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@c Copyright (C) 1988-2016 Free Software Foundation, Inc.
@c This is part of the GCC manual.
@c For copying conditions, see the file gcc.texi.
@node Collect2
@chapter @code{collect2}
GCC uses a utility called @code{collect2} on nearly all systems to arrange
to call various initialization functions at start time.
The program @code{collect2} works by linking the program once and
looking through the linker output file for symbols with particular names
indicating they are constructor functions. If it finds any, it
creates a new temporary @samp{.c} file containing a table of them,
compiles it, and links the program a second time including that file.
@findex __main
@cindex constructors, automatic calls
The actual calls to the constructors are carried out by a subroutine
called @code{__main}, which is called (automatically) at the beginning
of the body of @code{main} (provided @code{main} was compiled with GNU
CC)@. Calling @code{__main} is necessary, even when compiling C code, to
allow linking C and C++ object code together. (If you use
@option{-nostdlib}, you get an unresolved reference to @code{__main},
since it's defined in the standard GCC library. Include @option{-lgcc} at
the end of your compiler command line to resolve this reference.)
The program @code{collect2} is installed as @code{ld} in the directory
where the passes of the compiler are installed. When @code{collect2}
needs to find the @emph{real} @code{ld}, it tries the following file
names:
@itemize @bullet
@item
a hard coded linker file name, if GCC was configured with the
@option{--with-ld} option.
@item
@file{real-ld} in the directories listed in the compiler's search
directories.
@item
@file{real-ld} in the directories listed in the environment variable
@code{PATH}.
@item
The file specified in the @code{REAL_LD_FILE_NAME} configuration macro,
if specified.
@item
@file{ld} in the compiler's search directories, except that
@code{collect2} will not execute itself recursively.
@item
@file{ld} in @code{PATH}.
@end itemize
``The compiler's search directories'' means all the directories where
@command{gcc} searches for passes of the compiler. This includes
directories that you specify with @option{-B}.
Cross-compilers search a little differently:
@itemize @bullet
@item
@file{real-ld} in the compiler's search directories.
@item
@file{@var{target}-real-ld} in @code{PATH}.
@item
The file specified in the @code{REAL_LD_FILE_NAME} configuration macro,
if specified.
@item
@file{ld} in the compiler's search directories.
@item
@file{@var{target}-ld} in @code{PATH}.
@end itemize
@code{collect2} explicitly avoids running @code{ld} using the file name
under which @code{collect2} itself was invoked. In fact, it remembers
up a list of such names---in case one copy of @code{collect2} finds
another copy (or version) of @code{collect2} installed as @code{ld} in a
second place in the search path.
@code{collect2} searches for the utilities @code{nm} and @code{strip}
using the same algorithm as above for @code{ld}.
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