mirror of https://github.com/mkerrisk/man-pages
Various improvements.
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Michael Kerrisk
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@ -112,12 +112,23 @@ feature test macro requirements (this example from
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.fi
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.RE
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.PP
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This syntax is used in cases where only a single
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This format is employed in cases where only a single
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feature test macro can be used to expose the function
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declaration, and that macro is not defined by default.
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.SS Feature test macros understood by glibc
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Linux/glibc understands the following feature test macros:
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.\" FIXME ? add _ATFILE_SOURCE to this list
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.TP
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.B __STRICT_ANSI__
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ISO Standard C.
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This macro is implicitly defined by
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.BR gcc (1)
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when invoked with, for example, the
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.I -std=c99
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or
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.I -ansi
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flag.
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.TP
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.B _POSIX_C_SOURCE
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Defining this macro with the value 1 causes header files to expose
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@ -139,24 +150,12 @@ with the value 1.
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.B _XOPEN_SOURCE
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Defining this macro with any value causes header files to expose
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definitions conforming to POSIX.1, POSIX.2, and XPG4.
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Defining with the value 500 or greater additionally exposes
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Defining this macro with the value 500 or greater additionally exposes
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definitions for SUSv2 (UNIX 98).
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Defining with the value 600 or greater additionally exposes
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Since glibc 2.2, defining this macro with the value 600
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or greater additionally exposes
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definitions for SUSv3 (UNIX 03; i.e., the POSIX.1-2001 base specification
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plus the XSI extension) and C 99 definitions.
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.IP
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Defining
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.B _XOPEN_SOURCE
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also implicitly defines
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.BR _POSIX_C_SOURCE :
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with the value 2, if
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.B _XOPEN_SOURCE
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is less than 500;
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with the value 199506L, if 500 <=
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.B _XOPEN_SOURCE
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< 600; or with the value 200112L if
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.B _XOPEN_SOURCE
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>= 600.
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.TP
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.B _XOPEN_SOURCE_EXTENDED
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If this macro is defined with the value 1, and the
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@ -221,16 +220,25 @@ System V-derived definitions.
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(SVID == System V Interface Definition; see
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.BR standards (7).)
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.TP
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.B _ATFILE_SOURCE
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Defining this macro with any value cause header files to expose
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declarations of a range of functions with the suffix "at";
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see
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.BR openat (2).
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.TP
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.B _GNU_SOURCE
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Defining this macro (with any value) is equivalent to defining
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.BR _BSD_SOURCE ,
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.BR _SVID_SOURCE ,
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.BR _ATFILE_SOURCE ,
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.BR _LARGEFILE64_SOURCE ,
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.BR _ISOC99_SOURCE
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.BR _ISOC99_SOURCE ,
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.BR _XOPEN_SOURCE_EXTENDED ,
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.BR _POSIX_SOURCE ,
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.BR _POSIX_C_SOURCE
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with the value 200112L, and
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with the value 200112L (199506L in glibc versions before 2.5), and
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.BR _XOPEN_SOURCE
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with the value 600.
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with the value 600 (500 in glibc versions before 2.2).
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In addition, various GNU-specific extensions are also exposed.
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Where standards conflict, BSD definitions are disfavored.
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.TP
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@ -290,24 +298,70 @@ and result in a run-time error if the check fails.
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Use of this macro requires compiler support, available with
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.BR gcc (1)
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since version 4.0.
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.SS Feature test macros defined by default
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.SS Default definitions, implicit definitions, and combining definitions
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.PP
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The following macros are defined by default:
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The following feature test macros are defined by default:
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.BR _BSD_SOURCE ,
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.BR _SVID_SOURCE ,
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.BR _POSIX_SOURCE ,
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and
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.BR _POSIX_C_SOURCE =199506L.
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If individual macros are defined, then other macros are disabled
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unless they are also explicitly defined.
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(Exception: if
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.BR _POSIX_C_SOURCE =200112L
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(199506L in glibc versions before 2.5).
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.PP
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If any of
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.BR __STRICT_ANSI__ ,
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.BR _ISOC99_SOURCE ,
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.BR _POSIX_SOURCE ,
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.BR _POSIX_C_SOURCE ,
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.BR _XOPEN_SOURCE ,
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.BR _XOPEN_SOURCE_EXTENDED ,
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.BR _BSD_SOURCE ,
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or
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.BR _SVID_SOURCE
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is explicitly defined, then
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.BR _BSD_SOURCE ,
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and
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.BR _SVID_SOURCE
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are not defined by default.
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If
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.BR _POSIX_SOURCE
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and
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.BR _POSIX_C_SOURCE
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is not otherwise defined,
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then it is always defined with the value 200112L
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(199506L in glibc versions before 2.4),
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unless the compiler is invoked in one of its standard modes, for example, the
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.I -std=c99
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flag.)
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are not explicitly defined,
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and either
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.BR __STRICT_ANSI__
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is not defined or
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.BR _XOPEN_SOURCE
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is defined with a value of 500 or more, then
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.RS 6
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.IP * 3
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.B _POSIX_SOURCE
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is defined with the value 1; and
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.IP *
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.B _POSIX_C_SOURCE
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is defined with one of the following values:
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.RS 6
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.IP \(bu 3
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2,
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if
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.BR XOPEN_SOURCE
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is defined with a value less than 500;
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.IP \(bu
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199506L,
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if
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.BR XOPEN_SOURCE
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is defined with a value greater than or equal to 500 and less than 600;
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or
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.IP \(bu
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200112L (199506L in glibc versions before 2.5),
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if
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.BR XOPEN_SOURCE
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is undefined, or
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is defined with a value greater than or equal to 600.
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.RE
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.RE
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.PP
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Multiple macros can be defined; the results are additive.
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.SH CONFORMING TO
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POSIX.1 specifies
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@ -317,9 +371,11 @@ and
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.BR _XOPEN_SOURCE .
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.BR _XOPEN_SOURCE_EXTENDED
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was specified by XPG4v2 (aka SUSv1).
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.BR _FILE_OFFSET_BITS
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is not specified by any standard,
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but is employed on some other implementations.
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.BR _BSD_SOURCE ,
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.BR _SVID_SOURCE ,
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.BR _GNU_SOURCE ,
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