I am new to C, so I am not exactly sure where my error is. However, I do know that the great portion of the issue lies either in how I am storing the doubles in the d_buffer (double) array or the way I am printing it.

Specifically, my output keeps printing extremely large numbers (with around 10-12 digits before the decimal point and a trail of zeros after it. Additionally, this is an adaptation of an older program to allow for double inputs, so I only really added the two if statements (in the "read" for loop and the "printf" for loop) and the d_buffer declaration.

I would appreciate any input whatsoever as I have spent several hours on this error.

#include <stdio.h>
#include <fcntl.h>
#include <sys/types.h>
#include <unistd.h>
#include <string.h>


struct DataDescription
{
   char fieldname[30];
   char fieldtype;
   int  fieldsize;
};




/* -----------------------------------------------
   eof(fd): returns 1 if file `fd' is out of data
   ----------------------------------------------- */
int eof(int fd)
{
   char c;

   if ( read(fd, &c, 1) != 1 )
      return(1);
   else
    { lseek(fd, -1, SEEK_CUR);
      return(0);
    }
}


void main()
{
   FILE *fp;    /* Used to access meta data */
   int fd;  /* Used to access user data */

   /* ----------------------------------------------------------------
      Variables to hold the description of the data  - max 10 fields
      ---------------------------------------------------------------- */
   struct DataDescription DataDes[10];  /* Holds data descriptions
                                           for upto 10 fields */
   int  n_fields;                       /* Actual # fields */

   /* ------------------------------------------------------
      Variables to hold the data  - max 10 fields....
      ------------------------------------------------------ */
   char c_buffer[10][100];  /* For character data */
   int  i_buffer[10];       /* For integer data */
   double d_buffer[10];

   int i, j;
   int found;

   printf("Program for searching a mini database:\n");

   /* =============================
      Read in meta information
      ============================= */
   fp = fopen("db-description", "r");
   n_fields = 0;
   while ( fscanf(fp, "%s %c %d", DataDes[n_fields].fieldname, 
          &DataDes[n_fields].fieldtype,
          &DataDes[n_fields].fieldsize) > 0 )
      n_fields++;

   /* ---
      Prints meta information
      --- */
   printf("\nThe database consists of these fields:\n");
   for (i = 0; i < n_fields; i++)
      printf("Index %d: Fieldname `%s',\ttype = %c,\tsize = %d\n", 
        i, DataDes[i].fieldname, DataDes[i].fieldtype,
        DataDes[i].fieldsize);
   printf("\n\n");

   /* ---
      Open database file
      --- */
   fd = open("db-data", O_RDONLY);

   /* ---
      Print content of the database file
      --- */
   printf("\nThe database content is:\n");
   while ( ! eof(fd) )
   {  /* ------------------
         Read next record
         ------------------ */
      for (j = 0; j < n_fields; j++)
      {  
     if ( DataDes[j].fieldtype == 'I' )
        read(fd, &i_buffer[j], DataDes[j].fieldsize);
     if ( DataDes[j].fieldtype == 'F' )
        read(fd, &d_buffer[j], DataDes[j].fieldsize);
     if ( DataDes[j].fieldtype == 'C' )
        read(fd, &c_buffer[j], DataDes[j].fieldsize);       
      }

      double d;
      /* ------------------
         Print it...
         ------------------ */
      for (j = 0; j < n_fields; j++)
      {   
     if ( DataDes[j].fieldtype == 'I' )
        printf("%d ", i_buffer[j]);
     if ( DataDes[j].fieldtype == 'F' )
        d = d_buffer[j];
        printf("%lf ", d);
     if ( DataDes[j].fieldtype == 'C' )
        printf("%s ", c_buffer[j]);
      }
      printf("\n");
   }
   printf("\n");
   printf("\n");


}

Post edits output:

16777216 0.000000 107245694331284094976.000000 107245694331284094976.000000 Pi 
33554432 107245694331284094976.000000 2954938175610156848888276006519501238173891974277081114627768841840801736306392481516295906896346039950625609765296207682724801406770458881439696544971142710292689518104183685723154223544599940711614138798312668264956190761622328617992192.000000 2954938175610156848888276006519501238173891974277081114627768841840801736306392481516295906896346039950625609765296207682724801406770458881439696544971142710292689518104183685723154223544599940711614138798312668264956190761622328617992192.000000 Secret Key 
50331648 2954938175610156848888276006519501238173891974277081114627768841840801736306392481516295906896346039950625609765296207682724801406770458881439696544971142710292689518104183685723154223544599940711614138798312668264956190761622328617992192.000000 -0.000000 -0.000000 The number E 

Expected Output: 3 rows of data ending with the number "e = 2.18281828"

To reproduce the problem, the following two files need to be in the same directory as the lookup-data.c file:
- db-data
- db-description