In-depth analysis: named Constants

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A named constant in C is a variable modified with Const.

This article will go deep into the nature, storage location, whether it can be changed, and how to change the name constant.

First, declare the environment I tested: Linux Enterprise 5.0.

 

Test non-static global variables

The test procedure is as follows:

# Include <stdio. h> <br/> # include <stdlib. h> <br/> # include <string. h> <br/> const int g_int_a = 1000; <br/> int main () <br/> {<br/> printf ("g_int_a is % d. /n ", g_int_a); </P> <p> printf (" g_int_a is % d. /n ", g_int_a); </P> <p> return 0; </P> <p >}< br/>

Compile the program, and then run the "size executable program name" command to check its segment size.

[Root @ SS test] # Size Test

Text Data BSS dec hex filename

1229 280 8 1517 5ed Test

Then, change the global variable g_int_a to a non-Const. Compile the variable and run the size command to view it:

[Root @ SS test] # Size Test

Text Data BSS dec hex filename

1225 284 8 1517 5ed Test

It can be found that the size of the test segment (Text Segment) is reduced by four bytes, and the size of the Data Segment (Data Segment) is increased by four bytes.

Conclusion 1: The global variables modified by const are stored in text segments.

Then, test whether the specified constant can be changed. Still the above Code, add the statement in main:

G_int_a = 10;

When compiling, an error message is displayed: "Main. C: 11: Error: assign a value to the read-only variable 'G _ int_a"

Conclusion 2:

The value of a global variable modified by const cannot be changed by the variable name.

Test whether the address can be used to change its value. Add a statement in main

Int * pint = (int *) & g_int_a;

* Pint = 10;

No errors are found during compilation. Then run, the system prompts a segment error, and the program exits: "segment error (core dumped)"

Conclusion 3:

Because the global variable modified by const is saved in a text segment, its value cannot be modified by pointer.

 

Test static global variables

It is consistent with the non-static global variables and conforms to the above three conclusions.

 

Test static local variables

It is consistent with the non-static global variables and conforms to the above three conclusions.

 

Test non-static local variables

Conclusion 1: The local variables modified by const are stored in the stack segment.

Conclusion 2:

The value of a local variable modified by const cannot be changed by the variable name.

Conclusion 3:

The value of a local variable modified by const cannot be modified by pointer.

 

To sum up:

	Non-static global variables	Static global variables	Non-static local variables	Static local variable
Storage location	Text Section	Text Section	Stack segment	Text Section
Change its value using the variable name	No	No	No	No
Use a pointer to change its value	No	No	Yes	No