Q44078: Allocation of Variables within Blocks in C

Article: Q44078
Product(s): See article
Version(s): 5.00 5.10 | 5.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | S_QuickC | mspl13_c
Last Modified: 23-MAY-1989

The "Microsoft C for the MS-DOS Operating System: Language Reference"
manual in Section 3.5.2 states the following:

   An item with a local lifetime (a "local item") has storage and a
   defined value only within the block where the item is defined or
   declared. A local item is allocated new storage each time the
   program enters that block, and it loses its storage (and hence its
   value) when the program exits the block.

This statement is true for function blocks. All nonstatic variables
within a function, whether they are within a block or not, are
allocated space from the stack upon entry to the function.

According to the ANSI standard, the C language should support jumps
into blocks. To ensure that variables always are allocated, regardless
of the entry point into the block, the space for local variables
within a block must be allocated upon entry of the function, not the
block. Consider the following code example:

int checkup (int var)
{
      .
      .
      .
    if (var)
       goto test2;
    else
    {
       char buffer[80];
         strcpy (buffer, "This is a different test");
       test1:
         strcpy (buffer, "This is a test");
           .
           .
           .
    }
    test2:
       printf("testing stuff is coming up next");
       goto test1;
    return (0);
}

Although this is not the best way to implement this code sample, this
method should be legal, according to the ANSI standard. If the block
is entered through the label test1, and the array of characters has
not been allocated on the stack previously, the strcpy after the test1
label does not work properly. To ensure that "buffer" is allocated
space, it is allocated stack space upon entry to the function, and not
upon entry to the block.

Note: The 80-character buffer is removed from the stack upon exit from
the block, and not upon exit from the function.

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