How to turn on (literally) ALL of GCC's warnings?

I would like to enable -- literally -- ALL of the warnings that GCC has. (You'd think it would be easy...)

• You'd think -Wall might do the trick, but nope! Still need -Wextra.

• You'd think -Wextra might do the trick, but nope! Not all of the warnings listed here (for example, -Wshadow) are enabled by this. And I still have no idea if this list is comprehensive.

How do I tell GCC to enable (no if's, and's, or but's!) all the warnings it has?

You can't.

The manual for GCC 4.4.0 is only comprehensive for that version, but it does list all the possible warnings for 4.4.0. They're not all on the page you link to though, for instance some language-specific options are on the pages for C++ options or Obj-C options. To find them all you're better off looking at the Options Summary

Turning on everything would include -Wdouble-promotion which is only relevant on CPUs with a 32-bit single-precision floating-point unit which implements float in hardware, but emulates double in software. Doing calculations as double would use the software emulation and be slower. That's relevant for some embedded CPUs, but completely irrelevant for modern desktop CPUs with hardware support for 64-bit floating-point.

Another warning that's not usually useful is -Wtraditional, which warns about perfectly well formed code that has a different meaning (or doesn't work) in traditional C, e.g. "string " "concatenation", or ISO C function definitions! Do you really care about compatibility with 30 year old compilers? Do you really want a warning for writing int inc(int i) { return i+1; } ?

I think -Weffc++ is too noisy to be useful, it's based on the outdated first edition of Effective C++ and warns about constructs which are perfectly valid C++ (and for which the guidelines changed in later editions of the book.) I don't want to be warned that I haven't initialized a std::string member in my constructor; it has a default constructor that does exactly what I want, why should I write m_str() to call it? The -Weffc++ warnings that would be helpful are too difficult for the compiler to detect accurately (giving false negatives), and the ones that aren't useful, such as initializing all members explicitly, just produce too much noise, giving false positives.

Luc Danton provided a great example of useless warnings from -Waggregate-return that almost certainly never makes sense for C++ code.

i.e. you don't really want all warnings, you just think you do.

Go through the manual, read about them, decide which you might want to enable, try them. Reading your compiler's manual is a Good Thing^TM anyway, taking a short cut and enabling warnings you don't understand is not a very good idea, especially if it's to avoid having to RTFM.

Anyone who just turns on everything is probably either doing so because they're clueless because or a pointy-haired boss said "no warnings."

Some warnings are important, and some aren't. You have to be discriminating or you mess up your program. Consider, for instance, -Wdouble-promotion. If you're working on an embedded system you might want this; if you're working on a desktop system you probably don't. And do you want -Wtraditional? I doubt it.

Edit: See also -Wall-all to enable all warnings which is closed as WONTFIX.

Edit 2: in response to DevSolar's complaint about makefiles needing to use different warnings depending on compiler version, if -Wall -Wextra isn't suitable then it's not difficult to use compiler-specific and version-specific CFLAGS:

compiler_name := $(notdir $(CC))
ifeq ($(compiler_name),gcc)
compiler_version := $(basename $(shell $(CC) -dumpversion))
endif
ifeq ($(compile_name),clang)
compiler_version := $(shell $(CC) --version | awk 'NR==1{print $$3}')
endif
# ...
wflags.gcc.base := -Wall -Wextra
wflags.gcc.4.7 := -Wzero-as-null-pointer-constant
wflags.gcc.4.8 := $(wflags.gcc.4.7)
wflags.clang.base := -Wall -Wextra
wflags.clang.3.2 := -Weverything
CFLAGS += $(wflags.$(compiler_name).base) $(wflags.$(compiler_name).$(compiler_version))

I would agree with the previous answers that it is probably not beneficial to enable literally all warnings, however GCC does provide a reasonably convenient way to achieve this. The command

gcc -Q --help=warning

provides a list of all supported warning options with information on whether they are active. This can by the way be used to find out which options are (not) enabled by e.g. -Wall and -Wextra

gcc -Wall -Wextra -Q --help=warning

To enable all the warnings you can use some regex to extract the command line parameters

gcc -Q --help=warning | sed -e 's/^\s*\(\-\S*\)\s*\[\w*\]/\1 /gp;d' | tr -d '\n'

For my current GCC this gives:

