GNU Compiler Collection (GCC) Internals: Initialization

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#### 17.21.5 How Initialization Functions Are Handled

The compiled code for certain languages includes constructors (also called initialization routines)—functions to initialize data in the program when the program is started. These functions need to be called before the program is “started”—that is to say, before `main` is called.

Compiling some languages generates destructors (also called termination routines) that should be called when the program terminates.

To make the initialization and termination functions work, the compiler must output something in the assembler code to cause those functions to be called at the appropriate time. When you port the compiler to a new system, you need to specify how to do this.

There are two major ways that GCC currently supports the execution of initialization and termination functions. Each way has two variants. Much of the structure is common to all four variations.

The linker must build two lists of these functions—a list of initialization functions, called `__CTOR_LIST__`, and a list of termination functions, called `__DTOR_LIST__`.

Each list always begins with an ignored function pointer (which may hold 0, -1, or a count of the function pointers after it, depending on the environment). This is followed by a series of zero or more function pointers to constructors (or destructors), followed by a function pointer containing zero.

Depending on the operating system and its executable file format, either `crtstuff.c` or `libgcc2.c` traverses these lists at startup time and exit time. Constructors are called in reverse order of the list; destructors in forward order.

The best way to handle static constructors works only for object file formats which provide arbitrarily-named sections. A section is set aside for a list of constructors, and another for a list of destructors. Traditionally these are called ‘`.ctors`’ and ‘`.dtors`’. Each object file that defines an initialization function also puts a word in the constructor section to point to that function. The linker accumulates all these words into one contiguous ‘`.ctors`’ section. Termination functions are handled similarly.

This method will be chosen as the default by `target-def.h` if `TARGET_ASM_NAMED_SECTION` is defined. A target that does not support arbitrary sections, but does support special designated constructor and destructor sections may define `CTORS_SECTION_ASM_OP` and `DTORS_SECTION_ASM_OP` to achieve the same effect.

When arbitrary sections are available, there are two variants, depending upon how the code in `crtstuff.c` is called. On systems that support a .init section which is executed at program startup, parts of `crtstuff.c` are compiled into that section. The program is linked by the `gcc` driver like this:

    ld -o output_file crti.o crtbegin.o … -lgcc crtend.o crtn.o

The prologue of a function (`__init`) appears in the `.init` section of `crti.o`; the epilogue appears in `crtn.o`. Likewise for the function `__fini` in the .fini section. Normally these files are provided by the operating system or by the GNU C library, but are provided by GCC for a few targets.

The objects `crtbegin.o` and `crtend.o` are (for most targets) compiled from `crtstuff.c`. They contain, among other things, code fragments within the `.init` and `.fini` sections that branch to routines in the `.text` section. The linker will pull all parts of a section together, which results in a complete `__init` function that invokes the routines we need at startup.

To use this variant, you must define the `INIT_SECTION_ASM_OP` macro properly.

If no init section is available, when GCC compiles any function called `main` (or more accurately, any function designated as a program entry point by the language front end calling `expand_main_function`), it inserts a procedure call to `__main` as the first executable code after the function prologue. The `__main` function is defined in `libgcc2.c` and runs the global constructors.

In file formats that don’t support arbitrary sections, there are again two variants. In the simplest variant, the GNU linker (GNU `ld`) and an ‘a.out’ format must be used. In this case, `TARGET_ASM_CONSTRUCTOR` is defined to produce a `.stabs` entry of type ‘`N_SETT`’, referencing the name `__CTOR_LIST__`, and with the address of the void function containing the initialization code as its value. The GNU linker recognizes this as a request to add the value to a set; the values are accumulated, and are eventually placed in the executable as a vector in the format described above, with a leading (ignored) count and a trailing zero element. `TARGET_ASM_DESTRUCTOR` is handled similarly. Since no init section is available, the absence of `INIT_SECTION_ASM_OP` causes the compilation of `main` to call `__main` as above, starting the initialization process.

The last variant uses neither arbitrary sections nor the GNU linker. This is preferable when you want to do dynamic linking and when using file formats which the GNU linker does not support, such as ‘ECOFF’. In this case, `TARGET_HAVE_CTORS_DTORS` is false, initialization and termination functions are recognized simply by their names. This requires an extra program in the linkage step, called `collect2`. This program pretends to be the linker, for use with GCC; it does its job by running the ordinary linker, but also arranges to include the vectors of initialization and termination functions. These functions are called via `__main` as described above. In order to use this method, `use_collect2` must be defined in the target in `config.gcc`.

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