Error in switch case, a label can only be part of a statement and a declaration is not a statement

Question

I am creating a simple menu with switch case so depending on the value written it can do different things,

void
menu_principal()
{
    int opciones;

    printf ("Ingrese su eleccion \n");

    scanf("%d", &opciones);

    switch(opciones) 
    {
        case 1: 
            
            int coord_x;
            int coord_y;
            
            printf("Ingrese las cordenadas de x \n: ");
            scanf( "%d", &coord_x);
            
            printf("Ingrese las cordenadas de y \n");
            scanf( "%d", &coord_y );
            
            break;
        
        
        case 0:        
            
            break;
        
        …
          
    }
    return 0;

}

but it just doesn't work, I get errors with the scanf and the int coord_x, coord_y.

Answer 1

Before C23

Prior to C23, you couldn't put a label before a declaration like int coord_x;.

You could label a block — add { before int coord_x; and } before break; (after it would work too).

C11 §6.8.1 Labeled statements shows that a label must be followed by a statement. §6.8 Statements shows what constitutes a statement; §6.8.2 Compound statements shows that declarations are different from statements, and 6.7 Declarations covers what a declaration consists of.

A label must precede a statement, but that can be a null statement — just a semicolon ;. This means you can't put a label immediately before the closing brace of a compound statement; you must put a semicolon after it to be the labelled statement.

C23 and later

C23 lifts the constraint that you cannot label a variable declaration. The summary in (new) Annex M.2 states:

• allowed placement of labels in front of declarations and at the end of compound statement;

It achieves this by refining the syntax:

• (6.8.1) labeled-statement: label statement
• (6.8.2) compound-statement: { block-item-list_opt }
• (6.8.2) block-item-list: block-item | block-item-list block-item
• (6.8.2) block-item: declaration | unlabeled-statement | label

The semantics section says:

Any statement or declaration in a compound statement may be preceded by a prefix that declares an identifier as a label name. The optional attribute specifier sequence appertains to the label. Labels in themselves do not alter the flow of control, which continues unimpeded across them.

So, in C23, a label can appear before declarations or immediately before the closing brace of a compound statement.

The 'optional attribute specifier' is primarily [[maybe_unused]] which indicates that the label to which it is attached might not be used. The compiler is encouraged not to give warnings about the either the use or non-use of labels tagged with that attribute.