Thread: write structure to a file

Originally Posted by tubby123

Because, all though the 'int' is saved in the file(in disk), a text doc wont be able to display because it only displays ascii characters.

Congratulations! You've just discovered the difference between a binary and text file.

Originally Posted by tubby123

3)This sound logically flawless. But after converting the number to a string, how do you make that string also a part of the structure so that i can still do my

Code:

fwrite(&s,sizeof(s),1,fp);

Decide if you want text or if you want binary. If you want text, write everything as text, and read it in as text and stuff it in your structure after you've read it. If you want binary, then fwrite is fine.

Originally Posted by tubby123

Guys , so basically can you help me with a code that will take any integer (PART OF A STRUCTURE) and display it on the text doc.

Code:

fprintf( file, "%d", mystruct.myint );

Quzah.

I would suggest having a pair of serialization functions.

One that takes as an argument your structure and returns a string that contains structure field values (somehow delimited); another that takes as an argument a string, parses it, fills in a structure, while making the necessary conversions, and returns that.

You can then serialize your structures, write them to human-readable files, and, when necessary, read the file back and fill in your structures.

No. I meant fscanf/fgets/whatever into a buffer, turn that into an int, and put the value in your structure. Why is it you think you need to do a single write instead of writing and reading multiple times? If you really want to write with a single write, sprintf into one big string and write that all at once.


Quzah.

Originally Posted by tubby123

@msh, thank you sir, thats a really good solution. I'll do that. Do you mean to say, basically have another structure that has only string instances.
Is this correct ?

Code:

struct s1
{
 char* str1;
 char* str2;
 int num;
}

convert this to 

struct s2
{
 char* str1;
 char* str2;
 char* num;
}

Ok @quzah , so now it makes sense to me what u said.
By stuff into your structure, you mean to say stuff into another structure ??? correct

No. Note that I wrote "while making the necessary conversions". That means that after you have read a record back from file and split it based on whatever delimiter you choose while serializing; I expect you to, for example, convert fields that should be int's to int's by using atoi() function and then assign it to the appropriate field in the structure.

There is no need for a second structure when implementing this solution.

This is what I was thinking of. Implementing the deserialization function for the provided prototype is left as an exercise for the OP.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MAX_RECORD_LEN 100

typedef struct
{
  char first_name[20];
  char last_name[20];
  int age;
  int height;
} record;

char* serialize(const record* p);
record deserialize(char* line);   /* prototype for deserialization function */


int main(void)
{
  FILE* file = fopen("out.txt", "w");
  if (!file) {
    fprintf(stderr, "!! could not open output file\n");
    return 1;
  }
    
  record person = {"John", "Doe", 40, 180};
  char* sdata = serialize(&person);
  fprintf(file, "%s\n", sdata);
  
  fclose(file);
  free(sdata);
  return 0;
}

char* serialize(const record* p)
{
  char* out = malloc(MAX_RECORD_LEN * sizeof(*out));
  if (!out) {
    fprintf(stderr, "!! could not allocate memory\n");
  } else {
    memset(out, '\0', MAX_RECORD_LEN);
    sprintf(out, "%s;%s;%d;%d", p->first_name, p->last_name, p->age, p->height);
  } 
  return out;
}

I can see the same discussion and responses over here
write structure to a file - C

Originally Posted by tubby123

@quzah, msh : thanks a lot for all the help.
I have 1 last question.
Suppose s is my structure instance,

Code:

struct abc
{
 char* name;
 int id;
}s;

And i use this to write mo my file.

Code:

fwrite(&s,sizeof(s),1,fp)

Then can i use format specifiers to specify the format/spacing between the variable inside the structure (WHILE USING FWRITE AND NOT FPRINTF)

Oh my, this is very simple... If you use fwrite() to write a struct to a binary file, you need to use fread() to read the binary file back into a struct. You can then access the data from the struct instead of the file. No big deal...

What you must change to make this work is the char* in your struct... it will need to be char name[30]; (or such) so that the data is actually stored in the struct and thus gets written to disk.

Originally Posted by tubby123

Then can i use format specifiers to specify the format/spacing between the variable inside the structure (WHILE USING FWRITE AND NOT FPRINTF)

I think this has already been strongly hinted at or explained in this thread, but if you do that, you will simply be printing the memory addresses in "name", and not the content it points to.

And I have no idea what "format specifiers" you want to use with "fwrite and not printf" since fwrite does not use format specifiers.

Using fwrite is only "easier" here, if by "easier" you mean: I did not have to code much and do not care whether what I am doing actually works -- because it won't.

Originally Posted by CommonTater

Oh my, this is very simple... If you use fwrite() to write a struct to a binary file, you need to use fread() to read the binary file back into a struct. You can then access the data from the struct instead of the file. No big deal...

Unless, of course, you count the string a char* points to as "data". In that case, it's a big deal because that data will have been irretrievably lost.