conf.texi

一个C源代码分析器

@node File Minimums
@section Minimum Values for File System Limits

Here are the names for the POSIX minimum upper bounds for some of the
above 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_LINK_MAX
The most restrictive limit permitted by POSIX for the maximum value of a
file's link count.  The value of this constant is @code{8}; thus, you
can always make up to eight names for a file without running into a
system limit.

@comment limits.h
@comment POSIX.1
@item _POSIX_MAX_CANON
The most restrictive limit permitted by POSIX for the maximum number of
bytes in a canonical input line from a terminal device.  The value of
this constant is @code{255}.

@comment limits.h
@comment POSIX.1
@item _POSIX_MAX_INPUT
The most restrictive limit permitted by POSIX for the maximum number of
bytes in a terminal device input queue (or typeahead buffer).
@xref{Input Modes}.  The value of this constant is @code{255}.

@comment limits.h
@comment POSIX.1
@item _POSIX_NAME_MAX
The most restrictive limit permitted by POSIX for the maximum number of
bytes in a file name component.  The value of this constant is
@code{14}.

@comment limits.h
@comment POSIX.1
@item _POSIX_PATH_MAX
The most restrictive limit permitted by POSIX for the maximum number of
bytes in a file name.  The value of this constant is @code{255}.

@comment limits.h
@comment POSIX.1
@item _POSIX_PIPE_BUF
The most restrictive limit permitted by POSIX for the maximum number of
bytes that can be written atomically to a pipe.  The value of this
constant is @code{512}.
@end table

@node Pathconf
@section Using @code{pathconf}

When your machine allows different files to have different values for a
file system parameter, you can use the functions in this section to find
out the value that applies to any particular file.

These functions and the associated constants for the @var{parameter}
argument are declared in the header file @file{unistd.h}.

@comment unistd.h
@comment POSIX.1
@deftypefun {long int} pathconf (const char *@var{filename}, int @var{parameter})
This function is used to inquire about the limits that apply to
the file named @var{filename}.

The @var{parameter} argument should be one of the @samp{_PC_} constants
listed below.

The normal return value from @code{pathconf} is the value you requested.
A value of @code{-1} is returned both if the implementation does not
impose a limit, and in case of an error.  In the former case,
@code{errno} is not set, while in the latter case, @code{errno} is set
to indicate the cause of the problem.  So the only way to use this
function robustly is to store @code{0} into @code{errno} just before
calling it.

Besides the usual file name syntax errors (@pxref{File Name Errors}),
the following error condition is defined for this function:

@table @code
@item EINVAL
The value of @var{parameter} is invalid, or the implementation doesn't
support the @var{parameter} for the specific file.
@end table
@end deftypefun

@comment unistd.h
@comment POSIX.1
@deftypefun {long int} fpathconf (int @var{filedes}, int @var{parameter})
This is just like @code{pathconf} except that an open file descriptor
is used to specify the file for which information is requested, instead
of a file name.

The following @code{errno} error conditions are defined for this function:

@table @code
@item EBADF
The @var{filedes} argument is not a valid file descriptor.

@item EINVAL
The value of @var{parameter} is invalid, or the implementation doesn't
support the @var{parameter} for the specific file.
@end table
@end deftypefun

Here are the symbolic constants that you can use as the @var{parameter}
argument to @code{pathconf} and @code{fpathconf}.  The values are all
integer constants.

@table @code
@comment unistd.h
@comment POSIX.1
@item _PC_LINK_MAX
Inquire about the value of @code{LINK_MAX}.

@comment unistd.h
@comment POSIX.1
@item _PC_MAX_CANON
Inquire about the value of @code{MAX_CANON}.

@comment unistd.h
@comment POSIX.1
@item _PC_MAX_INPUT
Inquire about the value of @code{MAX_INPUT}.

@comment unistd.h
@comment POSIX.1
@item _PC_NAME_MAX
Inquire about the value of @code{NAME_MAX}.

@comment unistd.h
@comment POSIX.1
@item _PC_PATH_MAX
Inquire about the value of @code{PATH_MAX}.

@comment unistd.h
@comment POSIX.1
@item _PC_PIPE_BUF
Inquire about the value of @code{PIPE_BUF}.

@comment unistd.h
@comment POSIX.1
@item _PC_CHOWN_RESTRICTED
Inquire about the value of @code{_POSIX_CHOWN_RESTRICTED}.

@comment unistd.h
@comment POSIX.1
@item _PC_NO_TRUNC
Inquire about the value of @code{_POSIX_NO_TRUNC}.

@comment unistd.h
@comment POSIX.1
@item _PC_VDISABLE
Inquire about the value of @code{_POSIX_VDISABLE}.
@end table

@node Utility Limits
@section Utility Program Capacity Limits

The POSIX.2 standard specifies certain system limits that you can access
through @code{sysconf} that apply to utility behavior rather than the
behavior of the library or the operating system.

The GNU C library defines macros for these limits, and @code{sysconf}
returns values for them if you ask; but these values convey no
meaningful information.  They are simply the smallest values that
POSIX.2 permits.

