LD(1) General Commands Manual LD(1)
ld - link editor (2BSD)
ld [ option ] ... file ...
Ld combines several object programs into one, resolves external references, and searches libraries. In the simplest case several object
files are given, and ld combines them, producing an object module which can be either executed or become the input for a further ld run.
(In the latter case, the -r option must be given to preserve the relocation bits.) The output of ld is left on a.out. This file is made
executable only if no errors occurred during the load.
The argument routines are concatenated in the order specified. The entry point of the output is the beginning of the first routine (unless
the -e option is specified).
If any argument is a library, it is searched exactly once at the point it is encountered in the argument list. Only those routines defin-
ing an unresolved external reference are loaded. If a routine from a library references another routine in the library, and the library
has not been processed by ranlib(1), the referenced routine must appear after the referencing routine in the library. Thus the order of
programs within libraries may be important. The first member of a library should be a file named `__.SYMDEF', which is understood to be a
dictionary for the library as produced by ranlib(1); the dictionary is searched iteratively to satisfy as many references as possible.
The symbols `_etext', `_edata' and `_end' (`etext', `edata' and `end' in C) are reserved, and if referred to, are set to the first location
above the program, the first location above initialized data, and the first location above all data respectively. It is erroneous to
define these symbols.
Ld understands several options. Except for -l, they should appear before the file names.
-D Take the next argument as a decimal number and pad the data segment with zero bytes to the indicated length.
-d Force definition of common storage even if the -r flag is present.
-e The following argument is taken to be the name of the entry point of the loaded program; location 0 is the default.
-Ldir Add dir to the list of directories in which libraries are searched for. Directories specified with -L are searched before the stan-
-lx This option is an abbreviation for the library name `libx.a', where x is a string. Ld searches for libraries first in any directo-
ries specified with -L options, then in the standard directories `/lib', `/usr/lib', and `/usr/local/lib'. A library is searched
when its name is encountered, so the placement of a -l is significant.
-M produce a primitive load map, listing the names of the files which will be loaded.
-n Arrange (by giving the output file a 0410 "magic number") that when the output file is executed, the text portion will be read-only
and shared among all users executing the file. This involves moving the data areas up to the first possible 8K byte boundary fol-
lowing the end of the text. This option creates a `pure executable' format.
-i When the output file is executed, the program text and data areas will live in separate address spaces. The only difference between
this option and -n is that here the text and data segments are in separate address spaces and both start at location 0. This option
creates a `separate executable' format.
-z This option is a synonym for the -i option. On other systems (4.3BSD for example) the -z option causes a demand paged executable to
be built. This option was added to 2.11BSD because some systems (those which use gcc) do not safely ignore (with a warning) the -i
option. Adding the -z option to 2.11BSD allows makefiles to be copied freely between multiple platforms once again.
-O This is a text replacement overlay file; only the text segment will be replaced by execve(2). Shared data must have the same size
as the program overlaid, otherwise the execve(2) will fail. The entry point to the overlay may be defined with the -e option. This
option allows the creation of a `replacement executable' format.
-o The name argument after -o is used as the name of the ld output file, instead of a.out.
-r Generate relocation bits in the output file so that it can be the subject of another ld run. This flag also prevents final defini-
tions from being given to common symbols, and suppresses the `undefined symbol' diagnostics. (Note that this option cannot be used
with overlays (-Z) since they cannot be reloaded.)
-s `Strip' the output, that is, remove the symbol table and relocation bits to save space (but impair the usefulness of the debuggers).
This information can also be removed by strip(1).
-q ("quiet") Suppress the reporting of undefined symbols. Normally only used when building networked kernels - the large number of
undefined symbols is normal (due to the three phase link proceedure) but can be distracting none the less.
-t ("trace") Print the name of each file as it is processed.
-u Take the following argument as a symbol and enter it as undefined in the symbol table. This is useful for loading wholly from a
library, since initially the symbol table is empty and an unresolved reference is needed to force the loading of the first routine.
