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Full Discussion: Reading a particular line from a .txt file
Top Forums Programming Reading a particular line from a .txt file Post 302425811 by achenle on Sunday 30th of May 2010 12:34:29 PM
Old 05-30-2010
That's only going to work if you can guarantee each line fits into your buffer.

If you can do that, great. If not, you pretty much have to count newline characters. This works, and should be fairly fast as you'd be relying on the OS to page in the data, which should be fast enough. If it's a really big file, and you know you're only going through it once, it'd be faster to use open() and read() with direct IO set so you bypass any page cache (if you're only looking once at each byte of a bunch of gigabytes of data, any caching is wasted cycles):

Code:
struct stat sb;
int fd = open( filename, O_RDONLY );
fstat( fd, &sb );
char *ptr = mmap( NULL, sb.st_size, PROT_READ, MAP_PRIVATE, fd, 0 );
uint64_t offset;
uint64_t line_count = 0;
uint64_t desired_line = 123455;

for ( offset = 0; offset < sb.st_size; offset++ )
{
    if ( '\n' == ptr[ offset ] )
    {
        line_count++;

        if ( line_count == desired_line )
        {
            break;
        }
    }
}

// if offset is less than file size, the desired line was found
if ( offset < sb.st_size )
{
    // line starts at offset + 1
    offset++;
        .
        .
        .
}

Note that has no error checking.
 

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LEARN ABOUT CENTOS

memcached_decrement_with_initial

MEMCACHED_DECREMENT_WITH_INITIAL(3)				   libmemcached 			       MEMCACHED_DECREMENT_WITH_INITIAL(3)

NAME

       memcached_decrement_with_initial - Incrementing and Decrementing Values

SYNOPSIS

       #include <libmemcached/memcached.h>

       memcached_return_t memcached_increment(memcached_st *ptr, const char *key, size_t key_length, uint32_t offset, uint64_t *value)

       memcached_return_t memcached_decrement(memcached_st *ptr, const char *key, size_t key_length, uint32_t offset, uint64_t *value)

       memcached_return_t  memcached_increment_with_initial(memcached_st *ptr,	const char *key, size_t key_length, uint64_t offset, uint64_t ini-
       tial, time_t expiration, uint64_t *value)

       memcached_return_t memcached_decrement_with_initial(memcached_st *ptr, const char *key, size_t key_length,  uint64_t offset,  uint64_t ini-
       tial, time_t expiration, uint64_t *value)

       memcached_return_t   memcached_increment_by_key(memcached_st *ptr,   const   char *group_key,   size_t group_key_length,  const	char *key,
       size_t key_length, uint32_t offset, uint64_t *value)

       memcached_return_t  memcached_decrement_by_key(memcached_st *ptr,  const   char *group_key,   size_t group_key_length,	const	char *key,
       size_t key_length, uint32_t offset, uint64_t *value)

       memcached_return_t   memcached_increment_with_initial_by_key(memcached_st *ptr,	 const	 char *group_key,  size_t group_key_length,  const
       char *key, size_t key_length, uint64_t offset, uint64_t initial, time_t expiration, uint64_t *value)

       memcached_return_t  memcached_decrement_with_initial_by_key(memcached_st *ptr,  const   char *group_key,   size_t group_key_length,   const
       char *key, size_t key_length, uint64_t offset, uint64_t initial, time_t expiration, uint64_t *value)

       Compile and link with -lmemcached

DESCRIPTION

       memcached(1) servers have the ability to increment and decrement keys (overflow and underflow are not detected). This gives you the ability
       to use memcached to generate shared sequences of values.

       memcached_increment takes a key and keylength and increments the value by the offset passed to it. The  value  is  then	returned  via  the
       uint32_t value pointer you pass to it.

       memcached_decrement  takes  a  key  and	keylength  and decrements the value by the offset passed to it. The value is then returned via the
       uint32_t value pointer you pass to it.

       memcached_increment_with_initial takes a key and keylength and increments the value by the offset passed to it. If the object specified	by
       key does not exist, one of two things may happen: If the expiration value is MEMCACHED_EXPIRATION_NOT_ADD, the operation will fail. For all
       other expiration values, the operation will succeed by seeding the value for that key with a initial value  to  expire  with  the  provided
       expiration time. The flags will be set to zero.The value is then returned via the uint32_t value pointer you pass to it.

       memcached_decrement_with_initial  takes a key and keylength and decrements the value by the offset passed to it. If the object specified by
       key does not exist, one of two things may happen: If the expiration value is MEMCACHED_EXPIRATION_NOT_ADD, the operation will fail. For all
       other  expiration  values,  the	operation  will succeed by seeding the value for that key with a initial value to expire with the provided
       expiration time. The flags will be set to zero.The value is then returned via the uint32_t value pointer you pass to it.

       memcached_increment_by_key(),		memcached_decrement_by_key(),		 memcached_increment_with_initial_by_key(),	       and
       memcached_decrement_with_initial_by_key() are master key equivalents of the above.

RETURN

       A  value  of  type  memcached_return_t  is returned.  On success that value will be MEMCACHED_SUCCESS.  Use memcached_strerror to translate
       this value to a printable string.

HOME

       To find out more information please check: http://libmemcached.org/

SEE ALSO

       memcached(1) libmemcached(3) memcached_strerror(3)

AUTHOR

       Brian Aker

COPYRIGHT

       2011-2013, Brian Aker DataDifferential, http://datadifferential.com/

1.0.16								 January 31, 2013			       MEMCACHED_DECREMENT_WITH_INITIAL(3)
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