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Three lifetimes are available to the programmer. They do not have predefined keywords for names as scopes do. The first is the lifetime of extern and static variables, whose lifetime is from before main() is called until the program exits. The second is the lifetime of function arguments and automatics, which is from the time the function is called untilit returns. The third lifetime is that of dynamically allocated data. It starts when the program calls malloc() or calloc() to allocate space for the data and ends when the program calls free() or when it exits, whichever comes first.
A local block is any portion of a C program that is enclosed by the left brace ({) and the right brace (}). A C function contains left and right braces, and therefore anything between the two braces is contained in a local block. An if statement or a switch statement can also contain braces, so the portion of code between these two braces would be considered a local block. Additionally, you might want to create your own local block without the aid of a C function or keyword construct. This is perfectly legal. Variables can be declared within local blocks, but they must be declared only at the beginning of a local block. Variables declared in this manner are visible only within the local block. Duplicate variable names declared within a local block take precedence over variables with the same name declared outside the local block. Here is an example of a program that uses local blocks:
#include<stdio.h>void main(void);
void main(){
/* Begin local block for function main() */int test_var = 10;
printf(“Test variable before the if statement: %d\n”, test_var);if (test_var>5)
{/* Begin local block for “if” statement */
int test_var = 5;printf(“Test variable within the if statement: %d\n”, test_var);
{/* Begin independent local block (not tied to any function or keyword) */
int test_var = 0;printf(“Test variable within the independent local block:%d\n”, test_var);
}/* End independent local block */
}/* End local block for “if” statement */
printf(“Test variable after the if statement: %d\n”, test_var);}
/* End local block for function main() */
This example program produces the following output:
Test variable before the if statement: 10
Test variable within the if statement: 5Test variable within the independent local block: 0
Test variable after the if statement: 10
Notice that as each test_var was defined, it took precedence over the previously defined test_var. Also notice that when the if statement local block had ended, the program had reentered the scope of the original test_var, and its value was 10.
The function in the listing above is declared with the storage class specifier extern. This is not strictly necessary, since extern is the default storage class for functions. An ordinary function definition that does not contain a static or inline specifier can be placed in any source file of a program. Such a function is available in all of the program's source files, because its name is an external identifier. You merely have to declare the function before its first use in a given translation unit. Furthermore, you can arrange functions in any order you wish within a source file. The only restriction is that you cannot define one function within another. C does not allow you to define "local functions" in this way.
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