I just came across a site in an ad on Embedded.com that every reader of this blog needs to check out:
The Power of Ten 10 Rules for Writing Safety Critical Code.
Their rule #10 comments supports our position you want every useful compiler warning you can get.
Something I just saw on the AVR-GCC list:
"I mean the compiler gives some of the most stupid warnings, such as , when a function that is declared but not used..."
or a past favorite of mine "It is only a warning, just ignore it". Yellow colored traffic lights are "only warnings", that most people do seem to ignore, and governments are gaming to enhance revenue; sorry wrong blog...
I have always had a zero tolerance for warnings in code. If you have a warning in your code, your code is broken.
If you are using GCC here are the warnings you can enable, that I use in my own Makefiles:
Make sure you have -W and -Wall in your CFLAGS.
CFLAGS += -W -Wall -Wstrict-prototypes -Wchar-subscripts
I generally run with every warning/error message turned on with the exception of pedantic and unreachable-code. The later frequently gives bogus results and the former goes off on commonly accepted code.
# -Werror : Make all warnings into errors.
CFLAGS += -Werror
# -pedantic : Issue all the mandatory diagnostics listed in the C
# standard. Some of them are left out by default, since they trigger frequently
# on harmless code.
#
# -pedantic-errors : Issue all the mandatory diagnostics, and make all
# mandatory diagnostics into errors. This includes mandatory diagnostics that
# GCC issues without -pedantic but treats as warnings.
#CFLAGS += -pedantic
#-Wunreachable-code
#Warn if the compiler detects that code will never be executed. [Seems
to give bogus results]
#CFLAGS += -Wunreachable-code
#Warn if an undefined identifier is evaluated in an `#if' directive.
CFLAGS += -Wundef
# Dump the address, size, and relative cost of each statement into comments in
# the generated assembler code. Used for debugging avr-gcc.
CFLAGS += -msize
# -Winline : Warn when a function marked inline could not be
# substituted, and will give the reason for the failure.
CFLAGS += -Winline
Most of the following are turnned on via -Wall:
# Functions prologues/epilogues expanded as call to appropriate
# subroutines. Code size will be smaller. Use subroutines for function
# prologue/epilogue. For complex functions that use many registers (that needs
# to be saved/restored on function entry/exit), this saves some space at the
# cost of a slightly increased execution time.
CFLAGS += -mcall-prologues
# Use rjmp/rcall (limited range) on >8K devices. On avr2 and avr4 architectures
# (less than 8 KB or flash memory), this is always the case. On avr3 and avr5
# architectures, calls and jumps to targets outside the current function will
# by default use jmp/call instructions that can cover the entire address range,
# but that require more flash ROM and execution time.
#CFLAGS += -mshort-calls
# Do not generate tablejump instructions. By default, jump tables can be used
# to optimize switch statements. When turned off, sequences of compare
# statements are used instead. Jump tables are usually faster to execute on
# average, but in particular for switch statements where most of the jumps
# would go to the default label, they might waste a bit of flash memory.
# CFLAGS += -mno-tablejump
# Allocate to an enum type only as many bytes as it needs for the declared
# range of possible values. Specifically, the enum type will be equivalent to
# the smallest integer type which has enough room.
# CFLAGS += -fshort-enums
# Dump the address, size, and relative cost of each statement into comments in
# the generated assembler code. Used for debugging avr-gcc.
CFLAGS += -msize
# Dump the internal compilation result called "RTL" into comments in the
# generated assembler code. Used for debugging avr-gcc.
# CFLAGS += -mrtl
# Generate lots of debugging information to stderr.
#CFLAGS += -mdeb
#-Wchar-subscripts
#Warn if an array subscript has type char. This is a common cause of
error, as programmers often forget that this type is signed on some
machines. This warning is enabled by -Wall.
#
#-Wcomment
#Warn whenever a comment-start sequence `/*' appears in a `/*'
comment, or whenever a Backslash-Newline appears in a `//' comment.
This warning is enabled by -Wall.
#
#-Wfatal-errors
#This option causes the compiler to abort compilation on the first
error occurred rather than trying to keep going and printing further
error messages.
#
#-Wformat
#Check calls to printf and scanf, etc., to make sure that the
arguments supplied have types appropriate to the format string
specified, and that the conversions specified in the format string
make sense.
#
#-Winit-self (C, C++, Objective-C and Objective-C++ only)
#Warn about uninitialized variables which are initialized with
themselves. Note this option can only be used with the -Wuninitialized
option, which in turn only works with -O1 and above.
#
#-Wimplicit-int
#Warn when a declaration does not specify a type. This warning is
enabled by -Wall.
