Mention that GCC reorders statements.

[pintos-anon] / doc / standards.texi

@node Coding Standards, Project Documentation, Multilevel Feedback Scheduling, Top
@appendix Coding Standards

All of you should have taken a class like CS 107, so we expect you to
be familiar with some set of coding standards such as
@uref{http://www.stanford.edu/class/cs140/projects/misc/CodingStandards.pdf,
, CS 107 Coding Standards}. Even if you've taken 107, we recommend
reviewing that document.  We expect code at the "Peer-Review Quality"
level as described there.

Our standards for coding are mostly important in grading.  More
information on our grading methodology can be found on the Course Info
page and the Grading page.  We also want to stress that aside from the
fact that we are explicitly basing part of your grade on these things,
good coding practices will improve the quality of your code.  This
makes it easier for your partners to interact with it, and ultimately,
will improve your chances of having a good working program.  That said
once, the rest of this document will discuss only the ways in which
our coding standards will affect our grading.

@menu
* Coding Style::                
* Conditional Compilation::     
* C99::                         
* Unsafe String Functions::     
@end menu

@node Coding Style
@section Style

Style, for the purposes of our grading, refers to how readable your
code is.  At minimum, this means that your code is well formatted, your
variable names are descriptive and your functions are decomposed and
well commented.  Any other factors which make it hard (or easy) for us
to read or use your code will be reflected in your style grade.

The existing Pintos code is written in the GNU style and largely
follows the @uref{http://www.gnu.org/prep/standards_toc.html, , GNU
Coding Standards}.  We encourage you to follow the applicable parts of
them too, especially chapter 5, ``Making the Best Use of C.''  Using a
different style won't cause actual problems, but it's ugly to see
gratuitous differences in style from one function to another.

Pintos comments sometimes refer to outside standards or
specifications by writing a name inside square brackets, like this:
@code{[IA32-v3]}.  These names refer to the reference names used in
this documentation (@pxref{References}).

@node Conditional Compilation
@section Conditional Compilation

Given the scope and complexity of your assignments this quarter, you
may find it convenient while coding and debugging (and we will find it
convenient while grading) to be able to independently turn different
parts of the assignments on and off.  To do this, choose a macro name
and use it in conditional
compilation directives, e.g.:

@example
#ifdef @var{NAME}
@dots{}your code@dots{}
#endif
@end example

In general, the code that you turn in must not depend on conditional
compilation directives.  Project code should be written so that all of
the subproblems for the project function together, and it should
compile properly without the need for any new macros to be defined.
There are a few exceptions:

@itemize @bullet
@item
Problem 1-2, @func{thread_join}.  Some other code expects
@code{THREAD_JOIN_IMPLEMENTED} to be defined once you've implemented
this function.

@item
Problem 1-4, the advanced scheduler.  We must be able to turn this on
and off with a compile-time directive.  You must use the macro name we
specify for that part.  @xref{Problem 1-4 Advanced Scheduler}, for
details.

@item
Code written for extra credit may be included conditionally.  If the
extra credit code changes the normally expected functionality of the
code, then it @emph{must} be included conditionally, and it must not
be enabled by default.
@end itemize

You can use @file{constants.h} in @file{pintos/src} to define macros
for conditional compilation.  We will replace the @file{constants.h}
that you supply with one of our own when we test your code, so do not
define anything important in it.

@node C99
@section C99

The Pintos source code uses a few features of the ``C99'' standard
library that were not in the original 1989 standard for C.  Because
they are so new, most classes do not cover these features, so this
section will describe them.  The new features used in Pintos are
mostly in new headers:

@table @file
@item <stdbool.h>
Defines macros @code{bool}, a 1-bit type that takes on only the values
0 and 1, @code{true}, which expands to 1, and @code{false}, which
expands to 0.

