lseek(2) System Calls Manual lseek(2)Name
lseek, tell - move read or write pointer
Syntax
#include <sys/types.h>
#include <unistd.h>
pos = lseek(d, offset, whence)
off_t pos;
int d, whence;
off_t offset;
pos = tell(d)
off_t pos;
int d;
Description
The system call moves the file pointer associated with a file or device open for reading or writing.
The descriptor d refers to a file or device open for reading or writing. The system call sets the file pointer of d as follows:
o If whence is SEEK_SET, the pointer is set to offset bytes.
o If whence is SEEK_CUR the pointer is set to its current location plus offset.
o If whence is SEEK_END, the pointer is set to the size of the file plus offset.
Seeking beyond the end of a file and then writing to the file creates a gap or hole that does not occupy physical space and reads as zeros.
The system call returns the offset of the current byte relative to the beginning of the file associated with the file descriptor.
Environment
System Five
If you compile a program in the System Five environment, an invalid whence argument causes SIGSYS to be sent. This complies with the
behavior described in the System V Interface Definition (SVID), Issue 1.
Return Values
Upon successful completion, a long integer (the current file pointer value) is returned. This pointer is measured in bytes from the begin-
ning of the file, where the first byte is byte 0. (Note that some devices are incapable of seeking. The value of the pointer associated
with such a device is undefined.) If a value of -1 is returned, errno is set to indicate the error.
Diagnostics
The system call fails and the file pointer remains unchanged under the following conditions:
[EBADF] The fildes is not an open file descriptor.
[EINVAL] The whence is not a proper value.
[ESPIPE] The fildes is associated with a pipe or a socket.
See Alsodup(2), open(2)lseek(2)
Check Out this Related Man Page
LSEEK(2) System Calls Manual LSEEK(2)NAME
lseek, tell - move read/write pointer
SYNOPSIS
long lseek(fildes, offset, whence)
long offset;
long tell(fildes)
DESCRIPTION
The file descriptor refers to a file open for reading or writing. The read (resp. write) pointer for the file is set as follows:
If whence is 0, the pointer is set to offset bytes.
If whence is 1, the pointer is set to its current location plus offset.
If whence is 2, the pointer is set to the size of the file plus offset.
The returned value is the resulting pointer location.
The obsolete function tell(fildes) is identical to lseek(fildes, 0L, 1).
Seeking far beyond the end of a file, then writing, creates a gap or `hole', which occupies no physical space and reads as zeros.
SEE ALSO open(2), creat(2), fseek(3)DIAGNOSTICS -1 is returned for an undefined file descriptor, seek on a pipe, or seek to a position before the beginning of file.
BUGS
Lseek is a no-op on character special files.
ASSEMBLER
(lseek = 19.)
(file descriptor in r0)
sys lseek; offset1; offset2; whence
Offset1 and offset2 are the high and low words of offset; r0 and r1 contain the pointer upon return.
LSEEK(2)
open, creat, read, write, lseek and close
Are they all primitive?
:confused:
*Another Question: is there a different between a system call, and an i/o system call? (2 Replies)
I tried to use lseek system call to determine the number of bytes in a file. To do so, I used open system call with O_APPEND flag to open a file. As lseek returns the current offset so I called lseek for opened file with offset as zero and whence as SEEK_CUR. So I guess it must return the number of... (3 Replies)