A Few Tricky C Questions

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 malloc()), just 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 each situation.

Register variables

What is the output of this program?

#include <stdio.h>

int main()
{
    register int foo;
    printf("%p\n", &foo);
    return 0;
}

The 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?

struct {
    int foo, bar;
} baz;

int *example()
{
    return &baz.foo;
}

Here we’re creating a struct called baz and take a pointer to one of its fields. According to K&R C, this is invalid. Overall, structs are really limited in K&R C: they can’t be function arguments, nor can they be returned from functions, 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?

struct {
    int foo : 4;
} baz;

int *example()
{
    return &baz.foo;
}

The 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.

Pointer arithmetic

We want to average two pointers to get a pointer in-between them. Is this reasonable code?

char *foo()
{
    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.

Arrays

Is this valid?

void foo()
{
    char hello[] = "hello";
    char *foo = hello;
}

hello is an array of chars and foo is a pointer to a char. In general, arrays are interchangeable with pointers of the same type so this is valid. Now how about this one?

void foo()
{
    char hello[6];
    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 foo into hello.

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.

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null program

Chris Wellons