<errno.h> is aheader file in thestandard library of theC programming language. It definesmacros for reporting and retrieving error conditions using the symbolerrno (short form for "error number").^[1]

errno acts like an integer variable. A value (the error number) is stored inerrno by certainlibrary 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, typicallyNULL for functions that returnpointers, and-1 for functions that return integers. A few functions require the caller to preseterrno to zero and test it afterwards to see if an error was detected.

Theerrno macro expands to anlvalue with typeint, sometimes with theextern and/orvolatile type specifiers depending upon the platform.^[3] Originally this was a static memory location, but macros are almost always used today to allow formulti-threading, so that each thread will see its ownthread-local error number.

The header file also defines macros that expand to integer constants that represent the error codes. TheC standard library only requires three to be defined:^[2]

POSIX compliantoperating systems likeAIX,Linux orSolaris include many other error values, many of which are used much more often than the above ones, such asEACCES 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 ofUnix system manuals, named intro(2), lists all errno.h macros, but this is not the case withLinux, where these macros are instead listed in the errno(3).^[7]

Anerrno can be translated to a descriptive string usingstrerror (defined in<string.h>) or a BSD extension calledsys_errlist. The translation can be printed directly to thestandard error stream usingperror (defined in<stdio.h>). Asstrerror in many Unix-like systems is not thread-safe, a thread-safe versionstrerror_r is used, but conflicting definitions from POSIX and GNU makes it even less portable than thesys_errlist table.^[8]

TheGNU C library (GLIBC) provides the additionalPOSIX error values macros in the header file<errno.h>.^[9] These are the descriptions of the macros provided bystrerror.

The macro names and meanings for error codes are defined in thePOSIX Standards definition however the numeric values are NOT, though by convention the values appear to be the same across different versions of Unix.^{[citation needed]} Programs should not rely on specific numeric values and should test code using the macro names specified in the ERRORS section of theman page of the associated function. For sourcecode 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. Retrieved27 February 2015.

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

• C standard library headers
• POSIX error codes

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