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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.