difference between declaration and definition of a variable?

Hi everybody

I am declaring and initializing variable n in my code. I do not understand what meaning of define the variable in c

Code:

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#include<stdio.h>void main()
{
    int n  //declar variable n
    int n = 10; // //declar and initialize variable n with 10 value
}

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Any one can tell me what is difference between declaration and definition of a variable?

Definitions take up real physical space in the memory of your program.

A declaration is just a 'promise' you make to the compiler that something will exist at some point in the future.
A good example being function prototypes in header files - they're all declarations.

Quote:

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Originally Posted by Salem

Definitions take up real physical space in the memory of your program.
.

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I did not understand your answer It would be even better if you gave an example as I gave

A variable definition is usually also a declaration, unless an extern declaration has already been seen by the compiler.

Code:

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#include <stdio.h>

extern int z;  // Declaration only, not a definition
                // This variable may be defined in another file
                // Usually this would be in a header file
                // z cannot be used in the code until it has been defined
int main(void)
{
  int x;      // Variable declaration and definition
  int y = 10;  // Same as x but with initialization

  return 0;
}

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Have you tried your favourite search engine? "c definition versus declaration" produces over 130,000,000 results, surely at least one of those links would answer your question.

I once reviewed a book for potential use in a course on C. The book was not just badly written, some of the text and code were plain WRONG! Please recycle the book you currently have.

Please see my list of three books above. Any of them are a good replacement. Available as either a paper book, or an e-book.