those directories found in 
the environment variable, then any directories specified by
.B \-L 
options, and finally the standard directories
.B /usr/lib
and
.BR /usr/local/lib .
.SB NOTE:
when running a set-user- or set-group-ID program,
.BR ld.so
will only search for libraries in directories it \*(lqtrusts\*(rq, which
are
.BR /usr/lib ,
.BR /usr/5lib ,
.BR /usr/local/lib ,
and any directories specified within the executable as a result of
.B \-L
options given when the executable was constructed.
.br
.ne 5
.TP
.SB LD_OPTIONS
A default set of options to
.BR ld .
.SB LD_OPTIONS
is interpreted by
.B ld
just as though its value had been placed on the command line,
immediately following the name used to invoke
.BR ld ,
as in:
.IP
.ft B
example% ld $LD_OPTIONS \fR.\|.\|. \fIother-arguments \fR.\|.\|.
.LP
Note: Environment variable-names beginning with the characters
.RB ` LD_ '
are reserved for possible future enhancements to 
.BR ld .
.SH FILES
.PD 0
.TP 20
.B /usr/lib/lib*.a
libraries
.TP
.B lib*.so.v
shared libraries
.TP
.B lib*.sa.v
exported, initialized shared library data
.TP
.B /usr/lib/ld.so
execution-time
.B ld
.TP
.B /usr/lib/*crt*.o
default program bootstraps
.TP
.B a.out
output file
.TP
.B /usr/local/lib
.PD
.SH SEE ALSO
.BR ar (1V),
.BR as (1),
.BR cc (1V),
.BR lorder (1),
.BR ranlib (1),
.BR strip (1),
.BR tsort (1),
.BR ldconfig (8)
.br
.ne 8
.SH BUGS
.LP
Options are being overloaded and are an inappropriate vehicle for describing
to
.B ld
the wide variety of things it can do. 
There needs to be
a link-editing language which can be used in the more complex situations.
.LP
The
.B \-r
option does not properly handle programs assembled with
the
.B \-k
(position-independent) flag, invoked from
.B cc
with
.B \-pic
or
.BR \-\s-1PIC\s0 .