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cs:c_language:write_and_read_a_binary_file
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Write and read a binary file (fwrite e fread)

Concepts:

Functions fwrite and fread for writing and reading a binary file.

Text:
After the writing of a vector of structures (the elements of the structure are a vector of characters containing the name of a student, a variable of type unsigned int containing the student identifier and a variable of type float containing the average value of the scores of the exams attended by the student), the program must:

  • open in write mode a file with name “out.bin”, in order to store in the file the vector of structures
  • written the file in a binary way by using the function fwrite
  • close the file
  • reopen the file for reading it
  • read the file by using the function fread and fill the vector of structures with the read data
  • print all the data contained in the vector of structures
  • close the file

Solution:

binary_file_fwrite_fread.c
/* Write and read a binary file (fwrite and fread) */
 
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
#define LEN 50
#define N_STUD 2
 
typedef struct stud{
  char name[LEN+1];
  unsigned int id;
  float average;
}stud_t;
 
 
int main() {
  FILE *fp;
  stud_t student[N_STUD];
  int i;
  char row[LEN+1];
  unsigned int n_stud;
 
  strcpy(student[0].name, "Steven");
  student[0].id = 120000;
  student[0].average = 25.5;
 
  strcpy(student[1].name, "Julia");
  student[1].id = 120001;
  student[1].average = 28.5;
 
  fp = fopen("out.bin", "w"); /* Open the file for writing */
  if (fp == NULL){
    printf("Error: file out.bin cannot be opened\n");
    exit(1);
  }
 
 
  /* Write the file */
  /* NOTE 2: a binary file sometimes cannot be readable 
     correctly in a PC that it is not the one which generates it, 
     because, for instance, integer numbers can be coded with a
     different number of bytes.
  */
 
  /* Write in binary all the data contained in the structure */
  fwrite(student, sizeof(stud_t), N_STUD, fp); 
 
  /* DIMENSION OF THE GENERATED FILE
     The dimension of the generated file will be:
     n_stud*sizeof(stud_t)
     in the case of the computer used to test the program:
     2*60 = 120 byte
   */
 
  fclose(fp); /* Close the file */
 
 
 
  fp = fopen("out.bin", "r"); /* Open the file for reading */
  if (fp == NULL){
    printf("Error: file out.bin cannot be opened\n");
    exit(1);
  }
 
  /* Read the file */
  n_stud = 0;
  while( fread(&student[n_stud], sizeof(stud_t), 1, fp) == 1 ) {
    n_stud++;
  }
 
  fclose(fp); /* Close the file */
 
 
  /* Print the read records */
  for (i=0; i<n_stud; i++) {
    printf("%s %d %f\n", student[i].name, student[i].id, student[i].average);
  }
 
 
  return 0;
}

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/membri/wiki/data/pages/cs/c_language/write_and_read_a_binary_file.txt · Last modified: 2015/10/28 15:22 (external edit)


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