#
#-Wimplicit-function-declaration
#-Werror-implicit-function-declaration
#Give a warning (or error) whenever a function is used before being
declared. The form -Wno-error-implicit-function-declaration is not
supported. This warning is enabled by -Wall (as a warning, not an
error).
#
#-Wimplicit
#Same as -Wimplicit-int and -Wimplicit-function-declaration. This
warning is enabled by -Wall.
#
#-Wmain
#Warn if the type of `main' is suspicious. `main' should be a function
with external linkage, returning int, taking either zero arguments,
two, or three arguments of appropriate types. This warning is enabled
by -Wall.
#
#-Wmissing-braces
#Warn if an aggregate or union initializer is not fully bracketed. In
the following example, the initializer for `a' is not fully bracketed,
but that for `b' is fully bracketed.
#
# int a[2][2] = { 0, 1, 2, 3 };
# int b[2][2] = { { 0, 1 }, { 2, 3 } };
#
#This warning is enabled by -Wall.
#
#-Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only)
#Warn if a user-supplied include directory does not exist.
#
#
#-Wparentheses
#Warn if parentheses are omitted in certain contexts, such as when
there is an assignment in a context where a truth value is expected,
or when operators are nested whose precedence people often get
confused about.
#
#This warning is enabled by -Wall.
#
#-Wsequence-point
#Warn about code that may have undefined semantics because of
violations of sequence point rules in the C standard.
#
#This warning is enabled by -Wall.
#
#-Wreturn-type
#Warn whenever a function is defined with a return-type that defaults
to int. Also warn about any return statement with no return-value in a
function whose return-type is not void.
#
#This warning is enabled by -Wall.
#
#-Wswitch
#Warn whenever a switch statement has an index of enumerated type and
lacks a case for one or more of the named codes of that enumeration.
(The presence of a default label prevents this warning.) case labels
outside the enumeration range also provoke warnings when this option
is used. This warning is enabled by -Wall.
#
#-Wswitch-default
#Warn whenever a switch statement does not have a default case.
#
#-Wswitch-enum
#Warn whenever a switch statement has an index of enumerated type and
lacks a case for one or more of the named codes of that enumeration.
case labels outside the enumeration range also provoke warnings when
this option is used.
#
#-Wtrigraphs
#Warn if any trigraphs are encountered that might change the meaning
of the program (trigraphs within comments are not warned about). This
warning is enabled by -Wall.
#
#-Wunused-function
#Warn whenever a static function is declared but not defined or a
non-inline static function is unused. This warning is enabled by
-Wall.
#
#-Wunused-label
#Warn whenever a label is declared but not used. This warning is
enabled by -Wall.
#
#-Wunused-parameter
#Warn whenever a function parameter is unused aside from its declaration.
#
#-Wunused-variable
#Warn whenever a local variable or non-constant static variable is
unused aside from its declaration This warning is enabled by -Wall.
#
#-Wunused-value
#Warn whenever a statement computes a result that is explicitly not
used. This warning is enabled by -Wall.
#
#To suppress this warning cast the expression to `void'.
#
#-Wunused
#All the above -Wunused options combined.
#
#-Wuninitialized
#Warn if an automatic variable is used without first being initialized
or if a variable may be clobbered by a setjmp call.
#
#This warning is enabled by -Wall.
#
#-Wstring-literal-comparison
#Warn about suspicious comparisons to string literal constants. In C,
direct comparisons against the memory address of a string literal,
such as if (x == "abc"), typically indicate a programmer error, and
even when intentional, result in unspecified behavior and are not
portable.
#
#-Wall
# All of the above `-W' options combined. This enables all the warnings about
# constructions that some users consider questionable, and that are easy to
# avoid (or modify to prevent the warning), even in conjunction with macros.
# This also enables some language-specific warnings described in C++ Dialect
# Options and Objective-C and Objective-C++ Dialect Options.
#
#
#
#
#-Wextra
#-Wfloat-equal
#Warn if floating point values are used in equality comparisons.
#
#-Wtraditional (C only)
#Warn about certain constructs that behave differently in traditional
and ISO C. Also warn about ISO C constructs that have no traditional C
equivalent, and/or problematic constructs which should be avoided.
#
#-Wdeclaration-after-statement (C only)
#Warn when a declaration is found after a statement in a block
#
#
#-Wshadow
#Warn whenever a local variable shadows another local variable,
parameter or global variable or whenever a built-in function is
shadowed.
#
#
#-Wunsafe-loop-optimizations
#Warn if the loop cannot be optimized because the compiler could not
assume anything on the bounds of the loop indices.