-Wabi -Wabi-tag -Waddress -Waggregate-return -Waggressive-loop-optimizations -Waliasing -Walign-commons -Wampersand -Warray-bounds -Warray-temporaries -Wassign-intercept -Wattributes -Wbad-function-cast -Wbool-compare -Wbuiltin-macro-redefined -Wc++-compat -Wc++0x-compat -Wc++14-compat -Wc-binding-type -Wc90-c99-compat -Wc99-c11-compat -Wcast-align -Wcast-qual -Wchar-subscripts -Wcharacter-truncation -Wchkp -Wclobbered -Wcomment -Wcompare-reals -Wconditionally-supported -Wconversion -Wconversion-extra -Wconversion-null -Wcoverage-mismatch -Wcpp -Wctor-dtor-privacy -Wdate-time -Wdeclaration-after-statement -Wdelete-incomplete -Wdelete-non-virtual-dtor -Wdeprecated -Wdeprecated-declarations -Wdesignated-init -Wdisabled-optimization -Wdiscarded-array-qualifiers -Wdiscarded-qualifiers -Wdiv-by-zero -Wdouble-promotion -Weffc++ -Wempty-body -Wendif-labels -Wenum-compare -Wextra -Wfloat-equal -Wformat-contains-nul -Wformat-extra-args -Wformat-nonliteral -Wformat-security -Wformat-signedness -Wformat-y2k -Wformat-zero-length -Wfree-nonheap-object -Wfunction-elimination -Wignored-qualifiers -Wimplicit -Wimplicit-function-declaration -Wimplicit-int -Wimplicit-interface -Wimplicit-procedure -Wincompatible-pointer-types -Winherited-variadic-ctor -Winit-self -Winline -Wint-conversion -Wint-to-pointer-cast -Wintrinsic-shadow -Wintrinsics-std -Winvalid-memory-model -Winvalid-offsetof -Winvalid-pch -Wjump-misses-init -Wline-truncation -Wliteral-suffix -Wlogical-not-parentheses -Wlogical-op -Wlong-long -Wmain -Wmaybe-uninitialized -Wmemset-transposed-args -Wmissing-braces -Wmissing-declarations -Wmissing-field-initializers -Wmissing-include-dirs -Wmissing-parameter-type -Wmissing-prototypes -Wmultichar -Wnarrowing -Wnested-externs -Wnoexcept -Wnon-template-friend -Wnon-virtual-dtor -Wnonnull -Wodr -Wold-style-cast -Wold-style-declaration -Wold-style-definition -Wopenmp-simd -Woverflow -Woverlength-strings -Woverloaded-virtual -Woverride-init -Wpacked -Wpacked-bitfield-compat -Wpadded -Wparentheses -Wpedantic -Wpmf-conversions -Wpointer-arith -Wpointer-sign -Wpointer-to-int-cast -Wpragmas -Wproperty-assign-default -Wprotocol -Wreal-q-constant -Wrealloc-lhs -Wrealloc-lhs-all -Wredundant-decls -Wreorder -Wreturn-local-addr -Wreturn-type -Wselector -Wsequence-point -Wshadow -Wshadow-ivar -Wshift-count-negative -Wshift-count-overflow -Wsign-compare -Wsign-promo -Wsized-deallocation -Wsizeof-array-argument -Wsizeof-pointer-memaccess -Wstack-protector -Wstrict-null-sentinel -Wstrict-prototypes -Wstrict-selector-match -Wsuggest-attribute=const -Wsuggest-attribute=format -Wsuggest-attribute=noreturn -Wsuggest-attribute=pure -Wsuggest-final-methods -Wsuggest-final-types -Wsuggest-override -Wsurprising -Wswitch -Wswitch-bool -Wswitch-default -Wswitch-enum -Wsync-nand -Wsynth -Wsystem-headers -Wtabs -Wtarget-lifetime -Wtraditional -Wtraditional-conversion -Wtrampolines -Wtrigraphs -Wtype-limits -Wundeclared-selector -Wundef -Wunderflow -Wuninitialized -Wunknown-pragmas -Wunsafe-loop-optimizations -Wunsuffixed-float-constants -Wunused -Wunused-but-set-parameter -Wunused-but-set-variable -Wunused-dummy-argument -Wunused-function -Wunused-label -Wunused-local-typedefs -Wunused-macros -Wunused-parameter -Wunused-result -Wunused-value -Wunused-variable -Wuse-without-only -Wuseless-cast -Wvarargs -Wvariadic-macros -Wvector-operation-performance -Wvirtual-move-assign -Wvla -Wvolatile-register-var -Wwrite-strings -Wzero-as-null-pointer-constant -Wzerotrip -frequire-return-statement

