At work I recently came across an abandoned copy of the first edition
of The C Programming Language by Ken Thompson and Dennis Ritchie —
often lovingly abbreviated as K&R. It’s a
significant piece of computer science history
and I highly recommend it to anyone who writes software. As far as
computing manuals go, it’s a thin book (228 pages) so I got through
the whole thing in about a week.
I’ve been programming in C for seven years now but it seems there’s
always something new for me to learn about it. The book cleared up
some incomplete concepts I had about C, particularly the relationship
between pointers and arrays as well as operator precedence — the
reason why function pointers look so weird. By the end I re-gained an
appreciation for the simplicity and power of C. All of the examples in
the book are written without heap allocation (no
static memory, and it manages to get by with rather few limitations.
As I was reading I realized a handful of “tricky” questions that I
wouldn’t have been able to answer with confidence before reading the
book. If you’re a C developer, pause and reflect just after each chunk
of example code and try to answer the question as correctly as you
can. Pretend you’re a compiler and think about what you need to do in
What is the output of this program?
register int foo;
register keyword hints to the compiler that the automatic
variable should be stored in a register rather than memory, making
access to the variable faster. This is only a hint so the compiler is
free to ignore it.
In the example we take a pointer to the variable. However, we declared
this variable to be stored in a register. Addresses only point to
locations in memory so registers can’t be addressed by a
pointer. While the compiler can ignore the optimization hint and
provide an address, this is ultimately an inconsistent request. The
compiler will produce and error and the code will not compile.
Pointers to struct fields
Is this program valid?
int foo, bar;
Here we’re creating a struct called (Update: I
misunderstood. This is allowed.) Overall, structs are really limited in
K&R C: they can’t be function arguments, nor can they be returned from
baz and take a pointer to one of
its fields. According to K&R C, this is invalid.
nor can pointers be taken to their fields. Only
pointers to structs are first-class. They acknowledged that this was
limiting and said they planned on fixing it in the future.
Fortunately, this was fixed with ANSI C and structs are first-class
objects. This means the above program is valid in ANSI C.
How about this one?
int foo : 4;
foo field is a 4-bit wide bit-field — smaller than a single
byte. Pointers can only address whole bytes, so this is
invalid. Even if
foo was 8 or 32 bits wide (full/aligned bytes
on modern architectures) this would still be invalid.
We want to average two pointers to get a pointer in-between them. Is
this reasonable code?
char *start = "hello";
char *end = start + 5;
return (start + end) / 2;
A thoughtful programmer should notice that adding together pointers is
likely to be disastrous. Pointers tend to be very large, addressing
high areas of memory. Adding two pointers together is very likely to
lead to an overflow. When I posed this question to
Brian, he realized this and came up with this
solution to avoid the overflow.
return start / 2 + end / 2;
However, this is still invalid. As a complete precaution for
overflowing pointer arithmetic, pointer addition is forbidden and
neither of these will compile. Pointer subtraction is perfectly valid,
so it can be done like so.
return (end - start) / 2 + start;
Subtracting two pointers produces an integer. Adding integers to
pointers is not only valid but also essential, so this is only a
restriction about adding pointers together.
Is this valid?
char hello = "hello";
char *foo = hello;
hello is an array of
foo is a pointer to a
general, arrays are interchangeable with pointers of the same type so
this is valid. Now how about this one?
char *foo = "hello";
hello = foo;
Here we’ve inverted the relationship are are trying to assign the
array as a pointer. This is invalid. Arrays are like pointer
constants in that they can’t be used as lvalues — they can’t be
reassigned to point to somewhere else. The closest you can get is to
copy the contents of
I think that about sums my questions. I (foolishly) didn’t write them
down as I came up with them and this is everything I can remember.