C create

The create() function is used to create a new empty file in C. We can specify the permission and the name of the file which we want to create using the create() function. 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 create() in C

int create(char *filename, mode_t mode);

Parameter

• filename: name of the file which you want to create
• mode: indicates permissions of the new file.

Return Value

• return first unused file descriptor (generally 3 when first creating use in the process because 0, 1, 2 fd are reserved)
• return -1 when an error

How C create() works in OS

• Create a new empty 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.

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.