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\n", str); \ goto exit; \ empty: \ printf("string is empty\n"); \ exit: ; \ } while(0); \ int main() { char string[20] = {'\0'}; //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
This article is contributed by Kashish Bhatia. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above