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20.5 Local Variables

Declaring a variable inside a function definition (see Function Definitions) makes the variable name local to the containing block—that is, the containing pair of braces. More precisely, the variable’s name is visible starting just after where it appears in the declaration, and its visibility continues until the end of the block.

Local variables in C are generally automatic variables: each variable’s storage exists only from the declaration to the end of the block. Execution of the declaration allocates the storage, computes the initial value, and stores it in the variable. The end of the block deallocates the storage.6

Warning: Two declarations for the same local variable in the same scope are an error.

Warning: Automatic variables are stored in the run-time stack. The total space for the program’s stack may be limited; therefore, in using very large arrays, it may be necessary to allocate them in some other way to stop the program from crashing.

Warning: If the declaration of an automatic variable does not specify an initial value, the variable starts out containing garbage. In this example, the value printed could be anything at all:

  int i;

  printf ("Print junk %d\n", i);

In a simple test program, that statement is likely to print 0, simply because every process starts with memory zeroed. But don’t rely on it to be zero—that is erroneous.

Note: Make sure to store a value into each local variable (by assignment, or by initialization) before referring to its value.



Due to compiler optimizations, allocation and deallocation don’t necessarily really happen at those times.

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