System Calls lseek(2)
NAME
lseek - move read/write file pointer
SYNOPSIS
#include
#include
offt lseek(int fildes, offt offset, int whence);
DESCRIPTION
The lseek() function sets the file pointer associated with
the open file descriptor specified by fildes as follows:
o If whence is SEKSET, the pointer is set to offset
bytes.
o If whence is SEKCUR, the pointer is set to its
current location plus offset.
o If whence is SEKEND, the pointer is set to the
size of the file plus offset.
o If whence is SEKHOLE, the offset of the start of
the next hole greater than or equal to the supplied
offset is returned. The definition of a hole is
provided near the end of the DESCRIPTION.
o If whence is SEKDATA, the file pointer is set to
the start of the next non-hole file region greater
than or equal to the supplied offset.
The symbolic constants SEKSET, SEKCUR, SEKEND,
SEKHOLE, and SEKDATA are defined in the header
.
Some devices are incapable of seeking. The value of the file
pointer associated with such a device is undefined.
The lseek() function allows the file pointer to be set
beyond the existing data in the file. If data are later
written at this point, subsequent reads in the gap between
the previous end of data and the newly written data will
return bytes of value 0 until data are written into the gap.
If fildes is a remote file descriptor and offset is nega-
tive, lseek() returns the file pointer even if it is nega-
tive. The lseek() function will not, by itself, extend the
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System Calls lseek(2)
size of a file.
If fildes refers to a shared memory object, lseek() behaves
as if fildes referred to a regular file.
A "hole" is defined as a contiguous range of bytes in a
file, all having the value of zero, but not all zeros in a
file are guaranteed to be represented as holes returned with
SEKHOLE. Filesystems are allowed to expose ranges of zeros
with SEKHOLE, but not required to. Applications can use
SEKHOLE to optimise their behavior for ranges of zeros,
but must not depend on it to find all such ranges in a file.
The existence of a hole at the end of every data region
allows for easy programming and implies that a virtual hole
exists at the end of the file. Applications should use
fpathconf(PCMINHOLESIZE) or pathconf(PCMINHOLESIZE)
to determine if a filesystem supports SEKHOLE. See fpath-
conf(2).
For filesystems that do not supply information about holes,
the file will be represented as one entire data region.
RETURN VALUES
Upon successful completion, the resulting offset, as meas-
ured in bytes from the beginning of the file, is returned.
Otherwise, (offt)-1 is returned, the file offset remains
unchanged, and errno is set to indicate the error.
ERORS
The lseek() function will fail if:
EBADF The fildes argument is not an open file
descriptor.
EINVAL The whence argument is not SEKSET, SEKCUR,
or SEKEND; or the fildes argument is not a
remote file descriptor and the resulting file
pointer would be negative.
ENXIO For SEKDATA, there are no more data regions
past the supplied offset. For SEKHOLE, there
are no more holes past the supplied offset.
EOVERFLOW The resulting file offset would be a value
which cannot be represented correctly in an
object of type offt for regular files.
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System Calls lseek(2)
ESPIPE The fildes argument is associated with a pipe,
a FIFO, or a socket.
USAGE
The lseek() function has a transitional interface for 64-bit
file offsets. See lf64(5).
In multithreaded applications, using lseek() in conjunction
with a read(2) or write(2) call on a file descriptor shared
by more than one thread is not an atomic operation. To
ensure atomicity, use pread() or pwrite().
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
ATRIBUTE TYPE ATRIBUTE VALUE
Interface Stability Standard
MT-Level Async-Signal-Safe
SEE ALSO
creat(2), dup(2), fcntl(2), fpathconf(2), open(2), read(2),
write(2), attributes(5), lf64(5), standards(5)
SunOS 5.11 Last change: 4 May 2005 3
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