Reading between two given line number of a File

 It is sometimes very important, to let program to read a big file only some specific part/s.

For example, I was and still trying with Openstreetmap with readosm library. Which really need such option.

As .OSM file is quite a large and readosm does read every single bit of the file, on other hand my projected implementation hardware is quite low in resources, badly needed to optimize readosm reading style.

I'd and still have a plan of doing indexing for .OSM file and apply program to read according it.

To do so, here comes the first part of virtually slicing the .OSM file and apply to program to read by given lines between.

My program is an extension of
http://rosettacode.org/wiki/Read_a_specific_line_from_a_file



Here is the modified version

 /* This file is a modification of   
      hxxp://rosettacode.org/wiki/Read_a_specific_line_from_a_file  
 */  
 #include <stdio.h>  
 #include <stdlib.h>  
 #define BUF_SIZE ( 256 )  
 char *get_nth_line (FILE *f, int first_line_no, int last_line_no)  
 {  
   char  buf[ BUF_SIZE ];  
   size_t curr_alloc = BUF_SIZE, curr_ofs = 0;  
   char  *line   = malloc( BUF_SIZE );  
   int  in_line  = last_line_no == 1;  
   size_t bytes_read;  
   /* Illegal to ask for a line before the first one. */  
   if ( first_line_no < 1 )  
     return NULL;  
   /* Handle out-of-memory by returning NULL */  
   if ( !line )  
     return NULL;  
   /* Scan the file looking for newlines */  
   int first_line_no_handler = first_line_no;  
      last_line_no = last_line_no + 2 ;  
  while ( (last_line_no - first_line_no) &&   
            ( bytes_read = fread( buf, 1, BUF_SIZE, f ) ) > 0 )  
   {  
     int i;  
     for ( i = 0 ; i < bytes_read ; i++ )  
     {  
       if ( in_line )  
       {  
         if ( curr_ofs >= curr_alloc )  
         {  
           curr_alloc <<= 1;  
           line = realloc( line, curr_alloc );  
           if ( !line )  /* out of memory? */  
             return NULL;  
         }  
         line[ curr_ofs++ ] = buf[i];  
       }  
       if ( buf[i] == '\n' )       
       {  
                      first_line_no_handler--;  
                     if(first_line_no_handler <= 1)  
                     {  in_line = 1;  
                          last_line_no--;  
                     }  
                     if ( (last_line_no - first_line_no) == 0)  
           break;  
                }  
     }  
   }  
   /* Didn't find the line? */  
   if ( (last_line_no - first_line_no) != 0 )   
   {  
     free( line );  
     return NULL;  
   }  
   /* Resize allocated buffer to what's exactly needed by the string   
     and the terminating NUL character. Note that this code *keeps*  
     the terminating newline as part of the string.   
    */  
   line = realloc( line, curr_ofs + 1 );  
   if ( !line ) /* out of memory? */  
     return NULL;  
   /* Add the terminating NUL. */  
   line[ curr_ofs ] = '\0';  
   /* Return the line. Caller is responsible for freeing it. */  
   return line;  
 }  
 int main( int argc, char *argv[] )  
 {  
      if(argc != 4)  
      {  
           fprintf(stderr, "Usage: line_read FILE NAME First_line Last_line\n");  
           return -1;  
      }  
      FILE *fp = fopen(argv[1], "r");  
   char *line_bw = get_nth_line( fp, atoi(argv[2]),  
                                         atoi(argv[3]));  
   if ( line_bw )  
   {  
     printf("The line between Text is:\n%s\n", line_bw );  
     free( line_bw );  
   } else  
   {  
     printf("Did not find the 7th line of input. Reason: ");  
     if ( feof( stdin ) )  
       puts("End of file reached.");  
     else if ( ferror( stdin ) )  
       puts("Error reading input.");  
     else  
       puts("Out of memory.");  
   }  
   return 0;  
 }  

The program is very easy to understand. Now I need to cast the pointer last as a FILE pointer.