#
#
#-Wpointer-arith
#Warn about anything that depends on the 'size of' a function type or
of void. GNU C assigns these types a size of 1, for convenience in
calculations with void * pointers and pointers to functions.
#
#-Wbad-function-cast (C only)
#Warn whenever a function call is cast to a non-matching type. For
example, warn if int malloc() is cast to anything *.
#
#-Wcast-qual
#Warn whenever a pointer is cast so as to remove a type qualifier from
the target type. For example, warn if a const char * is cast to an
ordinary char *.
#
#-Wcast-align
#Warn whenever a pointer is cast such that the required alignment of
the target is increased. For example, warn if a char * is cast to an
int * on machines where integers can only be accessed at two- or
four-byte boundaries.
#
#-Wwrite-strings
#When compiling C, give string constants the type const char[length]
so that copying the address of one into a non-const char * pointer
will get a warning; when compiling C++, warn about the deprecated
conversion from string constants to char *. These warnings will help
you find at compile time code that can try to write into a string
constant, but only if you have been very careful about using const in
declarations and prototypes. Otherwise, it will just be a nuisance;
this is why we did not make -Wall request these warnings.
#
#-Wconversion
#Warn if a prototype causes a type conversion that is different from
what would happen to the same argument in the absence of a prototype.
This includes conversions of fixed point to floating and vice versa,
and conversions changing the width or signedness of a fixed point
argument except when the same as the default promotion.
#
#
#-Wsign-compare
#Warn when a comparison between signed and unsigned values could
produce an incorrect result when the signed value is converted to
unsigned. This warning is also enabled by -Wextra; to get the other
warnings of -Wextra without this warning, use `-Wextra
-Wno-sign-compare'.
#
#
#-Waggregate-return
#Warn if any functions that return structures or unions are defined or
called. (In languages where you can return an array, this also elicits
a warning.)
#
#
#-Wstrict-prototypes (C only)
#Warn if a function is declared or defined without specifying the
argument types. (An old-style function definition is permitted without
a warning if preceded by a declaration which specifies the argument
types.)
#
#-Wold-style-definition (C only)
#Warn if an old-style function definition is used. A warning is given
even if there is a previous prototype.
#
#-Wmissing-prototypes (C only)
#Warn if a global function is defined without a previous prototype
declaration. This warning is issued even if the definition itself
provides a prototype. The aim is to detect global functions that fail
to be declared in header files.
#
#-Wmissing-declarations (C only)
#Warn if a global function is defined without a previous declaration.
Do so even if the definition itself provides a prototype. Use this
option to detect global functions that are not declared in header
files.
#
#-Wmissing-field-initializers
#Warn if a structure's initializer has some fields missing.
#
#-Wmissing-noreturn
#Warn about functions which might be candidates for attribute
noreturn. Note these are only possible candidates, not absolute ones.
Care should be taken to manually verify functions actually do not ever
return before adding the noreturn attribute, otherwise subtle code
generation bugs could be introduced. You will not get a warning for
main in hosted C environments.
#
#-Wmissing-format-attribute
#Warn about function pointers which might be candidates for format
attributes. Note these are only possible candidates, not absolute
ones.
#
#-Wpacked
#Warn if a structure is given the packed attribute, but the packed
attribute has no effect on the layout or size of the structure.
#
#-Wpadded
#Warn if padding is included in a structure, either to align an
element of the structure or to align the whole structure. Sometimes
when this happens it is possible to rearrange the fields of the
structure to reduce the padding and so make the structure smaller.
#
#-Wredundant-decls
#Warn if anything is declared more than once in the same scope, even
in cases where multiple declaration is valid and changes nothing.
#
#-Wnested-externs (C only)
#Warn if an extern declaration is encountered within a function.
#
#-Wunreachable-code
#Warn if the compiler detects that code will never be executed.
#
#-Winline
#Warn if a function can not be inlined and it was declared as inline.
Even with this option, the compiler will not warn about failures to
inline functions declared in system headers.
#
#-Winvalid-pch
#Warn if a precompiled header (see Precompiled Headers) is found in
the search path but can't be used.
#
#-Wvolatile-register-var
#Warn if a register variable is declared volatile. The volatile
modifier does not inhibit all optimizations that may eliminate reads
and/or writes to register variables.
#
#-Wdisabled-optimization
#Warn if a requested optimization pass is disabled. This warning does
not generally indicate that there is anything wrong with your code; it
merely indicates that GCC's optimizers were unable to handle the code
effectively. Often, the problem is that your code is too big or too
complex; GCC will refuse to optimize programs when the optimization
itself is likely to take inordinate amounts of time.
#
#-Wstack-protector
#This option is only active when -fstack-protector is active. It warns
#about functions that will not be protected against stack smashing.
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