Read and Write Structure to a File Using C

fwrite() and fread() is used to write to a file in C.

fwrite() syntax

fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream)

where

ptr - A pointer to array of elements to be written

size - Size in bytes of each element to be written

nmemb - Number of elements, each one with a size of bytes

stream – A pointer to a FILE object that specifies an output stream

fread() syntax

fread(void *ptr, size_t size, size_t nmemb, FILE *stream)

where

ptr - A pointer to a block of memory with a minimum size of size*nmemb bytes.

size - Size in bytes of each element to be read.

nmemb - Number of elements, each one with a size of bytes.

stream - A pointer to a FILE object that specifies an input stream.

Algorithm

Begin
   Create a structure Student to declare variables.
   Open file to write.
   Check if any error occurs in file opening.
   Initialize the variables with data.
   If file open successfully, write struct using write method.
      Close the file for writing.
   Open the file to read.
   Check if any error occurs in file opening.
   If file open successfully, read the file using read method.
      Close the file for reading.
   Check if any error occurs.
   Print the data.
End.

This is an example to read/write structure in C:

Example Code

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct Student {
   int roll_no;
   char name[20];
};
int main () {
   FILE *of;
   of= fopen ("c1.txt", "w");
   if (of == NULL) {
      fprintf(stderr, "
Error to open the file
");
      exit (1);
   }
   struct Student inp1 = {1, "Ram"};
   struct Student inp2 = {2, "Shyam"};
   fwrite (&inp1, sizeof(struct Student), 1, of);
   fwrite (&inp2, sizeof(struct Student), 1, of);
   if(fwrite != 0)
      printf("Contents to file written successfully !
");
   else
      printf("Error writing file !
");
   fclose (of);
   FILE *inf;
   struct Student inp;
   inf = fopen ("c1.txt", "r");
   if (inf == NULL) {
      fprintf(stderr, "
Error to open the file
");
      exit (1);
   }
   while(fread(&inp, sizeof(struct Student), 1, inf))
      printf ("roll_no = %d name = %s
", inp.roll_no, inp.name);
   fclose (inf);
}

Ouput

Contents to file written successfully !
roll_no = 1 name = Ram
roll_no = 2 name = Shyam