gcc -Wall -Wextra -Werror: enable compiler warnings

gcc -Wall -Wextra turns on the warnings that catch most real bugs in C code: unused variables, missing return values, implicit declarations, format string mismatches, comparisons between signed and unsigned values, and uninitialized variables. Despite its name, -Wall is not "all warnings"; -Wextra adds more. -Werror makes every warning an error, which is useful in CI. Clang accepts the same flags.

Usage

gcc -Wall -Wextra -o app main.c                    # recommended baseline
gcc -Wall -Wextra -Wpedantic -std=c17 -o app main.c # also warn about non-standard extensions
gcc -Wall -Wextra -Werror -o app main.c            # fail the build on any warning
gcc -Wall -Wextra -Werror=format -o app main.c     # make only one warning group fatal
gcc -Wall -Wextra -Wno-unused-parameter -o app main.c  # turn one warning off

What each flag adds

Compiling the example below with gcc -Wall -Wextra reports the unused variable, the signed/unsigned comparison and the format mismatch; plain gcc stays quieter. GCC names the option that produced each warning at the end of the message, for example [-Wunused-variable], so you know which -Wno-... would silence it.

Example

#include <stdio.h>

int main(void) {
    int unused;
    unsigned n = 3;
    for (int i = 0; i < n; i++)    /* signed vs unsigned comparison: -Wextra */
        printf("%s\n", i);         /* %s with an int: -Wformat (in -Wall) */
    return 0;
}

Errors by default in GCC 14

Some diagnostics used to be warnings that only showed up with -Wall, and now stop the build by default. GCC 14 turned implicit function declarations, implicit int, integer/pointer conversions and incompatible pointer types into errors for C99 and later, as described in its porting notes. A missing #include that older GCC merely warned about now fails with error: implicit declaration of function (see implicit declaration of function). The right fix is the missing header or prototype; -fpermissive or -Wno-error=... only hides the problem for legacy code.

Silencing a warning on purpose

Prefer fixing the code over turning a warning off. When a warning is a false positive, narrow the exception as far as possible:

• Cast an intentionally unused parameter to void: (void)argc;, or mark it with __attribute__((unused)) or C23's [[maybe_unused]].
• Disable one warning for one file with -Wno-NAME in that file's compile options.
• Disable it for a few lines with #pragma GCC diagnostic push, #pragma GCC diagnostic ignored "-Wformat" and #pragma GCC diagnostic pop; Clang accepts the same pragmas.
• Keep a warning visible but non-fatal under -Werror with -Wno-error=NAME.

In a build system

With CMake, add the flags per target so they do not leak into third-party code: target_compile_options(app PRIVATE -Wall -Wextra). Wrap them in a generator expression or a compiler check if the project also builds with MSVC, which does not understand them. With a plain Makefile, append them to CFLAGS.

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