Storage-class specifiers (advanced)

A variable declaration can also be preceded by a storage-class specifier: auto, extern, register or static. Here we will only describe static, and refer the reader to a more complete reference manual for descriptions of the others.

Variables declared at the start of a compound-statement are only visible inside that compound-statement. Normally, each time the thread of execution enters a compound-statement, space is allocated for these variables and, if necessary, they are initialized. When it leaves, the space occupied by the variables is reclaimed and the value held by the variable is lost. If the declaration is prefixed with static, the variable will not be destroyed when compound-statement is left, and its value will be preserved. For example:

float f(float x) { static int init = 0; if (init == 0) { /* put setup code here */ init = 1; } /* by now things will be setup */ }

Here we have a function f which must do some complex setup when its called for the first time. By using a static variable, we can tell when this happens. init will be initialized to zero as the program is loaded, and the first time f is called the setup code will run, and set init to one. On subsequent calls to f, init will still be one and the setup code will not be repeated.