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

Solution:

```/* 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;
char row[LEN+1];
unsigned int n_stud;

strcpy(student.name, "Steven");
student.id = 120000;
student.average = 25.5;

strcpy(student.name, "Julia");
student.id = 120001;
student.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:
2*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);
}

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;
}``` 