ISO/ANSI Compliance

The ANSI and ISO/IEC standard for the C Programming Language specifies that there are two forms of conforming compiler implementations: hosted and freestanding.

From the ISO/IEC 9899:1990 standard:

A conforming hosted implementation shall accept any strictly conforming program.
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A conforming freestanding implementation shall accept any strictly conforming program that does not use complex types and in which the use of the features specified in the library clause is confined to the contents of the standard headers: <float.h>, <iso646.h>, <limits.h>, <stdarg.h>, <stdbool.h>, <stddef.h>, and <stdint.h>. A conforming implementation may have extensions (including additional library functions), provided they do not alter the behavior of any strictly conforming program.

Compilers from Keil are conforming freestanding implementations of the C Programming Language as specified in:

Compilers from Keil support some functionality of the ISO/IEC 9899:1999 standard as specified in:

ISO/IEC 9899:1999 Programming Languages — C

Some functionality of the conforming hosted implementation has been implemented as well.