GCC Warning options (and clang)

date: 2021-12-25 18:16:20

categories: C

firstPublishDate: 2021-12-25 18:16:20

The options listed here are for compiling C programs. They are extra options to make the code less bug prone (like static analyzers).

GCC version 10.2

GCC has warning options helping programmers write better code. When I run tests, I have these options (compiling C):

-g3 -std=gnu11 -fPIC -pipe -mrdrnd -rdynamic -pthread -Wall -Wextra

`-g3` stores all debug symbols including #defines
`-std=gnu11` is C version I use: GNU C11, the `_cleanup` attribute is convenient and the macros can values...
`-mrdrnd` allows using the hardware random number generator
`-rdynamic` give us function names in stack traces
`-pthread` is for threads, it is no longer required with glibc 2.34 but I keep it to able to compile my projects on older linux systems
`-Wall` and `-Wextra` enable many useful warning but not all of them

To make my code more explicit and easier to update in the future, I enabled the following extra warning options:

`-Wdouble-promotion` warn implicit conversion to double
`-Wformat-security` warn not string literals for print functions
`-Wformat-signedness` warn format string and argument signedness mismatch
`-Winit-self` warn variables initialized with themselves
`-Wshift-overflow=2` warn left shifting into the sign bit
`-Wswitch-default` warn when switch is missing a default case, it helps handling all cases
`-Wstrict-overflow=4` warn about compiler optimizations
`-Walloca` warn alloca usage, this allow finding code using a lot of stack space
`-Warith-conversion` warn type conversion with constants
`-Wduplicated-branches` warn when an if-else has identical branches
`-Wduplicated-cond` warn about duplicated conditions in an if-else-if chain
`-Wshadow` warn whenever a local variable or type declaration shadows another variable, parameter, type
`-Wcast-qual` warn when const qualifier(and other qualifiers) is removed in a cast
`-Wconversion` warn for implicit conversions that may alter a value. After fixing these warnings, the conversions become explicit in the code
`-Wdate-time` warn when macros **TIME**, **DATE** or **TIMESTAMP** are encountered as they might prevent bit-wise-identical reproducible compilations
`-Wstrict-prototypes` warn if a function is declared or defined without specifying the argument types (old style)
`-Wmissing-prototypes` warn if a global function is defined without a previous prototype declaration
`-Winvalid-pch` warn if a precompiled header cannot be used

Here are the warning options I don't use:

`-Wundef` because there are warnings for the platform defines that are not set (`__FreeBSD__`) and it is ok.
`-Wredundant-decls`: in my project, there are lots of redundant declarations, so I don't use this option, but I think it is good warning to enable
`-Wformat-nonliteral`: it issues a warning for global `const char*`, I would like to have a warning when the format string is not readonly because it could be a user input.
`-Wbad-function-cast`: I get the warning when converting `int64_t` to double and I run my programs most of the time on x64. The x64 CPUs have the instruction cvtsi2sd so the conversion is done in hardware. I mostly dpn't need this warning.
`-Wjump-misses-init`: When a variable is declared and used in the jumped code between the goto and the label, the warning is issued. I think it is unnecessary and I would like to disable the warning when the goto is ok with #pragma GCC diagnostic ignored "-Wjump-misses-init", but the warning for the label is kept

For more detailed information about GCC warning options check out the

Clang has different options compare to GCC, I compile my programs with both GCC and Clang. When compiling with Clang, I use these warning options:

`-Wdouble-promotion` same as GCC
`-Wformat-security` same as GCC
`-Wformat-type-confusion` warn format and variable type mismatch
`-Winit-self` same as GCC
`-Wswitch-default` same as GCC
`-Wstrict-overflow=4` same as GCC
`-Walloca` same as GCC
`-Wshadow` same as GCC
`-Wcast-qual` same as GCC
`-Wconversion` same as GCC
`-Wdate-time` same as GCC
`-Wstrict-prototypes` same as GCC
`-Wmissing-prototypes` same as GCC
`-Winvalid-pch` same as GCC
`-Wmissing-variable-declarations`
`-Wunreachable-code-aggressive` warn about dead code
`-Wextra-semi-stmt` warn about empty expression statements, for example extra ; like this: `puts("string");;`
`-Wused-but-marked-unused` warn used variables marked as unused
`-Wcast-align` warn about memory alignment after cast
`-Wdocumentation` warn about comments

For more detailed information about Clang warning options check out

hashtags: #cprogramming