Reading ith structure from a binary file in C

Question

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 :)

If you've written code that can successfully write structs to a file, then you have the ability to at least make an effort at this. We're not a code writing service. — Carey Gregory, May 16 '15 at 23:07
Since you know the size of each structure it should be very easy. — Some programmer dude, May 16 '15 at 23:10
When you are writing your example to post, consider a file offset of `(ith - 1) * sizeof (struct yourstruct)`. — David C. Rankin, May 16 '15 at 23:19
Carey Gregory: Where did i ask to write code for me? I just ask for explainning procedure. I don't know how to use fread function for this purpose since there is no pointer specifier to refer to particular position in file. — paul867, May 16 '15 at 23:25

score 3 · Answer 1 · edited May 23 '17 at 11:58

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.

Except for the absence of error handling, which you explicitly mention, what you've got works nicely for simple structures with a fixed size and no pointers. Life gets trickier if you have to deal with variable size structures, or the structures contain pointers instead of, say, fixed size arrays. Structures using a FAM (flexible array member) are also problematic because they too are not a fixed size. Such issues can be dealt with; it requires a _lot_ more code to do so. — Jonathan Leffler, May 17 '15 at 15:23
One change worth thinking about is combining declaration with initialization. For example, you could replace: `FILE *file_pointer; file_pointer = fopen("filename.dat","rb");` (2 statements) with `FILE *file_pointer = fopen("filename.dat","rb");` (1 statement). That works in C89. Similarly, you could replace `struct my_struct this_struct; this_struct.prop1=property; this_struct.prop2=property*2;` (3 statements) with `struct my_struct this_struct = { property, property*2 };` (C89) or even `struct my_struct this_struct = { .prop1 = property, .prop2 = property*2 };` (C99 designated initializers). — Jonathan Leffler, May 17 '15 at 15:29

Reading ith structure from a binary file in C

1 Answers1