Chapter 2: Invocation 3
2 Invocation
The gnu linker ld is meant to cover a broad range of situations, and to be as compatible
as possible with other linkers. As a result, you have many choices to control its behavior.
2.1 Command Line Options
The linker supports a plethora of command-line options, but in actual practice few of them
are used in any particular context. For instance, a frequent use of ld is to link standard
Unix object files on a standard, supported Unix system. On such a system, to link a file
hello.o:
ld -o output /lib/crt0.o hello.o -lc
This tells ld to produce a file called output as the result of linking the file /lib/crt0.o
with hello.o and the library libc.a, which will come from the standard search directories.
(See the discussion of the ‘-l’ option below.)
Some of the command-line options to ld may be specified at any point in the command
line. However, options which refer to files, such as ‘-l’ or ‘-T’, cause the file to be read at
the point at which the option appears in the command line, relative to the object files and
other file options. Repeating non-file options with a different argument will either have no
further effect, or override prior occurrences (those further to the left on the command line)
of that option. Options which may be meaningfully specified more than once are noted in
the descriptions below.
Non-option arguments are object files or archives which are to be linked together. They
may follow, precede, or be mixed in with command-line options, except that an object file
argument may not be placed between an option and its argument.
Usually the linker is invoked with at least one object file, but you can specify other forms
of binary input files using ‘-l’, ‘-R’, and the script command language. If no binary input
files at all are specified, the linker does not produce any output, and iss ues the message ‘No
input files’.
If the linker can not recognize the format of an object file, it will assume that it is a linker
script. A script specified in this way augments the main linker script used for the link
(either the default linker script or the one specified by using ‘-T’). This feature permits
the linker to link against a file which appears to be an object or an archive, but actually
merely defines some s ymbol values, or uses INPUT or GROUP to load other objects. Note that
specifying a script in this way should only be used to augment the main linker script; if
you want to use some command that logically can only appear once, such as the SECTIONS
or MEMORY command, you must replace the default linker script using the ‘-T’ option. See
Chapter 3 [Scripts], page 25.
For options whose names are a single letter, option arguments must either follow the op-
tion letter without intervening whitespace, or be given as separate arguments immediately
following the option that requires them.
For options whose names are multiple letters, either one dash or two can precede the option
name; for example, ‘-trace-symbol’ and ‘--trace-symbol’ are equivalent. Note - there is
one exception to this rule. Multiple letter options that start with a lower case ’o’ can only be
preceeded by two dashes. This is to reduce confusion with the ‘-o’ option. So for example
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