Standards, Defacto Standards and Manual References

International Standard for the C Programming language

ISO/IEC 9899:2011 - aka C11 The third formal standard (current)
ISO/IEC 9899:1999 - aka C99 The second formal standard
ISO/IEC 9899:1990 - aka C90 The first formal standard

Application Programming Interface (API) Standards - Single UNIX Specification

Single UNIX Specification v4 Version 4, 2016 edition
Single UNIX Specification v3 Version 3, 2004 edition
Single UNIX Specification v2 Version 2, 1998 edition

The GNU project development resources

The GNU C Library Last updated 08/01/2018, for glibc 2.28
Using the GNU Compiler Collection (GCC) 2018 - for gcc version 8.2.0
Debugging with GDB 2018 - Version 8.2.50.20180929-git
GNU make May 2016 - GNU make Version 4.2
GNU coding standards Last updated 08/26/2018
The GNU C Reference Manual 2015 - v0.2.5 release