filesys.texi

一个C源代码分析器

@table @code
@item EACCES
One of the directories containing @var{newname} or @var{oldname}
refuses write permission; or @var{newname} and @var{oldname} are
directories and write permission is refused for one of them.

@item EBUSY
A directory named by @var{oldname} or @var{newname} is being used by
the system in a way that prevents the renaming from working.  This includes
directories that are mount points for filesystems, and directories
that are the current working directories of processes.

@item ENOTEMPTY
@itemx EEXIST
The directory @var{newname} isn't empty.  The GNU system always returns
@code{ENOTEMPTY} for this, but some other systems return @code{EEXIST}.

@item EINVAL
The @var{oldname} is a directory that contains @var{newname}.

@item EISDIR
The @var{newname} names a directory, but the @var{oldname} doesn't.

@item EMLINK
The parent directory of @var{newname} would have too many links.

Well-designed file systems never report this error, because they permit
more links than your disk could possibly hold.  However, you must still
take account of the possibility of this error, as it could result from
network access to a file system on another machine.

@item ENOENT
The file named by @var{oldname} doesn't exist.

@item ENOSPC
The directory that would contain @var{newname} has no room for another
entry, and there is no space left in the file system to expand it.

@item EROFS
The operation would involve writing to a directory on a read-only file
system.

@item EXDEV
The two file names @var{newname} and @var{oldnames} are on different
file systems.
@end table
@end deftypefun

@node Creating Directories
@section Creating Directories
@cindex creating a directory
@cindex directories, creating

@pindex mkdir
Directories are created with the @code{mkdir} function.  (There is also
a shell command @code{mkdir} which does the same thing.)

@comment sys/stat.h
@comment POSIX.1
@deftypefun int mkdir (const char *@var{filename}, mode_t @var{mode})
The @code{mkdir} function creates a new, empty directory whose name is
@var{filename}.

The argument @var{mode} specifies the file permissions for the new
directory file.  @xref{Permission Bits}, for more information about
this.

A return value of @code{0} indicates successful completion, and
@code{-1} indicates failure.  In addition to the usual file name syntax
errors (@pxref{File Name Errors}), the following @code{errno} error
conditions are defined for this function:

@table @code
@item EACCES
Write permission is denied for the parent directory in which the new
directory is to be added.

@item EEXIST
A file named @var{filename} already exists.

@item EMLINK
The parent directory has too many links.

Well-designed file systems never report this error, because they permit
more links than your disk could possibly hold.  However, you must still
take account of the possibility of this error, as it could result from
network access to a file system on another machine.

@item ENOSPC
The file system doesn't have enough room to create the new directory.

@item EROFS
The parent directory of the directory being created is on a read-only
file system, and cannot be modified.
@end table

To use this function, your program should include the header file
@file{sys/stat.h}.
@pindex sys/stat.h
@end deftypefun

@node File Attributes
@section File Attributes

@pindex ls
When you issue an @samp{ls -l} shell command on a file, it gives you
information about the size of the file, who owns it, when it was last
modified, and the like.  This kind of information is called the
@dfn{file attributes}; it is associated with the file itself and not a
particular one of its names.

This section contains information about how you can inquire about and
modify these attributes of files.

@menu
* Attribute Meanings::          The names of the file attributes, 
                                 and what their values mean.
* Reading Attributes::          How to read the attributes of a file.
* Testing File Type::           Distinguishing ordinary files,
                                 directories, links... 
* File Owner::                  How ownership for new files is determined,
			         and how to change it.
* Permission Bits::             How information about a file's access
                                 mode is stored. 
* Access Permission::           How the system decides who can access a file.
* Setting Permissions::         How permissions for new files are assigned,
			         and how to change them.
* Testing File Access::         How to find out if your process can
                                 access a file. 
* File Times::                  About the time attributes of a file.
@end menu

@node Attribute Meanings
@subsection What the File Attribute Values Mean
@cindex status of a file
@cindex attributes of a file
@cindex file attributes

When you read the attributes of a file, they come back in a structure
called @code{struct stat}.  This section describes the names of the
attributes, their data types, and what they mean.  For the functions
to read the attributes of a file, see @ref{Reading Attributes}.

The header file @file{sys/stat.h} declares all the symbols defined
in this section.
@pindex sys/stat.h

@comment sys/stat.h
@comment POSIX.1
@deftp {Data Type} {struct stat}
The @code{stat} structure type is used to return information about the
attributes of a file.  It contains at least the following members:

@table @code
@item mode_t st_mode
Specifies the mode of the file.  This includes file type information
(@pxref{Testing File Type}) and the file permission bits
(@pxref{Permission Bits}).

@item ino_t st_ino
The file serial number, which distinguishes this file from all other
files on the same device.

@item dev_t st_dev
Identifies the device containing the file.  The @code{st_ino} and
@code{st_dev}, taken together, uniquely identify the file.  The
@code{st_dev} value is not necessarily consistent across reboots or
system crashes, however.

@item nlink_t st_nlink
The number of hard links to the file.  This count keeps track of how many
directories have entries for this file.  If the count is ever
decremented to zero, then the file itself is discarded.  Symbolic links
are not counted in the total.

@item uid_t st_uid
The user ID of the file's owner.  @xref{File Owner}.

@item gid_t st_gid
The group ID of the file.  @xref{File Owner}.

@item off_t st_size
This specifies the size of a regular file in bytes.  For files that
are really devices and the like, this field isn't usually meaningful.

@item time_t st_atime
This is the last access time for the file.  @xref{File Times}.

@item unsigned long int st_atime_usec
This is the fractional part of the last access time for the file.
@xref{File Times}.

@item time_t st_mtime
This is the time of the last modification to the contents of the file.
@xref{File Times}.

