Difference between C standard library and C POSIX library

Question

I\'m a little confused by \"C standard lib\" and \"C POSIX lib\", because I found that, many header files defined in \"C POSIX lib\" are also part of \"C standard lib\".

Answer 1

POSIX 7 quote

http://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap01.html#tag_14_01

1.1 Relationship to Other Formal Standards

Great care has been taken to ensure that this volume of POSIX.1-2008 is fully aligned with the following standards:

ISO C (1999) ISO/IEC 9899:1999, Programming Languages - C, including ISO/IEC 9899:1999/Cor.1:2001(E), ISO/IEC 9899:1999/Cor.2:2004(E), and ISO/IEC 9899:1999/Cor.3.

Parts of the ISO/IEC 9899:1999 standard (hereinafter referred to as the ISO C standard) are referenced to describe requirements also mandated by this volume of POSIX.1-2008. Some functions and headers included within this volume of POSIX.1-2008 have a version in the ISO C standard; in this case CX markings are added as appropriate to show where the ISO C standard has been extended (see Codes). Any conflict between this volume of POSIX.1-2008 and the ISO C standard is unintentional.

I have listed some major API extensions at: I never really understood: what is POSIX?