1.7 Error Handling

When an error occurs in one of the UNIX System functions, a negative value is often returned, and the integer errno is usually set to a value that tells why.

But in an environment that supports threads, the process address space is shared among multiple threads, and each thread needs its own local copy of errno to prevent one thread from interfering with another. Linux, for example, supports multithreaded access to errno by defining it as

extern int *_ _errno_location(void);
#define errno (*_ _errno_location())

NOTE: tag-per-thread private私有-errno

Error Recovery

NOTE:

对error handling的总结非常好

The errors defined in <errno.h> can be divided into two categories: fatal and nonfatal.

A fatal error has no recovery action. The best we can do is print an error message on the user ’s screen or to a log file, and then exit.

Nonfatal errors, on the other hand, can sometimes be dealt with more robustly. Most nonfatal errors are temporary, such as a resource shortage(不足), and might not occur when there is less activity on the system. Resource-related nonfatal errors include EAGAIN, ENFILE, ENOBUFS, ENOLCK, ENOSPC, EWOULDBLOCK, and sometimes ENOMEM. EBUSY can be treated as nonfatal when it indicates that a shared resource is in use. Sometimes, EINTR can be treated as a nonfatal error when it interrupts a slow system call (more on this in Section 10.5).

The typical recovery action for a resource-related nonfatal error is to delay and retry later. This technique can be applied in other circumstances. For example, if an error indicates that a network connection is no longer functioning, it might be possible for the application to delay a short time and then reestablish the connection. Some applications use an exponential backoff algorithm, waiting a longer period of time in each subsequent iteration.

Ultimately, it is up to the application developer to determine the cases where an application can recover from an error. If a reasonable recovery strategy can be used, we can improve the robustness of our application by avoiding an abnormal exit.

NOTE:

相关内容

APUE 12.6 Thread-Specific Data

APUE 10.6 Reentrant Functions

APUE 12.5 Reentrancy

APUE 8.9 Race Conditions