4. Reading a File using System Calls 

Table of Contents

4. Reading a File using System Calls

This lab is an introduction to using systems call in C to interact with the operating system directly. This lab shows you how to read a file using the system call read().

Please create a file called lab4.c from the template C file for this assignment.

System Call overview 

In computing, a system call is the programmatic way in which a computer program requests a service from the kernel of the operating system it is executed on. A system call is a way for programs to interact with the operating system. A computer program makes a system call when it makes a request to the operating system’s kernel. A system call provides the services of the operating system to the user programs via Application Program Interface (API). It provides an interface between a process and operating system to allow user-level processes to request operating system services. System calls are the only entry points into the kernel system. All programs needing resources must use system calls.
Source: Introduction to System Calls (GeeksforGeeks)

Reading a file 

Reading a file is a three-step process using system calls:

Open the file: open() gets the filehandle or descriptor
Read the file: read() gets the bytes as a file stream
Close the file: close() closes the filehandle

Source: A Handy Guide To Handling Handles

4.1. `open()` System Call 

System call open() returns the file descriptor handle that is used to access the file.
More details at:
- open on Code Wiki for more details.
- open on Microsoft’s C Runtime Library reference.

Required Include Files

#include <fcntl.h>

Function Definition

int open(const char* path, int flags);
int open(const char* path, int flags, int file_permissions);

Example Usages

int file_descriptor;
file_descriptor = open("file_name.txt", O_RDONLY);

open
Field	Description
`const char *path`	The relative or absolute path to the file that is to be opened.
`int flags`	A bitwise ‘or’ separated list of values that determine the method in which the file is to be opened (whether it should be read only, read/write, whether it should be cleared when opened, etc).
`int file_permissions`	A bitwise ‘or’ separated list of values that determine the permissions of the file if it is created. See a list of legal values at the end.
`return value`	Returns the file descriptor for the new file. The file descriptor returned is always the smallest integer greater than zero that is still available. If a negative value is returned, then there was an error opening the file.

4.1.1. Task 

Your first task is to open a file and create a filehandle (a file descriptor object) to that file.

Create a text file and write data to it using echo and then verify using type
```
echo echo|set /p="My code works if we can read this text." > lab4.txt
type lab4.txt
```
Note

You should use CMD (not PowerShell) for this echo statement to work.

Expected Output:
Create file lab4.c or start with a template C file.
Create the file descriptor handle using open().
Verify that the file descriptor is valid. The return value of open() will be a positive number if the handle is valid.
Print an error message and exit the program with return 1 if the file is not valid.
Verify the code functions correctly.

Expected Output

4.2. `read()` System Call 

System call read() reads data from a file.
More details at:
- read on Code Wiki for more details.
- _read on Microsoft’s C Runtime Library reference.

Required Include Files

#include <fcntl.h>

Function Definition

int read(int file_descriptor, void* data, int num_bytes);

Example Usages

int file_size;     // Number of bytes returned
char data[128];    // Retrieved data
int num_bytes;     // Number of bytes to read

num_bytes = 128;   // Read in 128 bytes of the file

file_size = read(file_descriptor, data, num_bytes);

read
Field	Description
`int file_descriptor`	The file descriptor of where to read the input. You can either use a file descriptor obtained from the open system call, or you can use 0, 1, or 2, to refer to standard input, standard output, or standard error, respectively.
`const void *data`	A character array where the read content will be stored.
`int num_bytes`	The number of bytes to read before truncating the data. If the data to be read is smaller than num_bytes, all data is saved in the buffer.
`return value`	Returns the number of bytes that were read. If value is negative, then the system call returned an error.

4.2.1. Task: Print the results from `read()`

Your second task is to read the file and print the contents to the screen.

call read()
Print the results of data to the screen.

Expected Output:

You will notice some extra characters at the end of the text. The text should stop at the period, but the char array is not terminated using the null terminator.

4.2.2. Task: Insert the Null Terminator `\0`

The string that we print must be terminated:
My code works if we can read this text.\0

However, our string does not have a null terminator. The program reads in 128 bytes, but our text string is shorter than 128 bytes. The memory location still has leftover data that print_f writes to the output buffer.

//The length of the string 128 bytes:
char* buffer[128];

// read() obtained 128 bytes of data from memory:
num_bytes = 128;

The solution to the problem is determining how many bytes to read. We know the actual number of bytes read from variable file_size. We can insert the null terminator into the array after the last byte read.

printf("The number of bytes read is '%d'\n", file_size);
// We should set '\0' at in char array at index file_size

Before printing the string, set the value at array index file_size to \0.

Expected Output:

4.3. `close()` System Call 

System call close() closes the filehandle.
More details:
- on Code Wiki for more details.
- _close on Microsoft’s C Runtime Library reference.

Required Include Files

#include <unistd.h>

Function Definition

int close(int file_descriptor);

Example Usages

close(file_descriptor);


Field	Description
`int file_descriptor`	The file descriptor to be closed.
`return value`	Returns a 0 upon success, and a -1 upon failure. It is important to check the return value, because some network errors are not returned until the file is closed.

4.3.1. Task 

Your third task is to close the file.

call close() to close the file.
If the file closed successfully, exit the program normally.
If the file failed to close, write and error message and exit the program with a return value of 1.

Expected Output: