|C standard library|
errno.h is a header file in the standard library of the C programming language. It defines macros for reporting and retrieving error conditions through error codes stored in a static memory location called
errno (short for "error number").
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. Most functions indicate that they detected an error by returning a special value, typically NULL for functions that return pointers, and −1 for functions that return integers. A few functions require the caller to preset
errno to zero and test it afterwards to see if an error was detected.
errno macro expands to an lvalue with type
volatile int, containing the last error code generated in any function using the errno facility.  Originally this was a static memory location, but macros are almost always used today to allow for multi-threading, each thread will see its own error number.
The header file also defines macros that expand to integer constants that represent the error codes. The C standard library only requires three to be defined:
- Results from a parameter outside a function's domain, for example
- Results from a result outside a function's range, for example
EILSEQ [Required since 1994 Amendment 1 to C89 standard]
- Results from an illegal byte sequence, for example
mbstowcs(buf,"\xff", 1)(if the multi-byte encoding is UTF-8).
POSIX compliant operating systems like AIX, Linux or Solaris include many other error values, many of which are used much more often than the above ones, such as EACCES for when a file cannot be opened for reading.