What is errno in C?
You may see references to errno in C code. This is defined in <errno.h> as:
#define errno (*__error())That is, errno dereferences a call to a function __error. This is defined as
extern int * __error(void);That is, a function which takes no arguments and returns a non-null pointer to the error number.
The docs say
errnois defined by the ISO C standard to be a modifiable lvalue of typeint
That is, we should be able to assign to errno. This is possible, because *V is also an lvalue.
These details aside, errno is basically a global int value. Its value is set by some system calls. It’s an assistant to the “return -1” idiom.
Tagged #c, #programming.
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