Are there C/C++ compilers that do not require standard library includes? - c++

Question

All applicants to our company must pass a simple quiz using C as part of early screening process.
It consists of a C source file that must be modified to provide the desired functionality. We clearly state that we will attempt to compile the file as-is, with no changes.
Almost all applicants user "strlen" but half of them do not include "string.h", so it does not compile until I include it.
Are they just lazy or are there compilers that do not require you to include standard library files, such as "string.h"?

Answer 1

GCC will happily compile the following code as is:
main()
{
printf("%u\n",strlen("Hello world"));
}
It will complain about incompatible implicit declaration of built-in function ‘printf’ and strlen(), but it will still produce an executable.
If you compile with -Werror it won't compile.

Answer 2

I'm pretty sure it's non-conformant for a compiler to include headers that aren't asked for. The reason for this is that the C standard says that various names are reserved, if the relevant header is included. I think this implies they aren't reserved if they aren't included, since compilers aren't allowed to reserve names the standard doesn't say are reserved (unless of course they include a non-standard header which happens to be provided by the compiler, and is documented elsewhere to reserve extra names. Which is what happens when you use POSIX).
This doesn't fully answer your question - there do exist non-conformant compilers. As for your applicants, maybe they're just used to including "windows.h", and so have never thought before about what header strlen might be defined in by the C standard. I assume without testing that MSVC does in principle require you to include "string.h". But since "windows.h" does that for you, for the vast majority of practical Windows programs you don't need to know that you have to include "string.h".

Answer 3

They might be lazy, but you can't tell. Standard library implementations (as opposed to compilers, though of course each compiler usually has "its own" stdlib impl.) are allowed to include other headers. For example, #include <stdlib.h> could include every other library described in the standard. (I'm talking in the context of "C/C++", not strictly C.)
As a result, programmers get accustomed to such things, even if not strictly guaranteed, and it's easy to forget whether some function comes from a general catch-all like stdlib.h or something else—many people forget that memcpy is from string.h too.
If they do not include any headers, I would count them as wrong. If you don't allow them to test it with a particular implementation, however, it's hard to say they're wrong. And if you don't provide them with man pages (which represent the resources they'll need to know how to use on the job), then you're wrong.
At that point, you can certainly say the don't follow the exact letter of the standard; but do you want coders that get things done and know how to fix problems when they see them, or coders that worry about minutiea that won't matter?

Answer 4

If you provide a C file to start working with, make it have all the headers that could be needed from the beginning and ask the applicants to remove the unused ones.

Answer 5

The most common engineering experience is to add (or delete) a few lines of code to/from an application with thousands of lines already working correctly. It would be extremely rare in such a case to need another header file when adding a call to printf() or strlen().
It would be interesting to look over the shoulder of experienced engineers—not just graduated from school, but with extensive experience in the trenches—to see if they simply add strlen() and try compiling, or if they check to see if stdlib.h or string.h is already included before compiling. I bet the overwhelming majority do the former.

Answer 6

C implementations usually still allow implicit function declarations.
Anyway, I wouldn't consider all the boilerplate a required part of an interview, unless you specifically ask for it (e.g. "please don't omit anything you'd normally have in a source file").
(And with Visual Assist's "add ... include" I know less and less where they comde from ;))

Answer 7

Most compilers provide some kind of option to force headers inclusion.
Eg. the GCC compiler has the -include option which is the equivalent of #include preprocessor directive.

Answer 8

TCC will also happily compile a file such as the accepted answer's example:
int main()
{
printf("%u\n", strlen("hello world"));
}
without any warnings (unless you pass -Wall); as an added bonus, you can just call tcc -run file.c to execute file.c without compiling to an output file first.

Answer 9

In C89 (the most commonly applied standard), a function that is not declared will be assumed to return an int and have unknown arguments, and will compile (probably with warnings). If it does not, teh compiler is not compliant. If on the other hand you compiled the code as C++ it will fail, andf must do so if the C++ compiler is compliant.
Why not simply specify in the question that all necessary headers must be included (and perhaps irrelevant ones omitted).
Alternatively, sit the candidates at a machine with a compiler and have them check their own code, and state that the code must compile at the maximum warning level, without warnings.

Answer 10

I'm doing C/C++ for 20 years (even taught classes) and I guess there's a 50% probability that I'd forget the include too (99% of the time when I code, the include is already there somewhere). I know this isn't exactly answering your question, but if someone knows strlen() they will know within seconds what to do with that specific compiler error, so from a job qualification standpoint the slip is practically irrelevant.
Rather than putting emphasis on stuff like that, checking for the subtleties that require real understanding of the language should be far more relevant, i.e. buffer overruns, strncpy not appending a final \0 when hitting the limits, asking someone to make a safe (truncating) copy to a buffer of limited length, etc. Especially in C/C++ programming, the errors that do not generate a compiler error are the ones which will cause you/your company the real trouble.