The following options are useful when GCC is generated with the capability for more than one debugging format. Some of them warn about constructions that users generally do not consider questionable, but which occasionally you might wish to check for; others warn about constructions that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to suppress the warning.
Some of them are enabled by -Wextra but many of them must be enabled individually. This applies regardless to whatever sort of output is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code. If -o is not specified, the default is to put an executable file in a. You can use -u multiple times with different symbols to force loading of additional library modules. Options for Directory Search These options specify directories to search for header files, for libraries and for parts of the compiler: -l library -l library Search the library named library when linking.
The second alternative with the library as a separate argument is only for POSIX compliance and is not recommended. It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Email Required, but never shown. The Overflow Blog. Does ES6 make JavaScript frameworks obsolete?
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Question feed. Stack Overflow works best with JavaScript enabled. Accept all cookies Customize settings. If you would like to follow along compiling the examples presented in this article, all the code can be found on Github 1. To compile the examples, you will need the 8. Compiler warnings are the first line of defense against catching various errors that could arise at runtime and enforcing conventions that lead to a more readable codebase.
It is a lot easier to fix issues at compilation time than when unexpected behavior is encountered on your platform. GCC and Clang have several warning flags which will enable a collection of useful checks which we will explore in more detail below. Trying to fix thousands of warnings all at the same time is a recipe for introducing regressions or upsetting fellow developers trying to review your change. The compiler flag -Wall enables a base set of warnings generally agreed upon as being useful and easy to fix.
The exact list of flags varies between compiler versions but can easily be found by consulting the GCC documentation for the compiler version you are using 3. The issues can easily be caught at compile time by using the -Wall flag.
This can be incredibly useful to ensure that no new warnings slip into a codebase. When working with a team, not everyone may be as excited about fixing compiler warnings as you. I really consider this flag a must have to prevent code with warnings from being merged to master. Enabling -Werror does mean that when the compiler version changes, new compilation errors may appear since new sets of warnings are usually added to flags like -Wall and pre-existing checks are improved and can catch new errors.
That way everyone will see the exact same errors and behavior. As new versions become available, one team member can update to the new version, fix up any new issues, and then merge an update requiring that compiler version be used by the rest of the team. Sometimes, there may be warnings you do not wish to trigger an error or even a warning.
These can easily be controlled with the following compiler options:. By design, Clang generally matches the set of compiler flag options available in the GNU toolchain, but there are a few that are different.
One flag there are some strong opinions 4 about is -Weverything which literally enables every single warning available to Clang! Many suggest you should never flip on this flag in a build. The strategy I prefer, followed by other projects such as CppUTest 5 , is to enable the warning but liberally disable warnings from it that are not useful. For example,. In this section we will explore some of the best ones.
Shadowing variables at the very least makes code difficult to read and often can be indicative of a bug because the code is not operating on the value the programmer expects.
These can easily be detected by using -Wshadow. The C types float and double are not the same! A float represents an IEEE single precision floating point number whereas a double represents double precision floating point number.
This means when a double is used, the compiler will need to emulate the operation with a function instead of a native instruction. Using a double not only affects performance since the instruction is not supported natively but also bloats the codesize because it pulls in some not-so-small functions. The reason a double gets used above is because according to the C standard a floating point constant, i. To use a float, the constant must be declared as 2.
Fortunately, a warning can be enabled to easily catch when this implicit double promotion takes place:. By default, -Wall enables -Wformat which turns on a variety of checks to make sure any formatter strings used in standard library routines i. One interesting option is upgrading the -Wformat check level to 2 which includes a couple additional checks such as -Wformat-security.
This is accomplished using the format attribute:. Another interesting set of formatter warning flags not enabled by default is -Wformat-overflow and -Wformat-truncation.
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