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+====== Write and read a binary file (System Calls: ''open'', ''write'', ''read'' and ''close'') ======
+**Concepts:**\\
+Use the //system calls// ''open'', ''write'', ''read'' and ''close'' 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 ''write''
+  * close the file
+  * reopen the file for reading it
+  * read the file by using the function ''read'' and fill the vector of structures with the read data
+  * print all the data contained in the vector of structures
+  * close the file
+**Video solution:**\\
+Video with the explanation of the solution:
+<html><iframe width="560" height="315" src="https://www.youtube.com/embed/-Y2oF2PWauQ" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe></html>
+**Solution:**\\
+<file C binary_file_write_read.c>
+/* Write and read a binary file using system calls
+   (open, write, read e close) */
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h> /* For system calls write, read e close */
+#include <sys/types.h> /* For system call open */
+#include <sys/stat.h>  /* For system call open */
+#include <fcntl.h>     /* For system call open */
+#define LEN 50
+#define N_STUD 2
+typedef struct stud{
+  char name[LEN+1];
+  unsigned int id;
+  float average;
+}stud_t;
+int main() {
+  int fd; /* NOTE: fd is an integer, not a FILE* */
+  stud_t student[N_STUD];
+  int i;
+  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;
+  /* When in the function open the option O_CREAT is specified
+  as a second argument (i.e., create the file if it does not exist),
+  a third argument which specifies the permission used for the
+  creation of the new file is needed. The third argument is not
+  needed if the option O_CREAT has not been specified.
+  Examples of the third argument:
+  S_IRUSR 400 Read permission for the owner
+  S_IWUSR 200 Write permission for the owner
+  S_IRGRP 040 Read permission for the group
+  S_IROTH 004 Read permission for other users
+  Using the previous options, the file will be created with the
+  permissions 644, i.e., -rw-r--r--
+ */
+  fd = open("out.bin", O_WRONLY|O_CREAT|O_TRUNC, S_IRUSR|S_IWUSR|S_IRGRP|S_IROTH); /* Open the file for writing */
+  if (fd == -1) { /* In the case of error, open returns -1 ! */
+    printf("Errore: impossibile aprire il file out.bin\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 */
+  write(fd, student, N_STUD*sizeof(stud_t));
+  /* 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:
+*60 = 120 byte
+  */
+  close(fd); /* Close the file */
+  fd = open("out.bin", O_RDONLY); /* Open the file for reading */
+  if (fd == -1){
+    printf("Error: file out.bin cannot be opened\n");
+    exit(1);
+  }
+  /* Read the file */
+  n_stud = 0;
+  while( read(fd, &student[n_stud], sizeof(stud_t)) == sizeof(stud_t) ) {
+    n_stud++;
+  }
+  close(fd); /* 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;
+}
+</file>

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