conf.texi

一个C源代码分析器

@comment POSIX.1
@item _SC_CLK_TCK
Inquire about the parameter corresponding to @code{CLOCKS_PER_SEC};
@pxref{Basic CPU Time}.

@comment unistd.h
@comment POSIX.2
@item _SC_2_C_DEV
Inquire about whether the system has the POSIX.2 C compiler command,
@code{c89}.

@comment unistd.h
@comment POSIX.2
@item _SC_2_FORT_DEV
Inquire about whether the system has the POSIX.2 Fortran compiler
command, @code{fort77}.

@comment unistd.h
@comment POSIX.2
@item _SC_2_FORT_RUN
Inquire about whether the system has the POSIX.2 @code{asa} command to
interpret Fortran carriage control.

@comment unistd.h
@comment POSIX.2
@item _SC_2_LOCALEDEF
Inquire about whether the system has the POSIX.2 @code{localedef}
command.

@comment unistd.h
@comment POSIX.2
@item _SC_2_SW_DEV
Inquire about whether the system has the POSIX.2 commands @code{ar},
@code{make}, and @code{strip}.

@comment unistd.h
@comment POSIX.2
@item _SC_BC_BASE_MAX
Inquire about the maximum value of @code{obase} in the @code{bc}
utility.

@comment unistd.h
@comment POSIX.2
@item _SC_BC_DIM_MAX
Inquire about the maximum size of an array in the @code{bc}
utility.

@comment unistd.h
@comment POSIX.2
@item _SC_BC_SCALE_MAX
Inquire about the maximum value of @code{scale} in the @code{bc}
utility.

@comment unistd.h
@comment POSIX.2
@item _SC_BC_STRING_MAX
Inquire about the maximum size of a string constant in the
@code{bc} utility.

@comment unistd.h
@comment POSIX.2
@item _SC_COLL_WEIGHTS_MAX
Inquire about the maximum number of weights that can necessarily
be used in defining the collating sequence for a locale.

@comment unistd.h
@comment POSIX.2
@item _SC_EXPR_NEST_MAX
Inquire about the maximum number of expressions nested within
parentheses when using the @code{expr} utility.

@comment unistd.h
@comment POSIX.2
@item _SC_LINE_MAX
Inquire about the maximum size of a text line that the POSIX.2 text
utilities can handle.

@comment unistd.h
@comment POSIX.2
@item _SC_EQUIV_CLASS_MAX
Inquire about the maximum number of weights that can be assigned to an
entry of the @code{LC_COLLATE} category @samp{order} keyword in a locale
definition.  The GNU C library does not presently support locale
definitions.

@comment unistd.h
@comment POSIX.2
@item _SC_VERSION
Inquire about the version number of POSIX.1 that the library and kernel
support.

@comment unistd.h
@comment POSIX.2
@item _SC_2_VERSION
Inquire about the version number of POSIX.2 that the system utilities
support.

@comment unistd.h
@comment GNU
@item _SC_PAGESIZE
Inquire about the virtual memory page size of the machine.
@code{getpagesize} returns the same value.
@c @xref{XXX getpagesize}. !!! ???
@end table

@node Examples of Sysconf 
@subsection Examples of @code{sysconf}

We recommend that you first test for a macro definition for the
parameter you are interested in, and call @code{sysconf} only if the
macro is not defined.  For example, here is how to test whether job
control is supported:

@smallexample
@group
int
have_job_control (void)
@{
#ifdef _POSIX_JOB_CONTROL
  return 1;
#else
  int value = sysconf (_SC_JOB_CONTROL);
  if (value < 0)
    /* @r{If the system is that badly wedged,}
       @r{there's no use trying to go on.}  */
    fatal (strerror (errno));
  return value;
#endif
@}
@end group
@end smallexample

Here is how to get the value of a numeric limit:

@smallexample
int
get_child_max ()
@{
#ifdef CHILD_MAX
  return CHILD_MAX;
#else
  int value = sysconf (_SC_CHILD_MAX);
  if (value < 0)
    fatal (strerror (errno));
  return value;
#endif
@}
@end smallexample

@node Minimums
@section Minimum Values for General Capacity Limits

Here are the names for the POSIX minimum upper bounds for the system
limit parameters.  The significance of these values is that you can
safely push to these limits without checking whether the particular
system you are using can go that far.

