errno

errno is a macro used in C and POSIX-like environments to indicate the error status of the most recently invoked library or system call. It expands to a modifiable lvalue of type int and is typically defined in errno.h. In multithreaded programs, errno is thread-local, so each thread maintains its own error value.

When a function returns an error (commonly -1 for many system calls), it may set errno to one of a defined set of error codes. The typical pattern is to check the function’s return value first and then inspect errno to determine the cause. To obtain a human-readable description, programmers commonly use strerror(errno) or perror.

The exact set of error codes is implementation-defined, but many common codes are standardized by POSIX, such as ENOENT (No such file or directory), EACCES (Permission denied), ENOMEM (Out of memory), EINVAL (Invalid argument), EBADF (Bad file descriptor), and EEXIST (File exists). These macros are declared in errno.h and map to implementation-defined integers.

Values of errno are not guaranteed to be zero on success; some functions do not modify errno on success, and some may leave it unchanged. A common practice is to initialize errno to 0 before a sequence of calls you want to diagnose, but this is not universally required. The exact behavior can vary between platforms and C libraries, so portable error handling typically relies on checking return values first and then using errno only when an error is detected.