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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 -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

official GCC documentation

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)

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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