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 bytes as the file is ready to append and lseek seeks only 0 bytes. But result is showing 0 bytes. Please correct me if I have understood wrong.
Program is as following
Program gives the output as zero, though the file 'example.txt' do exist with so data in it.
Thanks.
Last edited by jim mcnamara; 09-24-2012 at 02:43 PM..
I just started shell coding and I'm a bit confused on how 'mv' works can someone explain to me how it works and if i did this correctly. Thanks.
echo "Enter Name of the first file:"
read file1
#echo $file1
if ; then
echo "Sorry, file does not exist."
exit 1
... (16 Replies)
recently my project needs me to lseek a position over 4G size....
i found in linux or unix the parameters are all ulong 32 bits...the limit dooms the movement of a position over 4G
I was told that i should lseek64 to meet my need... but i have no idea where i can get the function neither by "man... (8 Replies)
Hi Gurus:
I am trying to understand the following line of code.I did enough of googling to understand but no luck.Please help me understand the follow chunk of code:
X=$0
MOD=${X%/*}/env.ksh
X is the current script from which I am trying to execute.
Say if X=test.ksh
$MOD is echoing :... (3 Replies)
I know there is lseek() function that will allow to write or read from certain position in the file. Is there similar function that will let do same but for array rather then file? (9 Replies)
Hi everybody,
i've been googling for ages now and gotten kinda desperate... The question, however, might be rather trivial for the experts: What is it exactly, i.e. physically, the POSIX function (for a file) "lseek" does? Does it trigger some kind of synchronization on disk? Is it just for the... (4 Replies)
I think both write at the end of the file ......
but is there a sharp difference between those 2 instruction .....
thank you
this is my 3rd question today forgive me :D (1 Reply)
Hi,
We are facing issues while backing up our 1205 GB filesystem on LTO5 Tape. During backup the "backup: The lseek call failed." messages were generated, I want to know why these messages were generating
AIX version is: 6100-08-00-0000
backup: The date of this level 0 backup is Mon Mar 11... (4 Replies)
The following code:
int fd;
if (fd = open("mem", O_RDONLY) == -1)
return 1;
if (lseek(fd, 0, SEEK_SET) == -1) {
char *buf = malloc(512);
buf = strerror(errno);
printf("%s\n", buf);
return 1;
}
always returns with "illegal seek"... (2 Replies)
Discussion started by: Sir_Tomasz
2 Replies
LEARN ABOUT FREEBSD
lseek
LSEEK(2) BSD System Calls Manual LSEEK(2)NAME
lseek -- reposition read/write file offset
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <unistd.h>
off_t
lseek(int fildes, off_t offset, int whence);
DESCRIPTION
The lseek() system call repositions the offset of the file descriptor fildes to the argument offset according to the directive whence. The
argument fildes must be an open file descriptor. The lseek() system call repositions the file position pointer associated with the file
descriptor fildes as follows:
If whence is SEEK_SET, the offset is set to offset bytes.
If whence is SEEK_CUR, the offset is set to its current location plus offset bytes.
If whence is SEEK_END, the offset is set to the size of the file plus offset bytes.
If whence is SEEK_HOLE, the offset is set to the start of the next hole greater than or equal to the supplied offset. The definition
of a hole is provided below.
If whence is SEEK_DATA, the offset is set to the start of the next non-hole file region greater than or equal to the supplied offset.
The lseek() system call allows the file offset to be set beyond the end of the existing end-of-file of the file. If data is later written at
this point, subsequent reads of the data in the gap return bytes of zeros (until data is actually written into the gap).
Some devices are incapable of seeking. The value of the pointer associated with such a device is undefined.
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 SEEK_HOLE. File systems are allowed to expose ranges of zeros with SEEK_HOLE, but not required to.
Applications can use SEEK_HOLE 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(_PC_MIN_HOLE_SIZE) or pathconf(_PC_MIN_HOLE_SIZE) to determine if a file system supports SEEK_HOLE.
See pathconf(2).
For file systems that do not supply information about holes, the file will be represented as one entire data region.
RETURN VALUES
Upon successful completion, lseek() returns the resulting offset location as measured in bytes from the beginning of the file. Otherwise, a
value of -1 is returned and errno is set to indicate the error.
ERRORS
The lseek() system call will fail and the file position pointer will remain unchanged if:
[EBADF] The fildes argument is not an open file descriptor.
[EINVAL] The whence argument is not a proper value or the resulting file offset would be negative for a non-character special file.
[ENXIO] For SEEK_DATA, there are no more data regions past the supplied offset. For SEEK_HOLE, 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 off_t.
[ESPIPE] The fildes argument is associated with a pipe, socket, or FIFO.
SEE ALSO dup(2), open(2), pathconf(2)STANDARDS
The lseek() system call is expected to conform to ISO/IEC 9945-1:1990 (``POSIX.1'').
HISTORY
The lseek() function appeared in Version 7 AT&T UNIX.
BUGS
This document's use of whence is incorrect English, but is maintained for historical reasons.
BSD May 26, 2012 BSD