@comment limits.h
@comment POSIX.2
@deftypevr Macro int BC_BASE_MAX
The largest value of @code{obase} that the @code{bc} utility is
guaranteed to support.
@end deftypevr

@comment limits.h
@comment POSIX.2
@deftypevr Macro int BC_SCALE_MAX
The largest value of @code{scale} that the @code{bc} utility is
guaranteed to support.
@end deftypevr

@comment limits.h
@comment POSIX.2
@deftypevr Macro int BC_DIM_MAX
The largest number of elements in one array that the @code{bc} utility
is guaranteed to support.
@end deftypevr

@comment limits.h
@comment POSIX.2
@deftypevr Macro int BC_STRING_MAX
The largest number of characters in one string constant that the
@code{bc} utility is guaranteed to support.
@end deftypevr

@comment limits.h
@comment POSIX.2
@deftypevr Macro int BC_DIM_MAX
The largest number of elements in one array that the @code{bc} utility
is guaranteed to support.
@end deftypevr

@comment limits.h
@comment POSIX.2
@deftypevr Macro int COLL_WEIGHTS_MAX
The largest number of weights that can necessarily be used in defining
the collating sequence for a locale.
@end deftypevr

@comment limits.h
@comment POSIX.2
@deftypevr Macro int EXPR_NEST_MAX
The maximum number of expressions that can be nested within parenthesis
by the @code{expr} utility.
@end deftypevr

@comment limits.h
@comment POSIX.2
@deftypevr Macro int LINE_MAX
The largest text line that the text-oriented POSIX.2 utilities can
support.  (If you are using the GNU versions of these utilities, then
there is no actual limit except that imposed by the available virtual
memory, but there is no way that the library can tell you this.)
@end deftypevr

@comment limits.h
@comment POSIX.2
@deftypevr Macro int EQUIV_CLASS_MAX
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.
@end deftypevr

@node Utility Minimums
@section Minimum Values for Utility Limits

@table @code
@comment limits.h
@comment POSIX.2
@item _POSIX2_BC_BASE_MAX
The most restrictive limit permitted by POSIX.2 for the maximum value of
@code{obase} in the @code{bc} utility.  Its value is @code{99}.

@comment limits.h
@comment POSIX.2
@item _POSIX2_BC_DIM_MAX
The most restrictive limit permitted by POSIX.2 for the maximum size of
an array in the @code{bc} utility.  Its value is @code{2048}.

@comment limits.h
@comment POSIX.2
@item _POSIX2_BC_SCALE_MAX
The most restrictive limit permitted by POSIX.2 for the maximum value of
@code{scale} in the @code{bc} utility.  Its value is @code{99}.

@comment limits.h
@comment POSIX.2
@item _POSIX2_BC_STRING_MAX
The most restrictive limit permitted by POSIX.2 for the maximum size of
a string constant in the @code{bc} utility.  Its value is @code{1000}.

@comment limits.h
@comment POSIX.2
@item _POSIX2_COLL_WEIGHTS_MAX
The most restrictive limit permitted by POSIX.2 for the maximum number
of weights that can necessarily be used in defining the collating
sequence for a locale.  Its value is @code{2}.

@comment limits.h
@comment POSIX.2
@item _POSIX2_EXPR_NEST_MAX
The most restrictive limit permitted by POSIX.2 for the maximum number
of expressions nested within parenthesis when using the @code{expr} utility.
Its value is @code{32}.

@comment limits.h
@comment POSIX.2
@item _POSIX2_LINE_MAX
The most restrictive limit permitted by POSIX.2 for the maximum size of
a text line that the text utilities can handle.  Its value is
@code{2048}.

@comment limits.h
@comment POSIX.2
@item _POSIX2_EQUIV_CLASS_MAX
The most restrictive limit permitted by POSIX.2 for 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.  Its value is
@code{2}.  The GNU C library does not presently support locale
definitions.
@end table

@node String Parameters
@section String-Valued Parameters

POSIX.2 defines a way to get string-valued parameters from the operating
system with the function @code{confstr}:

@comment unistd.h
@comment POSIX.2
@deftypefun size_t confstr (int @var{parameter}, char *@var{buf}, size_t @var{len})
This function reads the value of a string-valued system parameter,
storing the string into @var{len} bytes of memory space starting at
@var{buf}.  The @var{parameter} argument should be one of the
@samp{_CS_} symbols listed below.

The normal return value from @code{confstr} is the length of the string
value that you asked for.  If you supply a null pointer for @var{buf},
then @code{confstr} does not try to store the string; it just returns
its length.  A value of @code{0} indicates an error.

If the string you asked for is too long for the buffer (that is, longer
than @code{@var{len} - 1}), then @code{confstr} stores just that much
(leaving room for the terminating null character).  You can tell that
this has happened because @code{confstr} returns a value greater than or
equal to @var{len}.

The following @code{errno} error conditions are defined for this function:

@table @code
@item EINVAL
The value of the @var{parameter} is invalid.
@end table
@end deftypefun

Currently there is just one parameter you can read with @code{confstr}:

@table @code
@comment unistd.h
@comment POSIX.2
@item _CS_PATH
This parameter's value is the recommended default path for searching for
executable files.  This is the path that a user has by default just
after logging in.
@end table

The way to use @code{confstr} without any arbitrary limit on string size
is to call it twice: first call it to get the length, allocate the
buffer accordingly, and then call @code{confstr} again to fill the
buffer, like this:

@smallexample
@group
char *
get_default_path (void)
@{
  size_t len = confstr (_CS_PATH, NULL, 0);
  char *buffer = (char *) xmalloc (len);

  if (confstr (_CS_PATH, buf, len + 1) == 0)
    @{
      free (buffer);
      return NULL;
    @}

  return buffer;
@}
@end group
@end smallexample