-v ("verbose") Print the VM statistics. Printing out the number of pages swapped to and from the VM tmp file is now optional and only
used when a problem is suspected (or if you are voyeuristic).
-X Save local symbols except for those whose names begin with `L'. This option is used by cc(1) to discard internally-generated labels
while retaining symbols local to routines.
-x Do not preserve local (non-.globl) symbols in the output symbol table; only enter external symbols. This option saves some space in
the output file. It also allows temporary labels to be discarded to prevent redefinition in sucessive ld's. Warning: adb uses
these local symbols, especially when debugging overlaid programs, so some debugging information is necessarily lost if this option
-Z Indicate the creation of an automatic-overlay format. In addition a -i or -n must be present as overlays only work with shared text
objects. Repeated instances of -Z bracket the modules that will be loaded into a given overlay. Modules before the first -Z or
after the concluding -Y will be loaded into the non-overlaid text (base) area. Note that there may be a maximum of NOVL (currently
15) overlays. This option produces the `overlaid pure executable' and the `overlaid separate executable' formats. The loader cre-
ates a small entry interface in the base segment for each subroutine in an overlay. This interface ("thunk") arranges for the cor-
rect overlay to be present before the actual routine is entered.
-Y Terminate text overlays. This allows any remaining modules or libraries to be loaded into the base area. Note that the -Y option
used to be -L, but had to be changed when the loader was brought up to date with the 4.3BSD loader which uses -L to indicate a
directory to be searched for library references.
AUTOMATIC TEXT OVERLAY EXAMPLE
To set up an automatic text overlay object with the loader, use a command of the form:
ld -n -X /lib/crt0.o base.o base2.o
-Z ov1a.o ov1b.o ...
-Z ov2a.o ov2b.o ...
-Y base3.o ... -lovc
Assembly source code must be compiled using the assembler overlay flags: "as -V prog.s" which causes the assembler to leave certain symbols
unresolved so that ld may rearrange them. The various system compilers automatically use this option.
When arranging modules into overlays, the following rules control the maximum sizes for an executable file. The magic numbers are due to
the granularity of PDP-11 segmentation registers (there are 8 registers, each controlling an 8192-byte segment). The program is made up of
four areas: base text, overlay text, data + bss, and stack sections. The size of the overlay section is controlled by the size of the
largest of the overlays. Each section starts at an address that is a multiple of 8Kb, thus the size of each section is rounded up to a
multiple of 8Kb.
In the case of separate overlaid executable files, the text and overlays share one 64Kb byte address space; and the data + bss and stack
share the other. Thus, the total of the base text size (rounded up to an 8Kb boundary) plus the maximum overlay size (similarly rounded)
must be less than or equal to 64Kb. Or, put another way, since there are only 8 segmentation registers available, the number of segmenta-
tion registers needed for an overlaid object must be less than or equal to 8. As an example, if the base text segment has 36800 bytes and
the largest overlay takes 14144, the base will fit in 5 segments and the overlays in 2 segments; leaving one to spare. The data and bss
together must fit in 7 segments (56K bytes), leaving one 8Kb segment for the stack. All of the limits can be checked by using checkobj(1).
For pure overlaid programs, the rules are similar except that all four sections share one 64K-byte address space. The number of segments
required by the text, overlay, data + bss and stack are calculated in the same way. The sum of the segments required, including one for
the stack, must be less than or equal to 8. Example: a program has 8128 bytes of base text, the largest overlay is 16248 bytes, and the
data and bss total 19500. The text requires 1 8Kb segment, the overlays 2, and data and bss use 4, leaving one for the stack.
/usr/lib/lib*.a more libraries
/usr/local/lib/lib*.a still more libraries
a.out output file
adb(1), ar(1), as(1), cc(1), checkobj(1), f77(1), file(1), ranlib(1), size(1), a.out(5)
The text overlay scheme presented is unique to the PDP-11 and 2BSD. The -i, -P, -Z, -Y options are specific to 2BSD. The -q and -v
options are new with 2.11BSD.
3rd Berkeley Distribution May 08, 1995 LD(1)