Detailed explanation of errno

effect

errno yes error number Abbreviation , It means the system call error code .

If the system call returns success ,errno It is possible, but not necessarily 0; When the system call fails ,errno It must be set to the corresponding error number . therefore , OCD patients can call the system call before , Manual will errno Set up 0. however , If the system call returns success , Who will be free to see errno Well ？

Correlation function

Generally speaking , Do we need to know each errno What does the number of mean . We will use two C Library functions help us translate errno That's what it means .

They are ：

#include <string.h>
char *strerror(int errnum);

#include <stdio.h>
void perror(const char *msg);

Multithreading

Before multithreading ,errno It's actually a global variable . It's easy to understand , But it's past tense .

Because every thread may call system calls , Then the subsequent mistakes , It will cover the previous errors . If we can't guarantee errno The reliability of the , that errno What's the point? ？

therefore , It has changed .

1. When multithreading , Need to include header file #include <errno.h>
2. errno It is no longer a global variable , Now it has become a macro

although errno Changed , But it's still an lvalue （ The left value can be written as “ The left value = Right value ” In the form of ）.
What the hell is this macro ？ Don't wait to decompose , Let's keep looking ：

extern int *__errno_location(void);

#define errno (*__errno_location())

First , Declared functions from outside __errno_location(); Watch out! , The return value of this function is a function pointer , This pointer points to the thread's own errno The location of , By dereferencing this pointer , You can access the thread only errno.

therefore ,errno Is defined as (*__errno_location()).