Initialization and Termination Sections

Dynamic objects can supply code that provides for runtime initialization and termination processing. The initialization code of a dynamic object is executed once each time the dynamic object is loaded in a process. The termination code of a dynamic object is executed once each time the dynamic object is unloaded from a process or at process termination. This code can be encapsulated in one of two section types, either an array of function pointers or a single code block. Each of these section types is built from a concatenation of like sections from the input relocatable objects.

The sections .pre_initarray, .init_array and .fini_array provide arrays of runtime pre-initialization, initialization, and termination functions, respectively. When creating a dynamic object, the link-editor identifies these arrays with the .dynamic tag pairs DT_PREINIT_[ARRAY/ARRAYSZ], DT_INIT_[ARRAY/ARRAYSZ], and DT_FINI_[ARRAY/ARRAYSZ] accordingly. These tags identify the associated sections so that the sections can be called by the runtime linker. A pre-initialization array is applicable to dynamic executables only.

Functions that are assigned to these arrays must be provided from the object that is being built.

The sections .init and .fini provide a runtime initialization and termination code block, respectively. The compiler drivers typically supply .init and .fini sections with files they add to the beginning and end of your input file list. These compiler provided files have the effect of encapsulating the .init and .fini code from your relocatable objects into individual functions. These functions are identified by the reserved symbol names _init and _fini respectively. When creating a dynamic object, the link-editor identifies these symbols with the .dynamic tags DT_INIT and DT_FINI accordingly. These tags identify the associated sections so they can be called by the runtime linker.

For more information about the execution of initialization and termination code at runtime see Initialization and Termination Routines.

The registration of initialization and termination functions can be carried out directly by the link-editor by using the -z initarray and -z finiarray options. For example, the following command places the address of foo() in an .init_array element, and the address of bar() in a .fini_array element.

$ cat main.c
#include    <stdio.h>

void foo()
{
        (void) printf("initializing: foo()\n");
}

void bar()
{
        (void) printf("finalizing: bar()\n");
}

void main()
{
        (void) printf("main()\n");
}

$ cc -o main -zinitarray=foo -zfiniarray=bar main.c
$ main
initializing: foo()
main()
finalizing: bar()

The creation of initialization and termination sections can be carried out directly using an assembler. However, most compilers offer special primitives to simplify their declaration. For example, the previous code example can be rewritten using the following #pragma definitions. These definitions result in a call to foo() being placed in an .init section, and a call to bar() being placed in a .fini section.

$ cat main.c
#include    <stdio.h>

#pragma init (foo)
#pragma fini (bar)

....
$ cc -o main main.c
$ main
initializing: foo()
main()
finalizing: bar()

Initialization and termination code, spread throughout several relocatable objects, can result in different behavior when included in an archive library or shared object. The link-edit of an application that uses this archive might extract only a fraction of the objects contained in the archive. These objects might provide only a portion of the initialization and termination code spread throughout the members of the archive. At runtime, only this portion of code is executed. The same application built against the shared object will have all the accumulated initialization and termination code executed when the dependency is loaded at runtime.

To determine the order of executing initialization and termination code within a process at runtime is a complex issue that involves dependency analysis. Limit the content of initialization and termination code to simplify this analysis. Simplified, self contained, initialization and termination code provides predictable runtime behavior. See Initialization and Termination Order for more details.

Data initialization should be independent if the initialization code is involved with a dynamic object whose memory can be dumped using dldump(3C).