(libc.info.gz) File Position Primitive
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13.3 Setting the File Position of a Descriptor
==============================================
Just as you can set the file position of a stream with 'fseek', you can
set the file position of a descriptor with 'lseek'. This specifies the
position in the file for the next 'read' or 'write' operation.
File Positioning, for more information on the file position and what
it means.
To read the current file position value from a descriptor, use 'lseek
(DESC, 0, SEEK_CUR)'.
-- Function: off_t lseek (int FILEDES, off_t OFFSET, int WHENCE)
Preliminary: | MT-Safe | AS-Safe | AC-Safe | POSIX Safety
Concepts.
The 'lseek' function is used to change the file position of the
file with descriptor FILEDES.
The WHENCE argument specifies how the OFFSET should be interpreted,
in the same way as for the 'fseek' function, and it must be one of
the symbolic constants 'SEEK_SET', 'SEEK_CUR', or 'SEEK_END'.
'SEEK_SET'
Specifies that OFFSET is a count of characters from the
beginning of the file.
'SEEK_CUR'
Specifies that OFFSET is a count of characters from the
current file position. This count may be positive or
negative.
'SEEK_END'
Specifies that OFFSET is a count of characters from the end of
the file. A negative count specifies a position within the
current extent of the file; a positive count specifies a
position past the current end. If you set the position past
the current end, and actually write data, you will extend the
file with zeros up to that position.
The return value from 'lseek' is normally the resulting file
position, measured in bytes from the beginning of the file. You
can use this feature together with 'SEEK_CUR' to read the current
file position.
If you want to append to the file, setting the file position to the
current end of file with 'SEEK_END' is not sufficient. Another
process may write more data after you seek but before you write,
extending the file so the position you write onto clobbers their
data. Instead, use the 'O_APPEND' operating mode; Operating
Modes.
You can set the file position past the current end of the file.
This does not by itself make the file longer; 'lseek' never changes
the file. But subsequent output at that position will extend the
file. Characters between the previous end of file and the new
position are filled with zeros. Extending the file in this way can
create a "hole": the blocks of zeros are not actually allocated on
disk, so the file takes up less space than it appears to; it is
then called a "sparse file".
If the file position cannot be changed, or the operation is in some
way invalid, 'lseek' returns a value of -1. The following 'errno'
error conditions are defined for this function:
'EBADF'
The FILEDES is not a valid file descriptor.
'EINVAL'
The WHENCE argument value is not valid, or the resulting file
offset is not valid. A file offset is invalid.
'ESPIPE'
The FILEDES corresponds to an object that cannot be
positioned, such as a pipe, FIFO or terminal device. (POSIX.1
specifies this error only for pipes and FIFOs, but on GNU
systems, you always get 'ESPIPE' if the object is not
seekable.)
When the source file is compiled with '_FILE_OFFSET_BITS == 64' the
'lseek' function is in fact 'lseek64' and the type 'off_t' has 64
bits which makes it possible to handle files up to 2^63 bytes in
length.
This function is a cancellation point in multi-threaded programs.
This is a problem if the thread allocates some resources (like
memory, file descriptors, semaphores or whatever) at the time
'lseek' is called. If the thread gets canceled these resources
stay allocated until the program ends. To avoid this calls to
'lseek' should be protected using cancellation handlers.
The 'lseek' function is the underlying primitive for the 'fseek',
'fseeko', 'ftell', 'ftello' and 'rewind' functions, which operate
on streams instead of file descriptors.
-- Function: off64_t lseek64 (int FILEDES, off64_t OFFSET, int WHENCE)
Preliminary: | MT-Safe | AS-Safe | AC-Safe | POSIX Safety
Concepts.
This function is similar to the 'lseek' function. The difference
is that the OFFSET parameter is of type 'off64_t' instead of
'off_t' which makes it possible on 32 bit machines to address files
larger than 2^31 bytes and up to 2^63 bytes. The file descriptor
'filedes' must be opened using 'open64' since otherwise the large
offsets possible with 'off64_t' will lead to errors with a
descriptor in small file mode.
When the source file is compiled with '_FILE_OFFSET_BITS == 64' on
a 32 bits machine this function is actually available under the
name 'lseek' and so transparently replaces the 32 bit interface.
You can have multiple descriptors for the same file if you open the
file more than once, or if you duplicate a descriptor with 'dup'.
Descriptors that come from separate calls to 'open' have independent
file positions; using 'lseek' on one descriptor has no effect on the
other. For example,
{
int d1, d2;
char buf[4];
d1 = open ("foo", O_RDONLY);
d2 = open ("foo", O_RDONLY);
lseek (d1, 1024, SEEK_SET);
read (d2, buf, 4);
}
will read the first four characters of the file 'foo'. (The
error-checking code necessary for a real program has been omitted here
for brevity.)
By contrast, descriptors made by duplication share a common file
position with the original descriptor that was duplicated. Anything
which alters the file position of one of the duplicates, including
reading or writing data, affects all of them alike. Thus, for example,
{
int d1, d2, d3;
char buf1[4], buf2[4];
d1 = open ("foo", O_RDONLY);
d2 = dup (d1);
d3 = dup (d2);
lseek (d3, 1024, SEEK_SET);
read (d1, buf1, 4);
read (d2, buf2, 4);
}
will read four characters starting with the 1024'th character of 'foo',
and then four more characters starting with the 1028'th character.
-- Data Type: off_t
This is a signed integer type used to represent file sizes. In the
GNU C Library, this type is no narrower than 'int'.
If the source is compiled with '_FILE_OFFSET_BITS == 64' this type
is transparently replaced by 'off64_t'.
-- Data Type: off64_t
This type is used similar to 'off_t'. The difference is that even
on 32 bit machines, where the 'off_t' type would have 32 bits,
'off64_t' has 64 bits and so is able to address files up to 2^63
bytes in length.
When compiling with '_FILE_OFFSET_BITS == 64' this type is
available under the name 'off_t'.
These aliases for the 'SEEK_...' constants exist for the sake of
compatibility with older BSD systems. They are defined in two different
header files: 'fcntl.h' and 'sys/file.h'.
'L_SET'
An alias for 'SEEK_SET'.
'L_INCR'
An alias for 'SEEK_CUR'.
'L_XTND'
An alias for 'SEEK_END'.
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