JavaScript is required to for searching.
Skip Navigation Links
Exit Print View
Oracle Solaris Studio 12.2: C User's Guide
search filter icon
search icon

Document Information


1.  Introduction to the C Compiler

2.  C-Compiler Implementation-Specific Information

3.  Parallelizing C Code

4.  lint Source Code Checker

5.  Type-Based Alias Analysis

6.  Transitioning to ISO C

6.1 Basic Modes

6.1.1 -Xc

6.1.2 -Xa

6.1.3 -Xt

6.1.4 -Xs

6.2 A Mixture of Old- and New-Style Functions

6.2.1 Writing New Code

6.2.2 Updating Existing Code

6.2.3 Mixing Considerations

6.3 Functions With Varying Arguments

6.4 Promotions: Unsigned Versus Value Preserving

6.4.1 Background

6.4.2 Compilation Behavior

6.4.3 First Example: The Use of a Cast

6.4.4 Bit-fields

6.4.5 Second Example: Same Result

6.4.6 Integral Constants

6.4.7 Third Example: Integral Constants

6.5 Tokenization and Preprocessing

6.5.1 ISO C Translation Phases

6.5.2 Old C Translation Phases

6.5.3 Logical Source Lines

6.5.4 Macro Replacement

6.5.5 Using Strings

6.5.6 Token Pasting

6.6 const and volatile

6.6.1 Types, Only for lvalue

6.6.2 Type Qualifiers in Derived Types

6.6.3 const Means readonly

6.6.4 Examples of const Usage

6.6.5 volatile Means Exact Semantics

6.6.6 Examples of volatile Usage

6.7 Multibyte Characters and Wide Characters

6.7.1 Asian Languages Require Multibyte Characters

6.7.2 Encoding Variations

6.7.3 Wide Characters

6.7.4 Conversion Functions

6.7.5 C Language Features

6.8 Standard Headers and Reserved Names

6.8.1 Standard Headers

6.8.2 Names Reserved for Implementation Use

6.8.3 Names Reserved for Expansion

6.8.4 Names Safe to Use

6.9 Internationalization

6.9.1 Locales

6.9.2 The setlocale() Function

6.9.3 Changed Functions

6.9.4 New Functions

6.10 Grouping and Evaluation in Expressions

6.10.1 Definitions

6.10.2 The K&R C Rearrangement License

6.10.3 The ISO C Rules

6.10.4 The Parentheses

6.10.5 The As If Rule

6.11 Incomplete Types

6.11.1 Types

6.11.2 Completing Incomplete Types

6.11.3 Declarations

6.11.4 Expressions

6.11.5 Justification

6.11.6 Examples

6.12 Compatible and Composite Types

6.12.1 Multiple Declarations

6.12.2 Separate Compilation Compatibility

6.12.3 Single Compilation Compatibility

6.12.4 Compatible Pointer Types

6.12.5 Compatible Array Types

6.12.6 Compatible Function Types

6.12.7 Special Cases

6.12.8 Composite Types

7.  Converting Applications for a 64-Bit Environment

8.  cscope: Interactively Examining a C Program

A.  Compiler Options Grouped by Functionality

B.  C Compiler Options Reference

C.  Implementation-Defined ISO/IEC C99 Behavior

D.  Supported Features of C99

E.  Implementation-Defined ISO/IEC C90 Behavior

F.  ISO C Data Representations

G.  Performance Tuning

H.  The Differences Between K&R Solaris Studio C and Solaris Studio ISO C


6.8 Standard Headers and Reserved Names

Early in the standardization process, the ISO Standards Committee chose to include library functions, macros, and header files as part of ISO C.

This section presents the various categories of reserved names and some rationale for their reservations. At the end is a set of rules to follow that can steer your programs clear of any reserved names.

6.8.1 Standard Headers

The standard headers are:

Table 6-2 Standard Headers


Most implementations provide more headers, but a strictly conforming 1990 ISO/IEC C program can only use these.

Other standards disagree slightly regarding the contents of some of these headers. For example, POSIX (IEEE 1003.1) specifies that fdopen is declared in stdio.h. To allow these two standards to coexist, POSIX requires the macro _POSIX_SOURCE to be #defined prior to the inclusion of any header to guarantee that these additional names exist. In its Portability Guide, X/Open has also used this macro scheme for its extensions. X/Open’s macro is _XOPEN_SOURCE.

ISO C requires the standard headers to be both self-sufficient and idempotent. No standard header needs any other header to be #included before or after it, and each standard header can be #included more than once without causing problems. The Standard also requires that its headers be #included only in safe contexts, so that the names used in the headers are guaranteed to remain unchanged.

6.8.2 Names Reserved for Implementation Use

The Standard places further restrictions on implementations regarding their libraries. In the past, most programmers learned not to use names like read and write for their own functions on UNIX Systems. ISO C requires that only names reserved by the Standard be introduced by references within the implementation.

Thus, the Standard reserves a subset of all possible names for implementations to use. This class of names consists of identifiers that begin with an underscore and continue with either another underscore or a capital letter. The class of names contains all names matching the following regular expression:


Strictly speaking, if your program uses such an identifier, its behavior is undefined. Thus, programs using _POSIX_SOURCE (or _XOPEN_SOURCE) have undefined behavior.

However, undefined behavior comes in different degrees. If, in a POSIX-conforming implementation you use _POSIX_SOURCE, you know that your program’s undefined behavior consists of certain additional names in certain headers, and your program still conforms to an accepted standard. This deliberate loophole in the ISO C standard allows implementations to conform to seemingly incompatible specifications. On the other hand, an implementation that does not conform to the POSIX standard is free to behave in any manner when encountering a name such as _POSIX_SOURCE.

The Standard also reserves all other names that begin with an underscore for use in header files as regular file scope identifiers and as tags for structures and unions, but not in local scopes. The common practice of having functions named _filbuf and _doprnt to implement hidden parts of the library is allowed.

6.8.3 Names Reserved for Expansion

In addition to all the names explicitly reserved, the 1990 ISO/IEC C standard also reserves (for implementations and future standards) names matching certain patterns:

Table 6-3 Names Reserved for Expansion

Reserved Name Pattern
current function names[fl]

In the above lists, names that begin with a capital letter are macros and are reserved only when the associated header is included. The rest of the names designate functions and cannot be used to name any global objects or functions.

6.8.4 Names Safe to Use

There are four simple rules you can follow to keep from colliding with any ISO C reserved names:

These rules are just a general guideline to follow, as most implementations will continue to add names to the standard headers by default.