Login or Register to Ask a Question and Join Our Community

Sponsored Content
Full Discussion: Understanding lseek
Top Forums UNIX for Dummies Questions & Answers Understanding lseek Post 302705301 by jim mcnamara on Monday 24th of September 2012 01:49:10 PM
Old 09-24-2012
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));

 

9 More Discussions You Might Find Interesting

1. Programming

lseek in c

sir, i used lssek as this lseek(fp,-10,2); i am not getting any output i dont now why can you explan sir.. Thanks in advance, Arunkumar (4 Replies)
Discussion started by: arunkumar_mca
4 Replies

2. Shell Programming and Scripting

need help understanding mv

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)
Discussion started by: taiL
16 Replies

3. Programming

Hairy Problem! lseek over 4G

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)
Discussion started by: macroideal
8 Replies

4. UNIX for Dummies Questions & Answers

understanding {%/*}/

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)
Discussion started by: vemana
3 Replies

5. UNIX for Dummies Questions & Answers

lseek() equivalent

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)
Discussion started by: joker40
9 Replies

6. UNIX for Dummies Questions & Answers

Lseek implementation

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)
Discussion started by: Humudituu
4 Replies

7. Programming

what is the main difference between difference between using nonatomic lseek and O_APPEND

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)
Discussion started by: fwrlfo
1 Replies

8. AIX

Backup: The lseek call failed

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)
Discussion started by: m_raheelahmed
4 Replies

9. Programming

Problem with lseek call.

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 V7

lseek

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)
All times are GMT -4. The time now is 08:03 PM.
Unix & Linux Forums Content Copyright 1993-2022. All Rights Reserved.
Privacy Policy