Everybody who has programmed in C programming language must be aware about “goto” and “labels” used in C to jump within a C function. GCC provides an extension to C called “local labels”.
Conventional Labels vs Local Labels
Conventional labels in C have function scope. Where as local label can be scoped to a inner nested block. Therefore, conventional Labels cannot be declared more than once in a function and that is where local labels are used.
A label can appear more than once in function when the label is inside a macro and macro is expanded multiple times in a function. For example if a macro funcMacro() has definition which involves jump instructions (goto statement) within the block and funcMacro is used by a function foo() several times.
#define funcMacro(params …) do { if (cond == true) goto x; <some code > x: <some code> } while(0); Void foo() { <some code> funcMacro(params …); <some code > funcMacro(params…); } |
In such a function foo() , the function macro will be expanded two times.
This will result in having more than one definition of label ‘x’ in a function, which leads to confusion to the compiler and results in compilation error. In such cases local labels are useful.
The above problem can be avoided by using local labels. A local label are declared as below:
__label__ label;
Local label declarations must come at the beginning of the block, before any ordinary declarations or statements.
Below is C example where a macro IS_STR_EMPTY() is expanded multiple times. Since local labels have block scope and every expansion of macro causes a new do while block, the program compiles and runs fine.
#include <stdio.h> #include <string.h> //Function macro using local labels #define IS_STR_EMPTY(str) do { __label__ empty, not_empty, exit; if (strlen(str)) goto not_empty; else goto empty; not_empty: printf("string = %s
", str); goto exit; empty: printf("string is empty
"); exit: ; } while(0); int main() { char string[20] = {' '}; //Pass empty string to Macro function IS_STR_EMPTY(string); //Pass non-empty string to Macro function strcpy(string, "geeksForgeeks"); IS_STR_EMPTY(string); return 0; } |
Output:
string is empty string = geeksForgeeks
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