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1. Opens file descriptor, fd.
2. Uses fstat(fd) to get information such as size.
3. Set out to read size bytes, i.e. 4096. That would be line 253 of the code linked by @mattdm. read_size == 4096
   1. Ask; read: 4096 bytes.
   2. A short string is read i.e. 18 bytes. nread == 18
   3. read_size = read_size - nread (4096 - 18 = 4078)
   4. Ask; read: 4078 bytes
   5. 0 bytes read (as first read consumed all bytes in file).
   6. nread == 0 , line 255
   7. Unable to read 4096 bytes. Zero out buffer.
   8. Set error ENODATA.
   9. Return.
4. Report error.
5. Retry. (Above loop).
6. Fail.
7. Report error.
8. FINE.

1. Opens file descriptor, fd.
2. Uses fstat(fd) to get information such as size.
3. Set out to read size bytes, i.e. 4096. That would be line 253 of the code linked by @mattdm. read_size == 4096
   1. Ask; read: 4096 bytes.
   2. A short string is read i.e. 18 bytes. nread == 18
   3. read_size = read_size - nread (4096 - 18 = 4078)
   4. Ask; read: 4078 bytes
   5. 0 bytes read (as first read consumed all bytes in file).
   6. nread == 0 , line 255
   7. Unable to read 4096 bytes. Zero out buffer.
   8. Set error ENODATA.
   9. Return.
4. Report error.
5. Retry. (Above loop).
6. Fail.
7. Report error.
8. FINE.

First off /sys is a pseudo file system. If you look at /proc/filesystems you will find a list of registered file systems where quite a few has nodev in front. This indicates they are pseudo filesystems. This means they exists on a running kernel. Further they do not require a block device.

Further you can do a stat on a file and notice another peculiar feature; it occupies 0 blocks. Also inode of root (stat /sys) is 1. /stat/fs typically has inode 2. etc.

Ref 2: http://140.120.7.20/LinuxKernel/LinuxKernel/node34.html

First off /sys is a pseudo file system. If you look at /proc/filesystems you will find a list of registered file systems where quite a few has nodev in front. This indicates they are pseudo filesystems. This means they exists on a running kernel as a RAM-based filesystem. Further they do not require a block device.

Further you can do a stat on a file and notice another distinct feature; it occupies 0 blocks. Also inode of root (stat /sys) is 1. /stat/fs typically has inode 2. etc.

1. Opens file descriptor, fd.
2. Uses fstat(fd) to get information such as size.
3. Set out to read size bytes, i.e. 4096.
   1. Ask; read: 4096 bytes.
   2. A short string is read i.e. 18 bytes.
   3. size = size - read_bytes (4096 - 18 = 4078)
   4. Ask; read: 4078 bytes
   5. 0 bytes read (as first read consumed all bytes in file).
   6. Unable to read all bytes. Return error.
4. Report error.
5. Retry.
6. Fail.
7. Report error.
8. FINE.

1. Opens file descriptor, fd.

2. Uses fstat(fd) to get information such as st_size (also uses lstat and stat).

3. Check if file is likely to be sparse. That is the file has holes etc.

    copy.c:1010
    /* Use a heuristic to determine whether SRC_NAME contains any sparse
     * blocks.  If the file has fewer blocks than would normally be
     * needed for a file of its size, then at least one of the blocks in
     * the file is a hole.  */
    sparse_src = is_probably_sparse (&src_open_sb);
   

   As stat reports file to have zero blocks it is categorized as sparse.

4. Tries to read file by extent-copy (a more efficient way to copy normal sparse files), and fails.

5. Copy by sparse-copy.

   1. Starts out with max read size of MAXINT.
      Typically 18446744073709551615 bytes on a 32 bit system.
   2. Ask; read 4096 bytes.
   3. A short string is read i.e. 18 bytes.
   4. Check if a hole is needed, nope.
   5. Write buffer to target.
   6. Subtract 18 from max read size.
   7. Ask; read 4096 bytes.
   8. 0 bytes as all got consumed in first read.
   9. Return success.

6. All OK. Update flags for file.

7. FINE.

1. Opens file descriptor, fd.
2. Uses fstat(fd) to get information such as size.
3. Set out to read size bytes, i.e. 4096. That would be line 253 of the code linked by @mattdm. read_size == 4096
   1. Ask; read: 4096 bytes.
   2. A short string is read i.e. 18 bytes. nread == 18
   3. read_size = read_size - nread (4096 - 18 = 4078)
   4. Ask; read: 4078 bytes
   5. 0 bytes read (as first read consumed all bytes in file).
   6. nread == 0 , line 255
   7. Unable to read 4096 bytes. Zero out buffer.
   8. Set error ENODATA.
   9. Return.
4. Report error.
5. Retry. (Above loop).
6. Fail.
7. Report error.
8. FINE.

1. Opens file descriptor, fd.

2. Uses fstat(fd) to get information such as st_size (also uses lstat and stat).

3. Check if file is likely to be sparse. That is the file has holes etc.

    copy.c:1010
    /* Use a heuristic to determine whether SRC_NAME contains any sparse
     * blocks.  If the file has fewer blocks than would normally be
     * needed for a file of its size, then at least one of the blocks in
     * the file is a hole.  */
    sparse_src = is_probably_sparse (&src_open_sb);
   

   As stat reports file to have zero blocks it is categorized as sparse.

4. Tries to read file by extent-copy (a more efficient way to copy normal sparse files), and fails.

5. Copy by sparse-copy.

   1. Starts out with max read size of MAXINT.
      Typically 18446744073709551615 bytes on a 32 bit system.
   2. Ask; read 4096 bytes. (Buffer size allocated in memory from stat information.)
   3. A short string is read i.e. 18 bytes.
   4. Check if a hole is needed, nope.
   5. Write buffer to target.
   6. Subtract 18 from max read size.
   7. Ask; read 4096 bytes.
   8. 0 bytes as all got consumed in first read.
   9. Return success.

6. All OK. Update flags for file.

7. FINE.