Understanding lseek Post: 302705501

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Top Forums UNIX for Dummies Questions & Answers Understanding lseek Post 302705501 by Deepak Raj on Tuesday 25th of September 2012 04:54:57 AM

09-25-2012

Registered User

Thanks Jim. It's an nice and simple idea for identifying the file size.

Quote:

Originally Posted by jim mcnamara

File pointers do not necesarily have any relevance to file size. Use fstat() on an open file. lseek() is meant to move file pointers, other uses may not work as you found out.

Code:

#include <sys/stat.h>
size_t filesize(int fd)
{
    struct stat st;
    if(fstat(fd, &st)==-1)
    {
        perror("Cannot stat file");
        exit(1);
    }
    return st.st_size;
}

// usage someplace else in your code

FILE *in=fopen(somefile.dat, "a");
size_t sz=filesize(fileno(in));

---------- Post updated at 02:17 PM ---------- Previous update was at 01:19 PM ----------

Thanks Alister

That was helpful. Open syscall used with O_APPEND option doesn't position the offset to end of the file. Rather offset is positioned to the 'seeked' place just before any write operation occurs. This is what I observed with with the following code.

#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>

int main(void)
{
int fd;

if((fd = open("./example.c", O_WRONLY | APPEND)) < 0) {
perror("Open failed: ");
exit(1);
}
printf("Offset after append flag: %jd\n", (intmax_t) lseek(fd, 0, SEEK_CUR));
if(write(fd, "end", 3) != 3)
perror("Write failed: ");
printf("Offset after write: %jd\n", (intmax_t) lseek(fd, 0, SEEK_CUR));
close(fd);

exit(0);
}

And Thanks for correcting me with format specifier %jd and type conversion of lseek's return value. It helps with compatibility issue, that's what I found out. It will be appreciable if you can explain it.

Regards
Deepak

---------- Post updated at 02:24 PM ---------- Previous update was at 02:17 PM ----------

Hi

Okay, now I thought of a way to use lseek to know number of bytes in file. I only need to simply replace whence SEEK_CUR to SEEK_END. I guess, it works because we can't make sure that offset is set to EOF using O_APPEND, but SEEK_END with zero as offset will set it to EOF, surly. Correct me if I'm incorrect.

Regards
Deepak

Deepak Raj

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LEARN ABOUT NETBSD

seek

LSEEK(2)						      BSD System Calls Manual							  LSEEK(2)

NAME

     lseek, seek -- 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() function 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.  lseek() repositions the file pointer 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.

     The lseek() function 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.

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

     lseek() will fail and the file pointer will remain unchanged if:

     [EBADF]		fildes is not an open file descriptor.

     [EINVAL]		whence is not a proper value, or the resulting file offset would be invalid.

     [ESPIPE]		fildes is associated with a pipe, socket, or FIFO.

SEE ALSO

     dup(2), open(2)

STANDARDS

     The lseek() function conforms to ISO/IEC 9945-1:1990 (``POSIX.1'').

HISTORY

     A seek() function appeared in Version 2 AT&T UNIX, later renamed into lseek() for ``long seek'' due to a larger offset argument type.

BUGS

     This document's use of whence is incorrect English, but is maintained for historical reasons.

BSD
								   April 3, 2010							       BSD