This can now be used to call the GCC, i.e.

gcc $(gcc -Q --help=warning | sed -e 's/^\s*\(\-\S*\)\s*\[\w*\]/\1 /gp;d' | tr -d '\n')

Note however that this results in warnings due to some warning options only being available for certain languages (e.g. C++). These could be avoided by using some more regex to only include the options allowed for the current language or by adding an appropriate -Wno-whatever at the end of the call.

It's simply impossible to program with all warnings enabled (unless you are going to ignore them, but then, why bother?). For example, let's assume you use following set of flags: -Wstrict-prototypes -Wtraditional.

Even with two warnings enabled, the following program would complain.

/tmp $ cat main.c 
int main(int argc, char **argv) {
    return 0;
}
/tmp $ gcc -Wstrict-prototypes -Wtraditional main.c 
main.c: In function ‘main’:
main.c:1:5: warning: traditional C rejects ISO C style function definitions [-Wtraditional]
 int main(int argc, char **argv) {
     ^

You may think "well, I'm going to use old style prototypes then". Nope, this won't work.

/tmp $ cat main.c 
int main(argc, argv)
    int argc;
    char **argv;
{
    return 0;
}
/tmp $ gcc -Wstrict-prototypes -Wtraditional main.c 
main.c:1:5: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
 int main(argc, argv)
     ^

And no, not specifying any prototype is also wrong, as the compiler will also complain.

/tmp $ cat main.c 
int main() {
    return 0;
}
/tmp $ gcc -Wstrict-prototypes -Wtraditional main.c 
main.c:1:5: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
 int main() {
     ^

If you define any functions inside your program, you cannot use all flags, because the compiler will complain about any imaginable function definition.

For C++, this is possible (the -Wtraditional flag doesn't exist), and very simple programs can be compiled. To enable all warnings, use following list of warnings (probably some warnings are duplicated, because I didn't bother to filter warnings enabled by -Wall).

-Wabi -Wctor-dtor-privacy -Wnon-virtual-dtor -Wreorder -Weffc++ -Wstrict-null-sentinel -Wno-non-template-friend -Wold-style-cast -Woverloaded-virtual -Wno-pmf-conversions -Wsign-promo -Wextra -Wall -Waddress -Waggregate-return -Warray-bounds -Wno-attributes -Wno-builtin-macro-redefined -Wc++0x-compat -Wcast-align -Wcast-qual -Wchar-subscripts -Wclobbered -Wcomment -Wconversion -Wcoverage-mismatch -Wno-deprecated -Wno-deprecated-declarations -Wdisabled-optimization -Wno-div-by-zero -Wempty-body -Wenum-compare -Wno-endif-labels -Wfatal-errors -Wfloat-equal -Wformat -Wformat=2 -Wno-format-contains-nul -Wno-format-extra-args -Wformat-nonliteral -Wformat-security -Wformat-y2k -Wignored-qualifiers -Winit-self -Winline -Wno-int-to-pointer-cast -Wno-invalid-offsetof -Winvalid-pch -Wunsafe-loop-optimizations -Wlogical-op -Wlong-long -Wmain -Wmissing-braces -Wmissing-field-initializers -Wmissing-format-attribute -Wmissing-include-dirs -Wmissing-noreturn -Wno-mudflap -Wno-multichar -Wnonnull -Wno-overflow -Woverlength-strings -Wpacked -Wpacked-bitfield-compat -Wpadded -Wparentheses -Wpointer-arith -Wredundant-decls -Wreturn-type -Wsequence-point -Wshadow -Wsign-compare -Wsign-conversion -Wstack-protector -Wstrict-aliasing=1 -Wstrict-overflow=5 -Wswitch -Wswitch-default -Wswitch-enum -Wsync-nand -Wsystem-headers -Wtrigraphs -Wtype-limits -Wundef -Wuninitialized -Wunknown-pragmas -Wno-pragmas -Wunreachable-code -Wunused -Wunused-function -Wunused-label -Wunused-parameter -Wunused-value -Wunused-variable -Wvariadic-macros -Wvla -Wvolatile-register-var -Wwrite-strings