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:
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:
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
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.
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.