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5.1 Static memory allocations

The first type of memory allocation is known as a static memory allocation, which corresponds to file scope variables and local static variables. The addresses and sizes of these allocations are fixed at the time of compilation1 and so they can be placed in a fixed-sized data area which then corresponds to a section within the final linked executable file. Such memory allocations are called static because they do not vary in location or size during the lifetime of the program.

There can be many types of data sections within an executable file; the three most common are normal data, BSS data and read-only data. BSS data contains variables and arrays which are to be initialised to zero at run-time and so is treated as a special case, since the actual contents of the section need not be stored in the executable file. Read-only data consists of constant variables and arrays whose contents are guaranteed not to change when a program is being run. For example, on a typical SVR4 UNIX system the following variable definitions would result in them being placed in the following sections:

     int a;           /* BSS data */
     int b = 1;       /* normal data */
     const int c = 2; /* read-only data */

In C the first example would be considered a tentative declaration, and if there was no subsequent definition of that variable in the current translation unit then it would become a common variable in the resulting object file. When the object file gets linked with other object files, any common variables with the same name become one variable, or take their definition from a non-tentative definition of that variable. In the former case, the variable is placed in the BSS section. Note that C++ has no support for tentative declarations.

As all static memory allocations have sizes and address offsets that are known at compile-time and are explicitly initialised, there is very little that can go wrong with them. Data can be read or written past the end of such variables, but that is a common problem with all memory allocations and is generally easy to locate in that case. On systems that separate read-only data from normal data, writing to a read-only variable can be quickly diagnosed at run-time.


[1] Or more accurately, at link time.