Thread: Saving information from a structure to a file

Google "serialization".

This may help:
SourceForge.net: Serialization - cpwiki
I wrote that, so it must be good.

Originally Posted by manasij7479

I read that just now and noticed that it 'somewhat' avoids stream serialization and focuses on binary.
Was that intentional?

I think "stream vs. binary" is ambiguous, perhaps that is derived from the use of C++ operators? After all, there is such a thing as a binary or bit stream, but I presume you mean a "text stream".

Anyway, yes it was intentional, but I just re-read it myself, lol, and you're right, it is slightly misleading that way, so I changed some of the text (eg, it now says, "Often serialized data files cannot be read as normal text in a file browser or editor, since the typed (ints, structs, etc) data need not be translated into strings. However, there are formal schemas for serialization (such as XML or JSON) that are based on such a translation; these are not much discussed here. "

Also...nice uncyclopedia link!

I am curious about the origin of that quote "Programmer (noun): An organism that can turn caffeine and alcohol into code". I'm sure it was actually laserlight

Originally Posted by manasij7479

Yes....
My point was that.. your block copying/dumping of memory could give interesting results when used with floating points.

Not if they are read in and out as floats, and the endianness is the same. As long as there are no pointers involved, you can write/transmit any C type out, including arrays and structs, arrays of structs, etc, and easily read them directly back in again (presuming you use the same types again).

There is a potential complication with struct padding, of course.

Here's a confusing thing about typedefs...this:

Code:

struct {
} whatever;

Defines a single instance of a struct called "whatever". But this:

Code:

typdef struct {
} whatever;

Defines a new type called "whatever", but not any instance of it. So with your code, you can either get rid of "typedef" or (preferably, IMO), declare an instance of type records:

Code:

records test;

So now you have an instance of type records called "test". That means you have to change "records" to "test" in main(); "records" is the typename, "test" is the actual variable.

In either case, you cannot refer to a member of a struct without the instance name, which you do in the two fwrite() calls. You need to use "records.age" (or "test.age") everytime.

Finally, records.p_name in your original code is an array, so it counts as a pointer for scanf() -- you should not use & there. But you do need & with record.age in the fwrite, because the first arg to fwrite is a pointer.

@teensicle... The others are giving you very good advice about the creation and use of your struct, listen to them carefully.

For my part in this I offer you the following demonstration program that uses a near identical struct... From my code word could be name and number could be age... so this is very like what you are probably trying to do. Pay paticular attention to how the records (structs) are written to and read from the data file... Copy and compile the program and run it to give yourself a demonstration of how it works... but don't even think about handing my code in as homework, it's been posted several times in a couple of different places so you'll just get yourself in trouble for cheating. However, if you follow the techniques used, you can finish your program.

Code:

//random access file demo
#include <stdlib.h>
#include <stdio.h>
#include <time.h>
#include <ctype.h>
#include <string.h>

#define FNAME "random.dat"

// test data struct
struct t_Record
  { int number;
    char word[16]; }
  Record;



///////////////////////////////////////////////////////
// Random Access File Handlers
//

// open or create the file
FILE *FileOpen(char* Filename)
  { FILE* pFile;
    pFile = fopen(Filename,"rb+");
    if (!pFile)
      pFile = fopen(Filename,"wb+");
    return pFile; }


// Write a record to the file
int WriteRecord(FILE *File, int RecNum)
  { if( fseek(File, RecNum * sizeof(Record), SEEK_SET) == 0 )
      if ( fwrite(&Record,sizeof(Record),1,File) )
        return 1;
    return 0; }


// read a record from the file
int ReadRecord(FILE *File, int RecNum)
  { if( fseek(File, RecNum * sizeof(Record), SEEK_SET) == 0 )
      if ( fread(&Record,sizeof(Record),1,File) )
        return 1;
    return 0; }


// add a new record to the file
int AddRecord(FILE *File)
  { fseek(File,0,SEEK_END);
    fwrite(&Record,sizeof(Record),1,File);
    return (ftell(File) / sizeof(Record)) - 1; }



