78325827.c

#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main( void ) {
   int pipefds[2];
   if ( pipe( pipefds ) == -1 )
      err( 1, "pipe" );
   
   pid_t pid = fork();
   if ( pid == -1 )
      err( 1, "fork" );

   if ( pid == 0 ) {
      close( pipefds[ 0 ] );

      FILE *fh = fdopen( pipefds[ 1 ], "w" );
      if ( fh == NULL)
         err( 1, "fdopen" );

      for ( unsigned i=1; i<=1000; ++i ) {
         if ( fprintf( fh, "This is line %u\n", i ) < 0 )
            err( 1, "fprintf" );
      }

      if ( fclose( fh ) == EOF )
         err( 1, "fclose" );

      exit( 0 );
   }

   close( pipefds[ 1 ] );

   int newfd = dup( pipefds[ 0 ] );
   if ( newfd == -1 )
      err( 1, "dup" );

   {
      FILE *fh = fdopen( pipefds[ 0 ], "r" );
      if ( fh == NULL )
         err( 1, "fdopen" );

      char *line = NULL;
      size_t n = 0;
      for ( unsigned i = 5; i--; ) {
         ssize_t bytes_read = getline( &line, &n, fh );
         if ( bytes_read == -1 )
            err( 1, "getline" );
         if ( bytes_read == 0 )
            errx( 1, "Unexpected EOF" );

         if ( line[ bytes_read-1 ] == '\n' )
            line[ bytes_read-1 ] = 0;

         printf( "<%s>\n", line );
      }

      if ( fclose( fh ) == EOF )
         err( 1, "fclose" );

      free( line );
   }

   printf( "Should follow with \"This is line 6\" if there's no data loss:\n" );

   {
      char s[ 31 ];
      ssize_t bytes_read = read( newfd, s, 30 );
      if ( bytes_read == -1 )
         err( 1, "read" );

      s[ bytes_read ] = 0;
      printf( "[%s]\n", s );
   }
}

/* ****************************************
Output:
god
<This is line 1>
<This is line 2>
<This is line 3>
<This is line 4>
<This is line 5>
Should follow with "This is line 6" if there's no data loss:
[is line 248              <--- Lines 6 to 247 are lost, as well as the start of line 248.
This is line 249
T]
**************************************** */