This chapter provides information that you can use to help you port applications for K&R (Kernigan and Ritchie) style C to conform with ISO/IEC C standard. The chapter was written specifically to aid in porting to 9899:1990 ISO/IEC C standard, but can effectively aid in porting to the 9899:1999 or 9899:2011 versions of ISO/IEC C standards.
This version of the C compiler defaults to accepting code that conforms with 9899:2011, i.e. -std=c11. To compile for 9899:1999 use -std=c99, and to compile for 9899:1990 use -std=c89.
To compile a program that is maximally conformant to the ISO C dialect specified by the -std flag, you must also specify the -pedantic flag.