Conformance has a three-fold definition:

  • strictly conforming program - uses only well-defined language constructs, that is constructs with a single behavior. It excludes unspecified, undefined, or implementation-defined behavior, and does not exceed any minimum implementation limit.
  • conforming program - acceptable to a conforming implementation.
  • conforming implementation -
    • A conforming hosted implementation shall accept any strictly conforming program.
    • A conforming freestanding implementation shall accept any strictly conforming program in which the use of the features specified in the library clause (clause 7) is confined to the contents of the freestanding standard library headers (see below).
    • A conforming implementation may have extensions (including additional library functions), provided they do not alter the behavior of any strictly conforming program.


The standard does not define any minimum implementation limit on translation units. A hosted environment has an operating system; a freestanding environment does not. A program running in a hosted environment may use all features described in the library clause (clause 7); a program running in a freestanding environment may use a subset of library features required by clause 4.

Freestanding standard library headers

All standard library features in every fully freestanding header are required to be provided by a freestanding implementation.

Some standard library headers are conditionally freestanding.

  • If the implementation predefines the macro __STDC_IEC_60559_BFP__ or __STDC_IEC_60559_BFP__, then
    • <math.h> and <fenv.h> are fully freestanding headers, and
    • <stdlib.h> is a partially freestanding header.

In a partially freestanding header, only a part of standard library features are required to be provided by a freestanding implementation.

  • strdup, strndup, strcoll, strxfrm, and strerror are not required to be provided by a freestanding implementation.
  • When __STDC_IEC_60559_BFP__ or __STDC_IEC_60559_BFP__ are predefined, in <stdlib.h>, only numeric conversion functions (atoX, strtoX, and strfromX) are required to be provided by a freestanding implementation.
(since C23)
Fully freestanding standard library headers
<float.h> Limits of floating-point types
<iso646.h> (C95) Alternative operator spellings
<limits.h> Ranges of integer types
<stdalign.h> (C11) alignas and alignof convenience macros
<stdarg.h> Variable arguments
<stdbool.h> (C99) Macros for boolean type
<stddef.h> Common macro definitions
<stdint.h> (C99) Fixed-width integer types
<stdnoreturn.h> (C11) noreturn convenience macro
Partially freestanding standard library headers
<string.h> (C23) String handling
Conditionally fully freestanding standard library headers
<fenv.h> (C23) Floating-point environment
<math.h> (C23) Common mathematics functions
Conditionally partially freestanding standard library headers
<stdlib.h> (C23) General utilities: memory management, program utilities, string conversions, random numbers, algorithms


  • C17 standard (ISO/IEC 9899:2018):
    • 4 Conformance (p: 4)
  • C11 standard (ISO/IEC 9899:2011):
    • 4 Conformance (p: 8-9)
  • C99 standard (ISO/IEC 9899:1999):
    • 4 Conformance (p: 7-8)
  • C89/C90 standard (ISO/IEC 9899:1990):
    • 1.7 Compliance

See also

C++ documentation for Freestanding and hosted implementation

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