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#### 17.21.6 Macros Controlling Initialization Routines

Here are the macros that control how the compiler handles initialization and termination functions:

Macro: INIT_SECTION_ASM_OP

  If defined, a C string constant, including spacing, for the assembler operation to identify the following data as initialization code. If not defined, GCC will assume such a section does not exist. When you are using special sections for initialization and termination functions, this macro also controls how `crtstuff.c` and `libgcc2.c` arrange to run the initialization functions.

Macro: HAS_INIT_SECTION

  If defined, `main` will not call `__main` as described above. This macro should be defined for systems that control start-up code on a symbol-by-symbol basis, such as OSF/1, and should not be defined explicitly for systems that support `INIT_SECTION_ASM_OP`.

Macro: LD_INIT_SWITCH

  If defined, a C string constant for a switch that tells the linker that the following symbol is an initialization routine.

Macro: LD_FINI_SWITCH

  If defined, a C string constant for a switch that tells the linker that the following symbol is a finalization routine.

Macro: COLLECT_SHARED_INIT_FUNC (stream, func)

  If defined, a C statement that will write a function that can be automatically called when a shared library is loaded. The function should call func, which takes no arguments. If not defined, and the object format requires an explicit initialization function, then a function called `_GLOBAL__DI` will be generated.

  This function and the following one are used by collect2 when linking a shared library that needs constructors or destructors, or has DWARF2 exception tables embedded in the code.

Macro: COLLECT_SHARED_FINI_FUNC (stream, func)

  If defined, a C statement that will write a function that can be automatically called when a shared library is unloaded. The function should call func, which takes no arguments. If not defined, and the object format requires an explicit finalization function, then a function called `_GLOBAL__DD` will be generated.

Macro: INVOKE__main

  If defined, `main` will call `__main` despite the presence of `INIT_SECTION_ASM_OP`. This macro should be defined for systems where the init section is not actually run automatically, but is still useful for collecting the lists of constructors and destructors.

Macro: SUPPORTS_INIT_PRIORITY

  If nonzero, the C++ `init_priority` attribute is supported and the compiler should emit instructions to control the order of initialization of objects. If zero, the compiler will issue an error message upon encountering an `init_priority` attribute.

Target Hook: bool TARGET_HAVE_CTORS_DTORS

  This value is true if the target supports some “native” method of collecting constructors and destructors to be run at startup and exit. It is false if we must use `collect2`.

Target Hook: void TARGET_ASM_CONSTRUCTOR (rtx symbol, int priority)

  If defined, a function that outputs assembler code to arrange to call the function referenced by symbol at initialization time.

  Assume that symbol is a `SYMBOL_REF` for a function taking no arguments and with no return value. If the target supports initialization priorities, priority is a value between 0 and `MAX_INIT_PRIORITY`; otherwise it must be `DEFAULT_INIT_PRIORITY`.

  If this macro is not defined by the target, a suitable default will be chosen if (1) the target supports arbitrary section names, (2) the target defines `CTORS_SECTION_ASM_OP`, or (3) `USE_COLLECT2` is not defined.

Target Hook: void TARGET_ASM_DESTRUCTOR (rtx symbol, int priority)

  This is like `TARGET_ASM_CONSTRUCTOR` but used for termination functions rather than initialization functions.

If `TARGET_HAVE_CTORS_DTORS` is true, the initialization routine generated for the generated object file will have static linkage.

If your system uses `collect2` as the means of processing constructors, then that program normally uses `nm` to scan an object file for constructor functions to be called.

On certain kinds of systems, you can define this macro to make `collect2` work faster (and, in some cases, make it work at all):

Macro: OBJECT_FORMAT_COFF

  Define this macro if the system uses COFF (Common Object File Format) object files, so that `collect2` can assume this format and scan object files directly for dynamic constructor/destructor functions.

  This macro is effective only in a native compiler; `collect2` as part of a cross compiler always uses `nm` for the target machine.

Macro: REAL_NM_FILE_NAME

  Define this macro as a C string constant containing the file name to use to execute `nm`. The default is to search the path normally for `nm`.

Macro: NM_FLAGS

  `collect2` calls `nm` to scan object files for static constructors and destructors and LTO info. By default, `-n` is passed. Define `NM_FLAGS` to a C string constant if other options are needed to get the same output format as GNU `nm -n` produces.

If your system supports shared libraries and has a program to list the dynamic dependencies of a given library or executable, you can define these macros to enable support for running initialization and termination functions in shared libraries:

Macro: LDD_SUFFIX

  Define this macro to a C string constant containing the name of the program which lists dynamic dependencies, like `ldd` under SunOS 4.

Macro: PARSE_LDD_OUTPUT (ptr)

  Define this macro to be C code that extracts filenames from the output of the program denoted by `LDD_SUFFIX`. ptr is a variable of type `char *` that points to the beginning of a line of output from `LDD_SUFFIX`. If the line lists a dynamic dependency, the code must advance ptr to the beginning of the filename on that line. Otherwise, it must set ptr to `NULL`.

Macro: SHLIB_SUFFIX

  Define this macro to a C string constant containing the default shared library extension of the target (e.g., ‘`".so"`’). `collect2` strips version information after this suffix when generating global constructor and destructor names. This define is only needed on targets that use `collect2` to process constructors and destructors.

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