GCC C/C++ compiler flags
By default, the gcc and g++ compilers issue some warnings, but it can produce more. It is always good practice to switch on "all warnings" with -Wall. Contrary to expectations, -Wall doesn't activate all warnings though. A number of extra warnings on top of those enabled by -Wall can be switched on by specifying -Wextra. It is highly recommended to always use these two compiler options, and eliminate all warnings reported by them, i.e.,
$ gcc -Wall -Wextra ...
Language specification conformance
The gcc compiler can also check for language specification conformity." The -Wpedantic flag will activate this, and you should specify the specification it should check, i.e.,
-std=c89-std=c99-std=c11-std=c17
For g++, these values are:
-std=c++98-std=c++11-std=c++14-std=c++17
For example, to check for C++14 compliance, use
$ g++ ... -Wpedantic -std=c++14 ...
Additional warnings
It is almost always a bad idea to test floating point numbers for equality, the -Wfloat-equal will warn you about this.
Although not necessarily a problem, shadowing outer scope variable declarations may lead to confusion. The -Wshadow compiler flag will alert you to this.
A warning can be issued when a switch statement is based on named enum values, but not all of those are cases in the switch. To enable, compile with -Wswitch-enum.
When you cast away a const qualifier from a pointer, you may inadvertently modify a value you really shouldn't. The compiler can check for this if you add the -Wcast-qual flag.
For C code, the options -Wbad-function-cast will warn when the result of a function call is cast to an inappropriate type.
Although some conversions from a larger to a smaller type are legal, they may be unintended, use the -Wconversion flag to be warned about these.
The preprocessor can also issue warnings when required. -Wundef will warn when a macro variable that is used has not been assigned a value. -Wunused-macros will warn about macros that were defined in the main file, but never used.
If-statements with many many branches are generally hard to read, and it is easy to make mistakes by repeating the same condition twice, or even duplicate code in branches. This typically results from copy/pasting code fragments. The options that help you detect such issues are -Wduplicated-cond and -Wduplicated-branches respectively.
Although the option -Wmaybe-uninitialized is activated by -Wall it is worth noting that it is only enabled when the compiler also optimises the code, i.e., at least at -O1.
C++ specific warnings
Specifically for C++, the -Wnon-virtual-dtor option is very useful as a class derived from a virtual base class that has no virtual destructor may lead to memory leaks.
The -Woverloaded-virtual option will warn you when a virtual function in a base class is overloaded, rather than overridden in the derived class, which may not have been your intention.
C++ has a safer way of performing type cast than C using static_cast, dynamic_cast and so on. It is good practice to use appropriate C++ style casting, rather than C h style casts. Warnings can be generated using -Wold-style-cast.