GCC Warning options (and clang)

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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` 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

official GCC documentation

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Clang version 11

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

Diagnostic flags in Clang (short descriptions)

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Guides

[https://github.com/ossf/wg-best-practices-os-developers/blob/main/docs/Compiler_Hardening_Guides/Compiler-Options-Hardening-Guide-for-C-and-C++.md](Compiler Options Hardening Guide for C and C++)

hashtags: #cprogramming

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