errno.h

<errno.h> is a header file in the standard library of the C programming language. It defines macros for reporting and retrieving error conditions using the symbol errno (short form for "error number").^[1]

errno acts like an integer variable. 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.^[2] 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.

The errno macro expands to an lvalue with type int, sometimes with the extern and/or volatile type specifiers depending upon the platform.^[3] Originally this was a static memory location, but macros are almost always used today to allow for multi-threading, so that each thread will see its own thread-local 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:^[2]

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.^[5] C++11 additionally defines many of the same values found within the POSIX specification.^[6]

Traditionally, the first page of Unix system manuals, named intro(2), lists all errno.h macros, but this is not the case with Linux, where these macros are instead listed in the errno(3).^[7]

An errno can be translated to a descriptive string using strerror (defined in <string.h>) or a BSD extension called sys_errlist. The translation can be printed directly to the standard error stream using perror (defined in <stdio.h>). As strerror in many Unix-like systems is not thread-safe, a thread-safe version strerror_r is used, but conflicting definitions from POSIX and GNU makes it even less portable than the sys_errlist table.^[8]

The GNU C library (GLIBC) provides the additional POSIX error values macros in the header file <errno.h>.^[9] These are the descriptions of the macros provided by strerror.

The macro names and meanings for error codes are defined in the POSIX Standards definition however the numeric values are NOT, though by convention the values appear to be the same across different versions of Unix. Programs should not rely on specific numeric values and should test code using the macro names specified in the ERRORS section of the man page of the associated function. For source code readability and portability the use of the standard macro names in code is highly recommended.^[11][12]

• perror
• strerror

• Stevens, W. Richard; Rago, Stephen A. (May 24, 2013). Advanced Programming in the UNIX Environment (Third ed.). Addison-Wesley Professional. ISBN 978-0321637734. Retrieved 27 February 2015.

• intro(2) – FreeBSD System Calls Manual
• errno(3) – Linux Programmer's Manual – Library Functions from Manned.org
• GNU C library manual: Error codes
• Lists of errno values on Linux, both numeric and symbolic