The GNU C Library, described in this document, defines all of the library functions that are specified by the ISO C standard, as well as additional features specific to POSIX and other derivatives of the Unix operating system, and extensions specific to GNU systems. The purpose of this manual is to tell you how to use the facilities of the GNU C Library. We have mentioned which features belong to which standards to help you identify things that are potentially non-portable to other systems. But the emphasis in this manual is not on strict portability. As the GNU C Library became such a big project over the years, we had to split this reference manual in two parts that are two separate physical books. To keep it consistent with the digital manual, the references and page numbers cover both physical books as it were one. Therefore please note that you probably want to have both parts.