C open
The open() function in C is used to open the file for reading, writing, or both. It is also capable of creating the file if it does not exist. It is defined inside <unistd.h> header file and the flags that are passed as arguments are defined inside <fcntl.h> header file.
Syntax of open() in C
int open (const char* Path, int flags);
Parameters
- Path: Path to the file which we want to open.
- Use the absolute path beginning with “/” when you are not working in the same directory as the C source file.
- Use relative path which is only the file name with extension, when you are working in the same directory as the C source file.
- flags: It is used to specify how you want to open the file. We can use the following flags.
|
Flags |
Description |
|---|---|
| O_RDONLY | Opens the file in read-only mode. |
| O_WRONLY | Opens the file in write-only mode. |
| O_RDWR | Opens the file in read and write mode. |
| O_CREAT | Create a file if it doesn’t exist. |
| O_EXCL | Prevent creation if it already exists. |
| O_ APPEND | Opens the file and places the cursor at the end of the contents. |
| O_ASYNC | Enable input and output control by signal. |
| O_CLOEXEC | Enable close-on-exec mode on the open file. |
| O_NONBLOCK | Disables blocking of the file opened. |
| O_TMPFILE | Create an unnamed temporary file at the specified path. |
How C open() works in OS
- Find the existing file on the disk.
- Create file table entry.
- Set the first unused file descriptor to point to the file table entry.
- Return file descriptor used, -1 upon failure.
Example of C open()
C
// C program to illustrate// open system call#include <errno.h>#include <fcntl.h>#include <stdio.h>#include <unistd.h>extern int errno;int main(){ // if file does not have in directory // then file foo.txt is created. int fd = open("foo.txt", O_RDONLY | O_CREAT); printf("fd = %d\n", fd); if (fd == -1) { // print which type of error have in a code printf("Error Number % d\n", errno); // print program detail "Success or failure" perror("Program"); } return 0;} |
Output
fd = 3
Input-output system calls in C | Create, Open, Close, Read, Write
System calls are the calls that a program makes to the system kernel to provide the services to which the program does not have direct access. For example, providing access to input and output devices such as monitors and keyboards. We can use various functions provided in the C Programming language for input/output system calls such as create, open, read, write, etc.
Before we move on to the I/O System Calls, we need to know about a few important terms.
Important Terminology
What is the File Descriptor?
The file descriptor is an integer that uniquely identifies an open file of the process.
File Descriptor table: A file descriptor table is the collection of integer array indices that are file descriptors in which elements are pointers to file table entries. One unique file descriptors table is provided in the operating system for each process.
File Table Entry: File table entries are a structure In-memory surrogate for an open file, which is created when processing a request to open the file and these entries maintain file position.
Standard File Descriptors: When any process starts, then that process file descriptors table’s fd(file descriptor) 0, 1, 2 open automatically, (By default) each of these 3 fd references file table entry for a file named /dev/tty
/dev/tty: In-memory surrogate for the terminal.
Terminal: Combination keyboard/video screen.
Read from stdin => read from fd 0: Whenever we write any character from the keyboard, it reads from stdin through fd 0 and saves to a file named /dev/tty.
Write to stdout => write to fd 1: Whenever we see any output to the video screen, it’s from the file named /dev/tty and written to stdout in screen through fd 1.
Write to stderr => write to fd 2: We see any error to the video screen, it is also from that file write to stderr in screen through fd 2.