These options come into play when the compiler links object files into an executable output file. They are meaningless if the compiler is not doing a link step.
It makes a difference where in the command you write this option; the
linker searches and processes libraries and object files in the order they
are specified. Thus,
foo.o -lz bar.o searches library
foo.o but before
to functions in
z, those functions may not be loaded.
The linker searches a standard list of directories for the library,
which is actually a file named
.a. The linker
then uses this file as if it had been specified precisely by name.
The directories searched include several standard system directories
plus any that you specify with
Normally the files found this way are library files--archive files
whose members are object files. The linker handles an archive file by
scanning through it for members which define symbols that have so far
been referenced but not defined. But if the file that is found is an
ordinary object file, it is linked in the usual fashion. The only
difference between using an
-l option and specifying a file name
-l surrounds library with
and searches several directories.
-loption in order to link an Objective-C program.
-nostartfilesis used. The compiler may generate calls to memcmp, memset, and memcpy for System V (and ISO C) environments or to bcopy and bzero for BSD environments. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified.
One of the standard libraries bypassed by
libgcc.a, a library of internal subroutines
that GCC uses to overcome shortcomings of particular machines, or special
needs for some languages.
(See Interfacing to GCC Output,
for more discussion of
In most cases, you need
libgcc.a even when you want to avoid
other standard libraries. In other words, when you specify
-nodefaultlibs you should usually specify
-lgcc as well.
This ensures that you have no unresolved references to internal GCC
library subroutines. (For example,
__main, used to ensure C++
constructors will be called; see
-fPIC, or model suboptions) when you specify this option.1
libgccas a shared library, these options force the use of either the shared or static version respectively. If no shared version of
libgccwas built when the compiler was configured, these options have no effect.
There are several situations in which an application should use the
libgcc instead of the static version. The most common
of these is when the application wishes to throw and catch exceptions
across different shared libraries. In that case, each of the libraries
as well as the application itself should use the shared
Therefore, the G++ and GCJ drivers automatically add
-shared-libgcc whenever you build a shared library or a main
executable, because C++ and Java programs typically use exceptions, so
this is the right thing to do.
If, instead, you use the GCC driver to create shared libraries, you may
find that they will not always be linked with the shared
If GCC finds, at its configuration time, that you have a GNU linker that
does not support option
--eh-frame-hdr, it will link the shared
libgcc into shared libraries by default. Otherwise,
it will take advantage of the linker and optimize away the linking with
the shared version of
libgcc, linking with the static version of
libgcc by default. This allows exceptions to propagate through such
shared libraries, without incurring relocation costs at library load
However, if a library or main executable is supposed to throw or catch
exceptions, you must link it using the G++ or GCJ driver, as appropriate
for the languages used in the program, or using the option
-shared-libgcc, such that it is linked with the shared
-Xlinker -z -Xlinker defs). Only a few systems support this option.
If you want to pass an option that takes an argument, you must use
-Xlinker twice, once for the option and once for the argument.
For example, to pass
-assert definitions, you must write
-Xlinker -assert -Xlinker definitions. It does not work to write
-Xlinker "-assert definitions", because this passes the entire
string as a single argument, which is not what the linker expects.
-umultiple times with different symbols to force loading of additional library modules.
On some systems,
needs to build supplementary stub code for constructors to work. On
gcc -shared must select the correct support
libraries to link against. Failing to supply the correct flags may lead
to subtle defects. Supplying them in cases where they are not necessary