@table @code
@comment limits.h
@comment POSIX.1
@item _POSIX_ARG_MAX
The value of this macro is the most restrictive limit permitted by POSIX
for the maximum combined length of the @var{argv} and @var{environ}
arguments that can be passed to the @code{exec} functions.
Its value is @code{4096}.

@comment limits.h
@comment POSIX.1
@item _POSIX_CHILD_MAX
The value of this macro is the most restrictive limit permitted by POSIX
for the maximum number of simultaneous processes per real user ID.  Its
value is @code{6}.

@comment limits.h
@comment POSIX.1
@item _POSIX_NGROUPS_MAX
The value of this macro is the most restrictive limit permitted by POSIX
for the maximum number of supplementary group IDs per process.  Its
value is @code{0}.

@comment limits.h
@comment POSIX.1
@item _POSIX_OPEN_MAX
The value of this macro is the most restrictive limit permitted by POSIX
for the maximum number of files that a single process can have open
simultaneously.  Its value is @code{16}.

@comment limits.h
@comment POSIX.1
@item _POSIX_SSIZE_MAX
The value of this macro is the most restrictive limit permitted by POSIX
for the maximum value that can be stored in an object of type
@code{ssize_t}.  Its value is @code{32767}.

@comment limits.h
@comment POSIX.1
@item _POSIX_STREAM_MAX
The value of this macro is the most restrictive limit permitted by POSIX
for the maximum number of streams that a single process can have open
simultaneously.  Its value is @code{8}.

@comment limits.h
@comment POSIX.1
@item _POSIX_TZNAME_MAX
The value of this macro is the most restrictive limit permitted by POSIX
for the maximum length of a time zone name.  Its value is @code{3}.

@comment limits.h
@comment POSIX.2
@item _POSIX2_RE_DUP_MAX
The value of this macro is the most restrictive limit permitted by POSIX
for the numbers used in the @samp{\@{@var{min},@var{max}\@}} construct
in a regular expression.  Its value is @code{255}.
@end table

@node Limits for Files
@section Limits on File System Capacity

The POSIX.1 standard specifies a number of parameters that describe the
limitations of the file system.  It's possible for the system to have a
fixed, uniform limit for a parameter, but this isn't the usual case.  On
most systems, it's possible for different file systems (and, for some
parameters, even different files) to have different maximum limits.  For
example, this is very likely if you use NFS to mount some of the file
systems from other machines.

@pindex limits.h
Each of the following macros is defined in @file{limits.h} only if the
system has a fixed, uniform limit for the parameter in question.  If the
system allows different file systems or files to have different limits,
then the macro is undefined; use @code{pathconf} or @code{fpathconf} to
find out the limit that applies to a particular file.  @xref{Pathconf}.

Each parameter also has another macro, with a name starting with
@samp{_POSIX}, which gives the lowest value that the limit is allowed to
have on @emph{any} POSIX system.  @xref{File Minimums}.