@item unsigned long int st_mtime_usec
This is the fractional part of the time of last modification to the
contents of the file.  @xref{File Times}.

@item time_t st_ctime
This is the time of the last modification to the attributes of the file.
@xref{File Times}.

@item unsigned long int st_ctime_usec
This is the fractional part of the time of last modification to the
attributes of the file.  @xref{File Times}.

@item unsigned int st_nblocks
This is the amount of disk space that the file occupies, measured in
units of 512-byte blocks.

The number of disk blocks is not strictly proportional to the size of
the file, for two reasons: the file system may use some blocks for
internal record keeping; and the file may be sparse---it may have
``holes'' which contain zeros but do not actually take up space on the
disk.

You can tell (approximately) whether a file is sparse by comparing this
value with @code{st_size}, like this:

@smallexample
(st.st_blocks * 512 < st.st_size)
@end smallexample

This test is not perfect because a file that is just slightly sparse
might not be detected as sparse at all.  For practical applications,
this is not a problem.

@item unsigned int st_blksize
The optimal block size for reading of writing this file, in bytes.  You
might use this size for allocating the buffer space for reading of
writing the file.  (This is unrelated to @code{st_blocks}.)
@end table
@end deftp

  Some of the file attributes have special data type names which exist
specifically for those attributes.  (They are all aliases for well-known
integer types that you know and love.)  These typedef names are defined
in the header file @file{sys/types.h} as well as in @file{sys/stat.h}.
Here is a list of them.

@comment sys/types.h
@comment POSIX.1
@deftp {Data Type} mode_t
This is an integer data type used to represent file modes.  In the
GNU system, this is equivalent to @code{unsigned int}.
@end deftp

@cindex inode number
@comment sys/types.h
@comment POSIX.1
@deftp {Data Type} ino_t
This is an arithmetic data type used to represent file serial numbers.
(In Unix jargon, these are sometimes called @dfn{inode numbers}.)
In the GNU system, this type is equivalent to @code{unsigned long int}.
@end deftp

@comment sys/types.h
@comment POSIX.1
@deftp {Data Type} dev_t
This is an arithmetic data type used to represent file device numbers.
In the GNU system, this is equivalent to @code{int}.
@end deftp

@comment sys/types.h
@comment POSIX.1
@deftp {Data Type} nlink_t
This is an arithmetic data type used to represent file link counts.
In the GNU system, this is equivalent to @code{unsigned short int}.
@end deftp

@node Reading Attributes
@subsection Reading the Attributes of a File

To examine the attributes of files, use the functions @code{stat},
@code{fstat} and @code{lstat}.  They return the attribute information in
a @code{struct stat} object.  All three functions are declared in the
header file @file{sys/stat.h}.

@comment sys/stat.h
@comment POSIX.1
@deftypefun int stat (const char *@var{filename}, struct stat *@var{buf})
The @code{stat} function returns information about the attributes of the
file named by @w{@var{filename}} in the structure pointed at by @var{buf}.

If @var{filename} is the name of a symbolic link, the attributes you get
describe the file that the link points to.  If the link points to a
nonexistent file name, then @code{stat} fails, reporting a nonexistent
file.

The return value is @code{0} if the operation is successful, and @code{-1}
on failure.  In addition to the usual file name syntax errors
(@pxref{File Name Errors}, the following @code{errno} error conditions
are defined for this function:

@table @code
@item ENOENT
The file named by @var{filename} doesn't exist.
@end table
@end deftypefun

@comment sys/stat.h
@comment POSIX.1
@deftypefun int fstat (int @var{filedes}, struct stat *@var{buf})
The @code{fstat} function is like @code{stat}, except that it takes an
open file descriptor as an argument instead of a file name.
@xref{Low-Level I/O}.

Like @code{stat}, @code{fstat} returns @code{0} on success and @code{-1}
on failure.  The following @code{errno} error conditions are defined for
@code{fstat}:

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

@comment sys/stat.h
@comment BSD
@deftypefun int lstat (const char *@var{filename}, struct stat *@var{buf})
The @code{lstat} function is like @code{stat}, except that it does not
follow symbolic links.  If @var{filename} is the name of a symbolic
link, @code{lstat} returns information about the link itself; otherwise,
@code{lstat} works like @code{stat}.  @xref{Symbolic Links}.
@end deftypefun

@node Testing File Type
@subsection Testing the Type of a File

The @dfn{file mode}, stored in the @code{st_mode} field of the file
attributes, contains two kinds of information: the file type code, and
the access permission bits.  This section discusses only the type code,
which you can use to tell whether the file is a directory, whether it is
a socket, and so on.  For information about the access permission,
@ref{Permission Bits}.

There are two predefined ways you can access the file type portion of
the file mode.  First of all, for each type of file, there is a 
@dfn{predicate macro} which examines a file mode value and returns
true or false---is the file of that type, or not.  Secondly, you can
mask out the rest of the file mode to get just a file type code.
You can compare this against various constants for the supported file
types.

All of the symbols listed in this section are defined in the header file
@file{sys/stat.h}.
@pindex sys/stat.h

The following predicate macros test the type of a file, given the value
@var{m} which is the @code{st_mode} field returned by @code{stat} on
that file:

@comment sys/stat.h
@comment POSIX
@deftypefn Macro int S_ISDIR (mode_t @var{m})
This macro returns nonzero if the file is a directory.
@end deftypefn

@comment sys/stat.h
@comment POSIX
@deftypefn Macro int S_ISCHR (mode_t @var{m})
This macro returns nonzero if the file is a character special file (a
device like a terminal).
@end deftypefn

@comment sys/stat.h
@comment POSIX
@deftypefn Macro int S_ISBLK (mode_t @var{m})