Where is errno stored

Warning: Many library functions may set errno to some meaningless non-zero value even if they did not encounter any errors, and even if they return error codes directly.

Therefore, it is usually incorrect to check whether an error occurred by inspecting the value of errno. The proper way to check for error is documented for each function. There are a few library functions, like sqrt and atan , that return a perfectly legitimate value in case of an error, but also set errno.

For these functions, if you want to check to see whether an error occurred, the recommended method is to set errno to zero before calling the function, and then check its value afterward. All the error codes have symbolic names; they are macros defined in errno.

This is the case in GNU, where it is a per-thread variable. This redeclaration using the macro still works, but it will be a function declaration without a prototype and may trigger a -Wstrict-prototypes warning.

You don't need to do anything. If you are using Linux 2. Just start programming. Show 7 more comments. Yes Errno isn't a simple variable anymore, it's something complex behind the scenes, specifically for it to be thread-safe.

DigitalRoss DigitalRoss k 23 23 gold badges silver badges bronze badges. Therefore, the signal handlers should save and restore errno. Hi Craig, thanks for the info about the typo, now is corrected.

Regarding the other issue, I am not sure if I understand correctly what you are asking for. Whatever call that modifies errno in the signal handler could interfere with the errno being used by the same thread that was interrupted e. Jonathan Leffler k gold badges silver badges bronze badges. Jonathan Leffler Jonathan Leffler k gold badges silver badges bronze badges. Please refer to discussion on other answer for same question.

But you still have to be careful - on each platform - to ensure you get the desired behaviour. Is there a piece of documentation that indicates what standard ie: C99, ANSI, etc or at least which compilers ie: GCC version and onward that support this feature, and whether or not it is a default?

Thank you. The C11 standard says If a macro definition is suppressed in order to access an actual object, or a program defines an identifier with the name errno , the behavior is undefined. The main text continues: The value of errno in the initial thread is zero at program startup the initial value of errno in other threads is an indeterminate value , but is never set to zero by any library function.

Show 1 more comment. Because errno holds the value for the last call that set it, this value may be changed by succeeding calls. Run-time library calls that set errno on an error do not clear errno on success. On an error, errno is not necessarily set to the same value as the error code returned by a system call. We recommend that you use the more secure, functional versions instead of the global macros, as shown here:.

For a complete list, see errno Constants. Namespaces Book Discussion. Views Read Edit Edit source View history. Reading room forum Community portal Bulletin Board Help out! Policies and guidelines Contact us.