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<title>GNU Compiler Collection (GCC) Internals: Initialization</title>
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<a name="Initialization"></a>
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<div class="header">
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<p>
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Next: <a href="Macros-for-Initialization.html#Macros-for-Initialization" accesskey="n" rel="next">Macros for Initialization</a>, Previous: <a href="Label-Output.html#Label-Output" accesskey="p" rel="prev">Label Output</a>, Up: <a href="Assembler-Format.html#Assembler-Format" accesskey="u" rel="up">Assembler Format</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
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<a name="How-Initialization-Functions-Are-Handled"></a>
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<h4 class="subsection">17.20.5 How Initialization Functions Are Handled</h4>
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<a name="index-initialization-routines"></a>
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<a name="index-termination-routines"></a>
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<a name="index-constructors_002c-output-of"></a>
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<a name="index-destructors_002c-output-of"></a>
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<p>The compiled code for certain languages includes <em>constructors</em>
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(also called <em>initialization routines</em>)—functions to initialize
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data in the program when the program is started. These functions need
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to be called before the program is “started”—that is to say, before
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<code>main</code> is called.
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</p>
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<p>Compiling some languages generates <em>destructors</em> (also called
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<em>termination routines</em>) that should be called when the program
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terminates.
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</p>
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<p>To make the initialization and termination functions work, the compiler
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must output something in the assembler code to cause those functions to
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be called at the appropriate time. When you port the compiler to a new
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system, you need to specify how to do this.
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</p>
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<p>There are two major ways that GCC currently supports the execution of
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initialization and termination functions. Each way has two variants.
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Much of the structure is common to all four variations.
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</p>
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<a name="index-_005f_005fCTOR_005fLIST_005f_005f"></a>
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<a name="index-_005f_005fDTOR_005fLIST_005f_005f"></a>
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<p>The linker must build two lists of these functions—a list of
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initialization functions, called <code>__CTOR_LIST__</code>, and a list of
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termination functions, called <code>__DTOR_LIST__</code>.
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</p>
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<p>Each list always begins with an ignored function pointer (which may hold
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0, -1, or a count of the function pointers after it, depending on
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the environment). This is followed by a series of zero or more function
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pointers to constructors (or destructors), followed by a function
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pointer containing zero.
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</p>
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<p>Depending on the operating system and its executable file format, either
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<samp>crtstuff.c</samp> or <samp>libgcc2.c</samp> traverses these lists at startup
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time and exit time. Constructors are called in reverse order of the
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list; destructors in forward order.
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</p>
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<p>The best way to handle static constructors works only for object file
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formats which provide arbitrarily-named sections. A section is set
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aside for a list of constructors, and another for a list of destructors.
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Traditionally these are called ‘<samp>.ctors</samp>’ and ‘<samp>.dtors</samp>’. Each
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object file that defines an initialization function also puts a word in
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the constructor section to point to that function. The linker
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accumulates all these words into one contiguous ‘<samp>.ctors</samp>’ section.
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Termination functions are handled similarly.
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</p>
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<p>This method will be chosen as the default by <samp>target-def.h</samp> if
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<code>TARGET_ASM_NAMED_SECTION</code> is defined. A target that does not
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support arbitrary sections, but does support special designated
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constructor and destructor sections may define <code>CTORS_SECTION_ASM_OP</code>
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and <code>DTORS_SECTION_ASM_OP</code> to achieve the same effect.
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</p>
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<p>When arbitrary sections are available, there are two variants, depending
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upon how the code in <samp>crtstuff.c</samp> is called. On systems that
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support a <em>.init</em> section which is executed at program startup,
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parts of <samp>crtstuff.c</samp> are compiled into that section. The
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program is linked by the <code>gcc</code> driver like this:
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</p>
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<div class="smallexample">
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<pre class="smallexample">ld -o <var>output_file</var> crti.o crtbegin.o … -lgcc crtend.o crtn.o
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</pre></div>
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<p>The prologue of a function (<code>__init</code>) appears in the <code>.init</code>
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section of <samp>crti.o</samp>; the epilogue appears in <samp>crtn.o</samp>. Likewise
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for the function <code>__fini</code> in the <em>.fini</em> section. Normally these
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files are provided by the operating system or by the GNU C library, but
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are provided by GCC for a few targets.
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</p>
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<p>The objects <samp>crtbegin.o</samp> and <samp>crtend.o</samp> are (for most targets)
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compiled from <samp>crtstuff.c</samp>. They contain, among other things, code
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fragments within the <code>.init</code> and <code>.fini</code> sections that branch
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to routines in the <code>.text</code> section. The linker will pull all parts
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of a section together, which results in a complete <code>__init</code> function
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that invokes the routines we need at startup.
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</p>
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<p>To use this variant, you must define the <code>INIT_SECTION_ASM_OP</code>
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macro properly.
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</p>
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<p>If no init section is available, when GCC compiles any function called
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<code>main</code> (or more accurately, any function designated as a program
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entry point by the language front end calling <code>expand_main_function</code>),
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it inserts a procedure call to <code>__main</code> as the first executable code
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after the function prologue. The <code>__main</code> function is defined
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in <samp>libgcc2.c</samp> and runs the global constructors.
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</p>
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<p>In file formats that don’t support arbitrary sections, there are again
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two variants. In the simplest variant, the GNU linker (GNU <code>ld</code>)
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and an ‘a.out’ format must be used. In this case,
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<code>TARGET_ASM_CONSTRUCTOR</code> is defined to produce a <code>.stabs</code>
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entry of type ‘<samp>N_SETT</samp>’, referencing the name <code>__CTOR_LIST__</code>,
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and with the address of the void function containing the initialization
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code as its value. The GNU linker recognizes this as a request to add
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the value to a <em>set</em>; the values are accumulated, and are eventually
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placed in the executable as a vector in the format described above, with
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a leading (ignored) count and a trailing zero element.
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<code>TARGET_ASM_DESTRUCTOR</code> is handled similarly. Since no init
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section is available, the absence of <code>INIT_SECTION_ASM_OP</code> causes
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the compilation of <code>main</code> to call <code>__main</code> as above, starting
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the initialization process.
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</p>
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<p>The last variant uses neither arbitrary sections nor the GNU linker.
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This is preferable when you want to do dynamic linking and when using
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file formats which the GNU linker does not support, such as ‘ECOFF’. In
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this case, <code>TARGET_HAVE_CTORS_DTORS</code> is false, initialization and
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termination functions are recognized simply by their names. This requires
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an extra program in the linkage step, called <code>collect2</code>. This program
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pretends to be the linker, for use with GCC; it does its job by running
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the ordinary linker, but also arranges to include the vectors of
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initialization and termination functions. These functions are called
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via <code>__main</code> as described above. In order to use this method,
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<code>use_collect2</code> must be defined in the target in <samp>config.gcc</samp>.
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</p>
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<div class="header">
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<p>
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Next: <a href="Macros-for-Initialization.html#Macros-for-Initialization" accesskey="n" rel="next">Macros for Initialization</a>, Previous: <a href="Label-Output.html#Label-Output" accesskey="p" rel="prev">Label Output</a>, Up: <a href="Assembler-Format.html#Assembler-Format" accesskey="u" rel="up">Assembler Format</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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