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C - Identifier Scope
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  Function Prototype Scope 
#include <stdio.h>

/* The parameter name, apple, has function prototype scope.  These names
   are not significant outside the prototype itself.  This is demonstrated
   below. */

int test_function(int apple);

int main(void)
{
    int orange = 5;

    orange = test_function(orange);
    printf("%d\r\n", orange); //orange = 6

    return 0;
}

int test_function(int fruit)
{
    fruit += 1;
    return fruit;
}

Note that you get puzzling error messages if you introduce a type name in a prototype:

int function(struct whatever *arg);

struct whatever
{
    int a;
    // ...
};

int function(struct whatever *arg)
{
    return arg->a;
}

With GCC 6.3.0, this code (source file dc11.c) produces:

$ gcc -O3 -g -std=c11 -Wall -Wextra -Werror -c dc11.c
dc11.c:1:25: error: ‘struct whatever’ declared inside parameter list will not be visible outside of this definition or declaration [-Werror]
     int function(struct whatever *arg);
                         ^~~~~~~~
dc11.c:9:9: error: conflicting types for ‘function’
     int function(struct whatever *arg)
         ^~~~~~~~
dc11.c:1:9: note: previous declaration of ‘function’ was here
     int function(struct whatever *arg);
         ^~~~~~~~
cc1: all warnings being treated as errors
$

Place the structure definition before the function declaration, or add struct whatever; as a line before the function declaration, and there is no problem. You should not introduce new type names in a function prototype because there's no way to use that type, and hence no way to define or use that function.
  Block Scope

An identifier has block scope if its corresponding declaration appears inside a block (parameter declaration in function definition apply). The scope ends at the end of the corresponding block.

No different entities with the same identifier can have the same scope, but scopes may overlap. In case of overlapping scopes the only visible one is the one declared in the innermost scope.

#include <stdio.h>

void test(int bar)                   // bar has scope test function block
{
    int foo = 5;                     // foo has scope test function block
    {
        int bar = 10;                // bar has scope inner block, this overlaps with previous test:bar declaration, and it hides test:bar
        printf("%d %d\n", foo, bar); // 5 10
    }                                // end of scope for inner bar
    printf("%d %d\n", foo, bar);     // 5 5, here bar is test:bar
}                                    // end of scope for test:foo and test:bar

int main(void)
{
    int foo = 3;         // foo has scope main function block

    printf("%d\n", foo); // 3
    test(5);
    printf("%d\n", foo); // 3
    return 0;
}                        // end of scope for main:foo
  File Scope 
#include <stdio.h>

/* The identifier, foo, is declared outside all blocks.
   It can be used anywhere after the declaration until the end of 
   the translation unit. */
static int foo;

void test_function(void)
{
    foo += 2;
}

int main(void)
{
    foo = 1;

    test_function();
    printf("%d\r\n", foo); //foo = 3;

    return 0;
}
  Function scope

Function scope is the special scope for labels. This is due to their unusual property. A label is visible through the entire function it is defined and one can jump (using instruction gotolabel) to it from any point in the same function. While not useful, the following example illustrate the point:

#include <stdio.h>

int main(int argc,char *argv[]) {
    int a = 0;
    goto INSIDE;
  OUTSIDE:
    if (a!=0) {
        int i=0;
      INSIDE:
        printf("a=%d\n",a);
        goto OUTSIDE;
    }
}

INSIDE may seem defined inside the if block, as it is the case for i which scope is the block, but it is not. It is visible in the whole function as the instruction goto INSIDE; illustrates. Thus there can't be two labels with the same identifier in a single function.

A possible usage is the following pattern to realize correct complex cleanups of allocated ressources:

#include <stdlib.h>
#include <stdio.h>

void a_function(void) {
   double* a = malloc(sizeof(double[34]));
   if (!a) {
      fprintf(stderr,"can't allocate\n");
      return;                 /* No point in freeing a if it is null */
   }
   FILE* b = fopen("some_file","r");
   if (!b) {
      fprintf(stderr,"can't open\n");
      goto CLEANUP1;          /* Free a; no point in closing b */
   }
   /* do something reasonable */
   if (error) {
      fprintf(stderr,"something's wrong\n");
      goto CLEANUP2;       /* Free a and close b to prevent leaks */
   }
   /* do yet something else */
CLEANUP2:
   close(b);
CLEANUP1:
   free(a);
}

Labels such as CLEANUP1 and CLEANUP2 are special identifiers that behave differently from all other identifiers. They are visible from everywhere inside the function, even in places that are executed before the labeled statement, or even in places that could never be reached if none of the goto is executed. Labels are often written in lower-case rather than upper-case.

 

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