@item <stdint.h>
On systems that support them, this header defines types
@code{int@var{n}_t} and @code{uint@var{n}_t} for @var{n} = 8, 16, 32,
64, and possibly others.  These are 2's complement signed and unsigned
types, respectively, with the given number of bits.  

On systems where it is possible, this header also defines types
@code{intptr_t} and @code{uintptr_t}, which are integer types big
enough to hold a pointer.

On all systems, this header defines types @code{intmax_t} and
@code{uintmax_t}, which are the system's signed and unsigned integer
types with the widest ranges.

For every signed integer type @code{@var{type}_t} it defines, as well
as for @code{ptrdiff_t} defined in @file{<stddef.h>}, this header also
defines macros @code{@var{type}_MAX} and @code{@var{type}_MIN} that
give the type's range.  Similarly, for every unsigned integer type
@code{@var{type}_t} defined here, as well as for @code{size_t} defined
in @file{<stddef.h>}, this header defines a @code{@var{type}_MAX}
macro giving its maximum value.

@item <inttypes.h>
@file{<stdint.h>} is useful on its own, but it provides no way to pass
the types it defines to @func{printf} and related functions.  This
header provides macros to help with that.  For every
@code{int@var{n}_t} defined by @file{<stdint.h>}, it provides macros
@code{PRId@var{n}} and @code{PRIi@var{n}} for formatting values of
that type with @code{"%d"} and @code{"%i"}.  Similarly, for every
@code{uint@var{n}_t}, it provides @code{PRIo@var{n}},
@code{PRIu@var{n}}, @code{PRIu@var{x}}, and @code{PRIu@var{X}}.

You use these something like this, taking advantage of the fact that
the C compiler concatenates adjacent string literals:
@example
#include <inttypes.h>
@dots{}
int32_t value = @dots{};
printf ("value=%08"PRId32"\n", value);
@end example
@noindent
The @samp{%} is not supplied by the @code{PRI} macros.  As shown
above, you supply it yourself and follow it by any flags, field
widths, etc.

@item <stdio.h>
The @func{printf} function has some new type modifiers for printing
standard types:

@table @samp
@item j
For @code{intmax_t} (e.g.@: @samp{%jd}) or @code{uintmax_t} (e.g.@:
@samp{%ju}).

@item z
For @code{size_t} (e.g.@: @samp{%zu}).

@item t
For @code{ptrdiff_t} (e.g.@: @samp{%td}).
@end table

Pintos @func{printf} also implements a nonstandard @samp{'} flag that
group large numbers with commas to make them easier to read.
@end table

@node Unsafe String Functions
@section Unsafe String Functions

A few of the string functions declared in the standard
@file{<string.h>} and @file{<stdio.h>} headers are notoriously unsafe.
The worst offenders are intentionally not included in the Pintos C
library:

@table @func
@item strcpy
When used carelessly this function can overflow the buffer reserved
for its output string.  Use @func{strlcpy} instead.  Refer to
comments in its source code in @code{lib/string.c} for documentation.

@item strncpy
This function can leave its destination buffer without a null string
terminator and it has performance problems besides.  Again, use
@func{strlcpy}.

@item strcat
Same issue as @func{strcpy}, but substitute @func{strlcat}.
Again, refer to comments in its source code in @code{lib/string.c} for
documentation.

@item strncat
The meaning of its buffer size argument often leads to problems.
Again, use @func{strlcat}.

@item strtok
Uses global data, so it is unsafe in threaded programs such as
kernels.  Use @func{strtok_r} instead, and see its source code in
@code{lib/string.c} for documentation and an example.

@item sprintf
Same issue as @func{strcpy}.  Use @func{snprintf} instead.  Refer
to comments in @code{lib/stdio.h} for documentation.

@item vsprintf
Same issue as @func{strcpy}.  Use @func{vsnprintf} instead.
@end table

If you try to use any of these functions, you should get a hint from
the error message, which will refer to an identifier like
@code{dont_use_sprintf_use_snprintf}.