how read to the end of /proc file

Question

I have /proc file. I don't understand some argument of read function. Read function look like that :

int procfile_read(char *buffer,  char **buffer_location,  off_t offset, int buffer_length, int *eof, void *data)

I don't know how use offset argument.

some example use offset argument like that :

if (offset > 0)
return 0;

and they explain : This is important because the standard read function from the library would continue to issuethe read system call until the kernel replies that it has no more information, or until its buffer is filled.

I have a file greater than buffer. how can i read to the end of file???

score 2 · Answer 1 · answered Apr 16 '12 at 19:05

See the proc_file_read function implementation and especially the comment:

  74                        /*
  75                         * How to be a proc read function
  76                         * ------------------------------
  77                         * Prototype:
  78                         *    int f(char *buffer, char **start, off_t offset,
  79                         *          int count, int *peof, void *dat)
  80                         *
  81                         * Assume that the buffer is "count" bytes in size.
  82                         *
  83                         * If you know you have supplied all the data you
  84                         * have, set *peof.
  85                         *
  86                         * You have three ways to return data:
  87                         * 0) Leave *start = NULL.  (This is the default.)
  88                         *    Put the data of the requested offset at that
  89                         *    offset within the buffer.  Return the number (n)
  90                         *    of bytes there are from the beginning of the
  91                         *    buffer up to the last byte of data.  If the
  92                         *    number of supplied bytes (= n - offset) is 
  93                         *    greater than zero and you didn't signal eof
  94                         *    and the reader is prepared to take more data
  95                         *    you will be called again with the requested
  96                         *    offset advanced by the number of bytes 
  97                         *    absorbed.  This interface is useful for files
  98                         *    no larger than the buffer.
  99                         * 1) Set *start = an unsigned long value less than
 100                         *    the buffer address but greater than zero.
 101                         *    Put the data of the requested offset at the
 102                         *    beginning of the buffer.  Return the number of
 103                         *    bytes of data placed there.  If this number is
 104                         *    greater than zero and you didn't signal eof
 105                         *    and the reader is prepared to take more data
 106                         *    you will be called again with the requested
 107                         *    offset advanced by *start.  This interface is
 108                         *    useful when you have a large file consisting
 109                         *    of a series of blocks which you want to count
 110                         *    and return as wholes.
 111                         *    (Hack by Paul.Russell@rustcorp.com.au)
 112                         * 2) Set *start = an address within the buffer.
 113                         *    Put the data of the requested offset at *start.
 114                         *    Return the number of bytes of data placed there.
 115                         *    If this number is greater than zero and you
 116                         *    didn't signal eof and the reader is prepared to
 117                         *    take more data you will be called again with the
 118                         *    requested offset advanced by the number of bytes
 119                         *    absorbed.
 120                         */

how read to the end of /proc file

1 Answers1