Variable declaration after goto Label

Question

Today I found one interesting thing. I didn't know that one can't declare a variable after a goto label.

Compiling the following code

#include <stdio.h>
int main() {
    int x = 5;
    goto JUMP;
    printf("x is : %d\n",x);
JUMP:
    int a = 0;  <=== giving me all sorts of error..
    printf("%d",a);
}

gives errors like

temp.c: In function ‘main’:
temp.c:7: error: expected expression before ‘int’
temp.c:8: error: ‘a’ undeclared (first use in this function)
temp.c:8: error: (Each undeclared identifier is reported only once
temp.c:8: error: for each function it appears in.)

Now what is the logic behind that? I heard that one cannot create variables inside the case statements of switch. Since JUMP is inside the same scope (the scope of main function, in my case) of the goto statement, I believe that scope is not an issue here. But then, why am I getting this error?

score 101 · Accepted Answer · answered Dec 05 '11 at 11:19

The syntax simply doesn't allow it. §6.8.1 Labeled Statements:

labeled-statement:
    identifier : statement
    case constant-expression : statement
    default : statement

Note that there is no clause that allows for a "labeled declaration". It's just not part of the language.

You can trivially work around this, of course, with an empty statement.

JUMP:;
int a = 0;

score 18 · Answer 2 · answered Dec 05 '11 at 11:21

You want a semi-colon after the label like this:

 #include <stdio.h>
 int main() {
     int x = 5;
     goto JUMP;
     printf("x is : %d\n",x);
 JUMP: ;     /// semicolon for empty statement
     int a = 0; 
     printf("%d",a);
 }

Then your code compiles correctly for the C99 standard, with gcc -Wall -std=c99 -c krishna.c (I'm using GCC 4.6 on Debian/Sid/AMD64).

score 7 · Answer 3 · answered Dec 05 '11 at 19:11

7

Simple explanation, other than the spec says not, is that the compiler is exepecting the code after the goto to be something that compiles into an operation which it can then calculate the offset of, and is kicking because your variable declaration isn't a statement/block that it can compile into such an offset.

answered Dec 05 '11 at 19:11

jmoreno

12,752
4
60
91

The standard (and gcc) allows a _compound statement_ after a label, i.e., `JUMP: { int a = 0; printf("%d", a); }`; do you consider that such a statement is compiled into an operation? – tbrk Sep 10 '20 at 13:09

score 6 · Answer 4 · answered Dec 05 '11 at 11:14

6

My gcc version (4.4) is giving this compile error:

t.c:7: error: a label can only be part of a statement and a declaration is not a statement

. This error-message says it all.

answered Dec 05 '11 at 11:14

Patrick B.

11,773
8
58
101

score 5 · Answer 5 · edited Dec 05 '11 at 11:35

If you know why you can't create variables inside case statement of switch, basically its the same reason why you cant do this too. As a fix, you can try this,

#include <stdio.h>
int main() {
    int x = 5;
    goto JUMP;
    printf("x is : %d\n",x);
JUMP:
    {                                              //Note this
       int a = 0;  // <=== no more error..
       printf("%d",a);
    }                                             //Note this
}

score 2 · Answer 6 · answered Dec 05 '11 at 11:13

2

Well, first you should be consistent. It's either LABEL or label. Second, label is a part of the statement and the declaration doesn't answer the description enough.

You can replace LABEL: with label: ; and then it is likelier to compile.

EDIT: Now that you edited your code all over, it should be JUMP: replaced with JUMP: ; ;-)

answered Dec 05 '11 at 11:13

Michael Krelin - hacker

138,757
24
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sorry about that..Changed to JUMP so that there is no confusion..And please consider this as an easy test program which reproduce what I am asking here..Those printf statements mean nothing. – Krishnabhadra Dec 05 '11 at 11:17
That's okay. `printf`s mean nothing, I pointed you to the problem (and solution) regardless of `printf`s. – Michael Krelin - hacker Dec 05 '11 at 11:18
You all are saying, 1) A label needs to be a part of a statement. And **in C a declaration is not a statement.** 2) The error is nothing related with scope (which is what I first thought), correct? – Krishnabhadra Dec 05 '11 at 11:36
No, it's not about the scope. – Michael Krelin - hacker Dec 05 '11 at 11:38
Then the same situation in switch case is also is not about the scope. right? – Krishnabhadra Dec 05 '11 at 11:39
1

Yes, most likely. There may be different problems with bypassing variable initialization due to the jump, but you should get a decent diagnostic for that. And if you do, just separate declaration and initialization. – Michael Krelin - hacker Dec 05 '11 at 11:41

score 0 · Answer 7 · answered Dec 05 '11 at 12:23

#include <stdio.h>
int main() {
    int x = 5;
    goto JUMP;
    printf("x is : %d\n",x);
JUMP:
    printf("Do anything after label but dont declare 
    anything. even empty statement will also work 
    because label can only be part of a statement");
    int a = 0;  
    printf("%d",a);
}

Variable declaration after goto Label

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