CODEWITHSUNDEEP
cfilebinarystructure
paul867
paul867

Reputation: 41

Reading ith structure from a binary file in C

I have some binary file that i write struct objects to(of one defined type). I want to be able to read particular (ith) "struct block" from a binary file to a struct and display it. The only idea that comes to my mind is to create an array of structures containning all of them so that i could have an access to the ordinary one but it doesn't seem to be an efficient way. I would appreciate if one could help me with this problem :)

Upvotes: 1

Views: 454

Answers (1)

user1717828
user1717828

Reputation: 7225

I am new to C but think I can help with this. Is this the kind of thing you were looking to do:

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

//just a struct for purposes of demonstration
struct my_struct{
  int prop1;
  int prop2;
};

//writes structs to filename.dat
void writeStruct(int property){
  FILE *file_pointer;
  file_pointer = fopen("filename.dat","ab");

  //define and assign variables to a quick dummy struct
  struct my_struct this_struct;
  this_struct.prop1=property;
  this_struct.prop2=property*2;

  //write struct to file
  fwrite(&this_struct, sizeof(this_struct), 1, file_pointer);
  fclose(file_pointer);
}

//returns the nth struct stored in "filename.dat"
struct my_struct getNthStruct(long int n){
  FILE *file_pointer;
  file_pointer = fopen("filename.dat","rb");

  //will be the struct we retrieve from the file
  struct my_struct nth_struct;

  //set read position of file to nth struct instance in file
  fseek(file_pointer, n*sizeof(struct my_struct), SEEK_SET);

  //copy specified struct instance to the 'nth_struct' variable
  fread(&nth_struct, sizeof(struct my_struct), 1, file_pointer);

  return nth_struct;
}

int main(){
  //write a bunch of structs to a file
  writeStruct(1);
  writeStruct(2);
  writeStruct(3);
  writeStruct(4);
  writeStruct(5);

  //get nth struct (2 is third struct, in this case)
  struct my_struct nth_struct;
  nth_struct=getNthStruct(2);

  printf("nth_struct.prop1=%d, nth_struct.prop2=%d\n",
      nth_struct.prop1, //outputs 3
      nth_struct.prop2); //outputs 6

  return 0;
}

I intentionally didn't check for obvious errors (FILE pointers returning NULL, length of files, etc) for brevity and isolating the core concept.

Feedback is welcome.

Upvotes: 3

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