C arrays: some oddities

[utzoo!decvax!ucbvax!menlo70!sri-unix!dan at BBN-UNIX]

From: Dan Franklin <dan at BBN-UNIX>
The recent correspondence on the subject of arrays in C spurred me into
investigating them a little closer. In particular, while "array", "&array",
"&array[0]", "&array[0][0]", and so on, all had the same value, I wondered
whether their types differed--whether they were pointers to objects of
different sizes, as revealed by attempts to do arithmetic on them. So I wrote a
small program which declared a one-dimensional and a two-dimensional array, and
tried adding 1 to all those forms. I found the results surprising. Here is the
program's output, first as compiled with all the PDP-11 (Ritchie) compilers I
could find and then with the 4.1BSD (Johnson) compiler:

--------------------------- PDP-11 compilers -----------------------------------
char array1[10]

array1 =   4050   &array1 =   4050   &array1[0] =   4050
array1+1 = 4051   &array1+1 = 4060   &array1[0]+1 = 4051

char array2[10][3]

array2 =   4060   &array2 =   4060   &array2[0] =   4060  &array2[0][0] =   4060
array2+1 = 4063   &array2+1 = 4090   &array2[0]+1 = 4063  &array2[0][0]+1 = 4061

---------------------------- VAX ---------------------------------------------
char array1[10]

array1 =   6776   &array1 =   6776   &array1[0] =   6776
array1+1 = 6777   &array1+1 = 6777   &array1[0]+1 = 6777

char array2[10][3]

array2 =   6788   &array2 =   6788   &array2[0] =   6788  &array2[0][0] =   6788
array2+1 = 6791   &array2+1 = 6791   &array2[0]+1 = 6789  &array2[0][0]+1 = 6789

So with the Ritchie compilers, "array" is equivalent to "&array[0]" (even for
multidimensional arrays), while "&array" is a pointer to the array itself--
usually not a very useful thing to have. The portable C compiler, on the other
hand, makes "array" equivalent to "&array" and "&array[0]" on one-dimensional
arrays, but multidimensional arrays are more obscure: "&array[0]" is like
"&array[0][0]", which implies that the compiler tacks on an extra "[0]"
whenever it can. I find that pretty disturbing, since I would have expected
"&array2[0]+1" to get me the second row of three chars.

Conclusion: when lint says "& before array or function: ignored" it should be
read as "& beore array or function: watch out!"