Global variables are helpful when many functions in the program use the same data. Storage for global variables is in a fixed region of memory set aside for this purpose by the compiler.

In the following program, the variable count has been declared outside of all functions.

Example:

#include "stdio.h"
int count;  /* count is global  */
void func1(void);
int main(void)
{
  count = 100;
  func1();
  return 0;
}
void func1(void)
{
  printf("count is %
d", count); /* will print 100 */
}

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Local variables exist only while the block of code in which they are declared is executing. That is, a local variable is created upon entry into its block and destroyed upon exit. Furthermore, a variable declared within one code block has no bearing on or relationship to another variable with the same name declared within a different code block.

Example:

void func1(void) 
{ 
  int y; 
  y = 10;
}
void func2(void)
{ 
  int y;
  y = -199;
}

The integer variable y is declared twice, once in func1( ) and once in func2( ). The y in func1( ) has no relationship to the y in func2( ). This is because each y is known only to the code within the block in which it is declared.

The C language contains the keyword auto, which can be used to declare local variables. However, since all variables are, by default, assumed to be auto, this keyword is virtually never used.

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