What is errno what is it for how is it used?

errno is a preprocessor macro used for error indication. It expands to a static (until C++11) thread-local (since C++11) modifiable lvalue of type int. Several standard library functions indicate errors by writing positive integers to errno .

What does errno 0 mean?

A value (the error number) is stored in errno by certain library functions when they detect errors. At program startup, the value stored is zero. Library functions store only values greater than zero. Any library function can alter the value stored before return, whether or not they detect errors.

What does errno 1 mean?

The errno(1) command can also be used to look up individual error numbers and names, and to search for errors using strings from the error description, as in the following examples: $ errno 2 ENOENT 2 No such file or directory $ errno ESRCH ESRCH 3 No such process $ errno -s permission EACCES 13 Permission denied List …

Is errno thread safe?

Yes, it is thread safe. On Linux, the global errno variable is thread-specific. POSIX requires that errno be threadsafe.

Should I set errno?

A program may set and check errno for these library functions but is not required to do so. The program should not check the value of errno without first verifying that the function returned an error indicator.

How do I access errno?

Your program can use the strerror() and perror() functions to print the value of errno. The strerror() function returns a pointer to an error message string that is associated with errno. The perror() function prints a message to stderr.

Does errno get reset?

Initializing Errno Your program should always initialize errno to 0 (zero) before calling a function because errno is not reset by any library functions. Check for the value of errno immediately after calling the function that you want to check. You should also initialize errno to zero after an error has occurred.

Where is Errno stored?

errno. h defines macros to report error conditions through error codes stored in a static location called errno . A value is stored in errno by certain library functions when they detect errors. At program startup, the value stored is zero.

Do I need to set errno to 0?

To detect an error, an application must set errno to 0 before calling the function and check whether it is nonzero after the call. Affected functions include strcoll() , strxfrm() , strerror() , wcscoll() , wcsxfrm() , and fwide() . The C Standard allows these functions to set errno to a nonzero value on success.

Should errno be cleared?

Clearing errno is only required when the function you called doesn’t signify an existence of an error with its return value (e.g., a function whose return type is void and a function whose return value’s domain is entirely meaningful for normal and not-error value).