//////////////////////////////////////////////////////////////
// View a Record
//

int ViewRecord (FILE *File, int RecNum)
  { if (! ReadRecord(File,RecNum))
      { printf("Invalid record\n"); 
        return -1; }
    printf("-----\n");
    printf("Record        : %d\n",RecNum);
    printf("Number Value  : %d\n",Record.number);
    printf("Word Value    : %s\n",Record.word);
    printf("-----\n");  
    return RecNum; }



//////////////////////////////////////////////////////////////
// Add a new record
//

int AddNewData(FILE *File)
  { memset(&Record,0,sizeof(Record));
    printf("\nEnter a number : ");
    scanf("%d", &Record.number);
    printf("Enter a word : ");
    scanf(" %s",Record.word);
    return AddRecord(File); }



//////////////////////////////////////////////////////////////
// Edit a record
//

int EditRecord(FILE *File, int RecNum)
  { if (! ReadRecord(File,RecNum))
      { printf("Invalid record\n");  
        return -1; }
    printf("\n-----\n");
    printf("Record        : %d\n",RecNum);
    printf("Number Value  : %d\n",Record.number);
    printf("Word Value    : %s\n",Record.word);
    printf("-----\n");  
    
    do
      { while(getchar() != '\n');
        printf("Change Values: Number, Word or Save (N, W or S) ? ");
        switch (toupper(getchar()))
          { case 'N' :
              printf("\nEnter new number : ");
              scanf("%d",&Record.number);
              break;
            case 'W' : 
              printf("Enter new word : ");
              scanf(" %15s", Record.word);
              break;
            case 'S' :
              if (WriteRecord(File,RecNum))
                printf("\nRecord #%d updated\n",RecNum);
              return RecNum; } }
    while(1);
    return -1; }


////////////////////////////////////////////////////////////////
// List records
// 

void ListRecords(FILE *File )
  { int i = 0;
    printf("\nRecord     Number\tWord\n\n");
    while (ReadRecord(File,i))
      { printf("%3d%16d\t%s\n",i,Record.number,Record.word); 
        i++; }
    printf("\n\n"); }



////////////////////////////////////////////////////////
// this is for demonstration purposes only
// you would not do this in a real program
void InitFile(FILE* File)
 { int x, y;
   memset(&Record,sizeof(Record),0);
   for (x = 0; x < 10; x++)
      { Record.number = rand();
        for (y = 0; y < ((Record.number % 15) + 1); y++)
          Record.word[y] = (rand() % 26) + 'a';
        Record.word[y] = 0;
        if (! WriteRecord(File,x))
          printf("Oh drat!");  } }
 


//////////////////////////////////////////////////////////
// program mains
//
int main (void)
  { int Rec = 0; // record number
    FILE *File;

    srand(time(NULL));

    File = FileOpen(FNAME); 
    if (!File)
      { printf("Curses foiled again!\n\n");
        exit(-1); }

    printf("Random Access File Demonstration\n\n");
 
    do
      { printf("Menu : Dummy, Add, Edit, View, List, Quit (D, A, E, V, L or Q) : ");
        switch(toupper(getchar()))
          { case 'D' :
              printf("Creating dummy file of 10 entries\n");
              InitFile(File);
              break;
            case 'A' :
              Rec = AddNewData(File);
              printf("Record #%d Added\n\n", Rec);
              break;              
            case 'E' :
              printf("\nRecord number (-1 Cancels): ");
              scanf("%d",&Rec);
              if (Rec > -1)
                EditRecord(File,Rec);
              break;
            case 'V' :
              printf("\nRecord number (-1 Cancels): ");
              scanf("%d",&Rec);
              if (Rec > -1)
                ViewRecord(File,Rec);
              break;
            case 'L' :
              ListRecords(File);
              break;
            case 'Q' :
              fclose(File);
              return 0; } 
              
         while(getchar() != '\n'); }
    while (1); 
    return 0; }