Lseek implementation Post: 302554895

Sponsored Content

Top Forums UNIX for Dummies Questions & Answers Lseek implementation Post 302554895 by Corona688 on Tuesday 13th of September 2011 10:00:03 AM

09-13-2011

Registered User

ext4 uses generic_file_llseek for lseek, and I find this implementation for that in fs/read_write.c:

Code:

/**
 * generic_file_llseek - generic llseek implementation for regular files
 * @file:       file structure to seek on
 * @offset:     file offset to seek to
 * @origin:     type of seek
 *
 * This is a generic implemenation of ->llseek useable for all normal local
 * filesystems.  It just updates the file offset to the value specified by
 * @offset and @origin under i_mutex.
 */
loff_t generic_file_llseek(struct file *file, loff_t offset, int origin)
{
        loff_t rval;

        mutex_lock(&file->f_dentry->d_inode->i_mutex);
        rval = generic_file_llseek_unlocked(file, offset, origin);
        mutex_unlock(&file->f_dentry->d_inode->i_mutex);

        return rval;
}

/**
 * generic_file_llseek_unlocked - lockless generic llseek implementation
 * @file:       file structure to seek on
 * @offset:     file offset to seek to
 * @origin:     type of seek
 *
 * Updates the file offset to the value specified by @offset and @origin.
 * Locking must be provided by the caller.
 */
loff_t
generic_file_llseek_unlocked(struct file *file, loff_t offset, int origin)
{
        struct inode *inode = file->f_mapping->host;

        switch (origin) {
        case SEEK_END:
                offset += inode->i_size;
                break;
        case SEEK_CUR:
                /*
                 * Here we special-case the lseek(fd, 0, SEEK_CUR)
                 * position-querying operation.  Avoid rewriting the "same"
                 * f_pos value back to the file because a concurrent read(),
                 * write() or lseek() might have altered it
                 */
                if (offset == 0)
                        return file->f_pos;
               break;
        }

        if (offset < 0 || offset > inode->i_sb->s_maxbytes)
                return -EINVAL;

        /* Special lock needed here? */
        if (offset != file->f_pos) {
                file->f_pos = offset;

                file->f_version = 0;
        }

        return offset;
}

So really, nothing to it, and the only thing that could be blocking is that mutex...

I think you've saturated the kernel with so many simultaneous system calls to the same inode that they're competing for i_mutex.

I don't think this'd happen if you hadn't opened it with O_DIRECT. Caching is your friend...

Last edited by Corona688; 09-13-2011 at 11:05 AM..

Corona688

View Public Profile for Corona688

Visit Corona688's homepage!

Find all posts by Corona688

10 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

2. Shell Programming and Scripting

Need help on AWK implementation

Hi, I am accepting a string from user. compare this output with the awk output as below... echo "\n\n\tDay : \c" read day awk '{ if($day == $2) { if ($mon == $1) { print "Yes" }}}' syslog.txt I am getting the follwoing error awk: Field $() is not correct. The input line...

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...

4. Programming

Malloc implementation in C

Hey Guys I am trying to implement the malloc function for my OS class and I am having a little trouble with it. I would be really grateful if I could get some hints on this problem. So I am using a doubly-linked list as my data structure and I have to allocate memory for it (duh...). The...

5. UNIX for Advanced & Expert Users

Malloc Implementation in C

Hey Guys Some of my friends have got together and we are trying to write a basic kernel similar to Linux. I am trying to implement the malloc function in C and I am using a doubly linked list as the primary data structure. I need to allocate memory for this link list (duh...) and I don't feel...

6. 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?

7. UNIX for Dummies Questions & Answers

Understanding lseek

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...

8. 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

9. 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...

10. 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"...

LEARN ABOUT OSX

lseek

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

NAME

     lseek -- reposition read/write file offset

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.

     [EINVAL]		The seek location (calculated from offset and whence) is negative.

     [EOVERFLOW]	The seek location is too large to be stored in an object of type off_t.

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

SEE ALSO

     dup(2), open(2)

BUGS

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

STANDARDS

     The lseek() function conforms to IEEE Std 1003.1-1988 (``POSIX.1'').

4th Berkeley Distribution					  April 19, 1994					 4th Berkeley Distribution

10 More Discussions You Might Find Interesting

1. Programming

lseek in c

Discussion started by: arunkumar_mca

2. Shell Programming and Scripting

Need help on AWK implementation

Discussion started by: EmbedUX

3. Programming

Hairy Problem! lseek over 4G

Discussion started by: macroideal

4. Programming

Malloc implementation in C

Discussion started by: Gambit_b