[Haifux] Intermingled code and declarations and gcc

[Yotam Medini יותם מדיני]

File: gcc-4.2.info,  Node: Variable Length,  Next: Empty Structures,  Prev:
Zero Length,  Up: C Extensions

5.14 Arrays of Variable Length
==============================

Variable-length automatic arrays are allowed in ISO C99, and as an
extension GCC accepts them in C89 mode and in C++.  (However, GCC's
implementation of variable-length arrays does not yet conform in detail
to the ISO C99 standard.)  These arrays are declared like any other
automatic arrays, but with a length that is not a constant expression.
The storage is allocated at the point of declaration and deallocated
when the brace-level is exited.  For example:

     FILE *
     concat_fopen (char *s1, char *s2, char *mode)
     {
       char str[strlen (s1) + strlen (s2) + 1];
       strcpy (str, s1);
       strcat (str, s2);
       return fopen (str, mode);
     }

 Jumping or breaking out of the scope of the array name deallocates the
storage.  Jumping into the scope is not allowed; you get an error
message for it.

 You can use the function `alloca' to get an effect much like
variable-length arrays.  The function `alloca' is available in many
other C implementations (but not in all).  On the other hand,
variable-length arrays are more elegant.

 There are other differences between these two methods.  Space allocated
with `alloca' exists until the containing _function_ returns.  The
space for a variable-length array is deallocated as soon as the array
name's scope ends.  (If you use both variable-length arrays and
`alloca' in the same function, deallocation of a variable-length array
will also deallocate anything more recently allocated with `alloca'.)

 You can also use variable-length arrays as arguments to functions:

     struct entry
     tester (int len, char data[len][len])
     {
       /* ... */
     }

 The length of an array is computed once when the storage is allocated
and is remembered for the scope of the array in case you access it with
`sizeof'.

 If you want to pass the array first and the length afterward, you can
use a forward declaration in the parameter list--another GNU extension.

     struct entry
     tester (int len; char data[len][len], int len)
     {
       /* ... */
     }

 The `int len' before the semicolon is a "parameter forward
declaration", and it serves the purpose of making the name `len' known
when the declaration of `data' is parsed.

 You can write any number of such parameter forward declarations in the
parameter list.  They can be separated by commas or semicolons, but the
last one must end with a semicolon, which is followed by the "real"
parameter declarations.  Each forward declaration must match a "real"
declaration in parameter name and data type.  ISO C99 does not support
parameter forward declarations.
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