@cindex limits, link count of files
@comment limits.h
@comment POSIX.1
@deftypevr Macro int LINK_MAX
The uniform system limit (if any) for the number of names for a given
file.  @xref{Hard Links}.
@end deftypevr

@cindex limits, terminal input queue
@comment limits.h
@comment POSIX.1
@deftypevr Macro int MAX_CANON
The uniform system limit (if any) for the amount of text in a line of
input when input editing is enabled.  @xref{Canonical or Not}.
@end deftypevr

@comment limits.h
@comment POSIX.1
@deftypevr Macro int MAX_INPUT
The uniform system limit (if any) for the total number of characters
typed ahead as input.  @xref{I/O Queues}.
@end deftypevr

@cindex limits, file name length
@comment limits.h
@comment POSIX.1
@deftypevr Macro int NAME_MAX
The uniform system limit (if any) for the length of a file name component.
@end deftypevr

@comment limits.h
@comment POSIX.1
@deftypevr Macro int PATH_MAX
The uniform system limit (if any) for the length of an entire file name (that
is, the argument given to system calls such as @code{open}).
@end deftypevr

@cindex limits, pipe buffer size
@comment limits.h
@comment POSIX.1
@deftypevr Macro int PIPE_BUF
The uniform system limit (if any) for the number of bytes that can be
written atomically to a pipe.  If multiple processes are writing to the
same pipe simultaneously, output from different processes might be
interleaved in chunks of this size.  @xref{Pipes and FIFOs}.
@end deftypevr

These are alternative macro names for some of the same information.

@comment dirent.h
@comment BSD
@deftypevr Macro int MAXNAMLEN
This is the BSD name for @code{NAME_MAX}.  It is defined in
@file{dirent.h}.
@end deftypevr

@comment stdio.h
@comment ANSI
@deftypevr Macro int FILENAME_MAX 
The value of this macro is an integer constant expression that
represents the maximum length of a file name string.  It is defined in
@file{stdio.h}.

Unlike @code{PATH_MAX}, this macro is defined even if there is no actual
limit imposed.  In such a case, its value is typically a very large
number.  @strong{This is always the case on the GNU system.}

@strong{Usage Note:} Don't use @code{FILENAME_MAX} as the size of an
array in which to store a file name!  You can't possibly make an array
that big!  Use dynamic allocation (@pxref{Memory Allocation}) instead.
@end deftypevr

@node Options for Files
@section Optional Features in File Support

POSIX defines certain system-specific options in the system calls for
operating on files.  Some systems support these options and others do
not.  Since these options are provided in the kernel, not in the
library, simply using the GNU C library does not guarantee any of these
features is supported; it depends on the system you are using.  They can
also vary between file systems on a single machine.

@pindex unistd.h
This section describes the macros you can test to determine whether a
particular option is supported on your machine.  If a given macro is
defined in @file{unistd.h}, then its value says whether the
corresponding feature is supported.  (A value of @code{-1} indicates no;
any other value indicates yes.)  If the macro is undefined, it means
particular files may or may not support the feature.

Since all the machines that support the GNU C library also support NFS,
one can never make a general statement about whether all file systems
support the @code{_POSIX_CHOWN_RESTRICTED} and @code{_POSIX_NO_TRUNC}
features.  So these names are never defined as macros in the GNU C
library.

@comment unistd.h
@comment POSIX.1
@deftypevr Macro int _POSIX_CHOWN_RESTRICTED
If this option is in effect, the @code{chown} function is restricted so
that the only changes permitted to nonprivileged processes is to change 
the group owner of a file to either be the effective group ID of the
process, or one of its supplementary group IDs.  @xref{File Owner}.
@end deftypevr

@comment unistd.h
@comment POSIX.1
@deftypevr Macro int _POSIX_NO_TRUNC
If this option is in effect, file name components longer than
@code{NAME_MAX} generate an @code{ENAMETOOLONG} error.  Otherwise, file
name components that are too long are silently truncated.
@end deftypevr

@comment unistd.h
@comment POSIX.1
@deftypevr Macro {unsigned char} _POSIX_VDISABLE
This option is only meaningful for files that are terminal devices.
If it is enabled, then handling for special control characters can
be disabled individually.  @xref{Special Characters}.
@end deftypevr

@pindex unistd.h
If one of these macros is undefined, that means that the option might be
in effect for some files and not for others.  To inquire about a
particular file, call @code{pathconf} or @code{fpathconf}.
@xref{Pathconf}.