Errno.h
C standard library header
<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").
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. 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.
“Errno.h” enters the record as c standard library header. Crown Archives preserves that source wording while asking what Errno, standard and library can confirm, complicate or overturn.
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“Errno.h” is worth following because a concise public description often conceals a longer documentary argument. Here, Errno, standard and library provides the most credible route into that argument.
Place names and jurisdictional language are key evidence: both can reveal earlier catalogue descriptions and overlooked record series. The source revision retrieved here is dated Jun 16, 2026. The linked authority identifier is Q306452. None of the 0 selected statements returned an explicit reference.
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This entry incorporates text from “Errno.h” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.