C read
From the file indicated by the file descriptor fd, the read() function reads the specified amount of bytes cnt of input into the memory area indicated by buf. A successful read() updates the access time for the file. The read() function is also defined inside the <unistd.h> header file.
Syntax of read() in C
size_t read (int fd, void* buf, size_t cnt);
Parameters
- fd: file descriptor of the file from which data is to be read.
- buf: buffer to read data from
- cnt: length of the buffer
Return Value
- return Number of bytes read on success
- return 0 on reaching the end of file
- return -1 on error
- return -1 on signal interrupt
Important Points
- buf needs to point to a valid memory location with a length not smaller than the specified size because of overflow.
- fd should be a valid file descriptor returned from open() to perform the read operation because if fd is NULL then the read should generate an error.
- cnt is the requested number of bytes read, while the return value is the actual number of bytes read. Also, some times read system call should read fewer bytes than cnt.
Example of read() in C
C
// C program to illustrate// read system Call#include <fcntl.h>#include <stdio.h>#include <unistd.h>int main(){ int fd, sz; char* c = (char*)calloc(100, sizeof(char)); fd = open("foo.txt", O_RDONLY); if (fd < 0) { perror("r1"); exit(1); } sz = read(fd, c, 10); printf("called read(% d, c, 10). returned that" " %d bytes were read.\n", fd, sz); c[sz] = '\0'; printf("Those bytes are as follows: % s\n", c); return 0;} |
Output
called read(3, c, 10). returned that 10 bytes were read.
Those bytes are as follows: 0 0 0 foo.
Suppose that foobar.txt consists of the 6 ASCII characters “foobar”. Then what is the output of the following program?
C
// C program to illustrate// read system Call#include <fcntl.h>#include <stdio.h>#include <stdlib.h>#include <unistd.h>int main(){ char c; int fd1 = open("sample.txt", O_RDONLY, 0); int fd2 = open("sample.txt", O_RDONLY, 0); read(fd1, &c, 1); read(fd2, &c, 1); printf("c = %c\n", c); exit(0);} |
Output
c = f
The descriptors fd1 and fd2 each have their own open file table entry, so each descriptor has its own file position for foobar.txt. Thus, the read from fd2 reads the first byte of foobar.txt, and the output is c = f, not c = o.
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.