R Kelly Can't Read Write or Add
Of import Terminology
What is the File Descriptor?
File descriptor is integer that uniquely identifies an open file of the process.
File Descriptor table: File descriptor tabular array is the collection of integer array indices that are file descriptors in which elements are pointers to file tabular array entries. I unique file descriptors tabular array is provided in operating system for each process.
File Table Entry: File table entries is a structure In-memory surrogate for an open file, which is created when process asking to opens file and these entries maintains file position.
Standard File Descriptors: When whatsoever process starts, and then that procedure file descriptors table'due south fd(file descriptor) 0, 1, ii open up automatically, (By default) each of these 3 fd references file table entry for a file named /dev/tty
/dev/tty: In-retention surrogate for the terminal
Last: Combination keyboard/video screen
Read from stdin => read from fd 0 : Whenever we write any character from keyboard, it read from stdin through fd 0 and salvage to file named /dev/tty.
Write to stdout => write to fd one : Whenever nosotros run into whatsoever 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 whatever mistake to the video screen, it is likewise from that file write to stderr in screen through fd 2.
I/O System calls
Basically there are full 5 types of I/O system calls:
1. Create: Used to Create a new empty file.
Syntax in C language: int create(char *filename, mode_t mode)
Parameter:
- filename : name of the file which you lot want to create
- mode : indicates permissions of new file.
Returns:
- return first unused file descriptor (more often than not 3 when first create use in process considering 0, i, 2 fd are reserved)
- return -ane when fault
How information technology work in Os
- Create new empty file on deejay
- Create file table entry
- Set up first unused file descriptor to point to file table entry
- Return file descriptor used, -ane upon failure
2. open: Used to Open up the file for reading, writing or both.
Syntax in C linguistic communication #include<sys/types.h> #include<sys/stat.h> #include <fcntl.h> int open up (const char* Path, int flags [, int style ]);
Parameters
- Path: path to file which you desire to use
- utilise absolute path begin with "/", when you are not work in same directory of file.
- Use relative path which is just file proper noun with extension, when you are work in same directory of file.
- flags : How you similar to employ
- O_RDONLY: read merely, O_WRONLY: write only, O_RDWR: read and write, O_CREAT: create file if it doesn't exist, O_EXCL: foreclose creation if it already exists
How it works in OS
- Find the existing file on disk
- Create file table entry
- Ready first unused file descriptor to signal to file tabular array entry
- Render file descriptor used, -one upon failure
C
#include<stdio.h>
#include<fcntl.h>
#include<errno.h>
extern int errno ;
int main()
{
int fd = open up( "foo.txt" , O_RDONLY | O_CREAT);
printf ( "fd = %d/due north" , fd);
if (fd ==-1)
{
printf ( "Fault Number % d\n" , errno );
perror ( "Program" );
}
return 0;
}
Output:
fd = iii
iii. close: Tells the operating system you lot are done with a file descriptor and Close the file which pointed by fd.
Syntax in C linguistic communication #include <fcntl.h> int close(int fd);
Parameter:
- fd :file descriptor
Return:
- 0 on success.
- -i on error.
How it works in the OS
- Destroy file tabular array entry referenced by element fd of file descriptor table
– As long every bit no other process is pointing to information technology! - Set element fd of file descriptor table to NULL
C
#include<stdio.h>
#include <fcntl.h>
int main()
{
int fd1 = open( "foo.txt" , O_RDONLY);
if (fd1 < 0)
{
perror ( "c1" );
exit (one);
}
printf ( "opened the fd = % d\n" , fd1);
if (close(fd1) < 0)
{
perror ( "c1" );
leave (ane);
}
printf ( "closed the fd.\north" );
}
Output:
opened the fd = three closed the fd.
C
#include<stdio.h>
#include<fcntl.h>
int main()
{
int fd1 = open up( "foo.txt" , O_RDONLY, 0);
close(fd1);
int fd2 = open( "baz.txt" , O_RDONLY, 0);
printf ( "fd2 = % d\northward" , fd2);
get out (0);
}
Output:
fd2 = iii
Here, In this code start open up() returns 3 because when chief process created, then fd 0, 1, 2 are already taken by stdin, stdout and stderr. So first unused file descriptor is iii in file descriptor table. Later that in close() system call is free it this 3 file descriptor and then afterward set iii file descriptor every bit null. So when nosotros called second open up(), and then first unused fd is likewise 3. And then, output of this program is 3.
four. read: From the file indicated past the file descriptor fd, the read() role reads cnt bytes of input into the retention area indicated by buf. A successful read() updates the access fourth dimension for the file.
Syntax in C language size_t read (int fd, void* buf, size_t cnt);
Parameters:
- fd: file descriptor
- buf: buffer to read data from
- cnt: length of buffer
Returns: How many bytes were actually read
- return Number of bytes read on success
- return 0 on reaching terminate of file
- render -1 on error
- return -1 on indicate interrupt
Important points
- buf needs to bespeak to a valid retentiveness location with length not smaller than the specified size because of overflow.
- fd should exist a valid file descriptor returned from open() to perform read operation because if fd is NULL then read should generate error.
- cnt is the requested number of bytes read, while the return value is the actual number of bytes read. Too, some times read system phone call should read less bytes than cnt.
C
#include<stdio.h>
#include <fcntl.h>
int primary()
{
int fd, sz;
char *c = ( char *) calloc (100, sizeof ( char ));
fd = open( "foo.txt" , O_RDONLY);
if (fd < 0) { perror ( "r1" ); go out (1); }
sz = read(fd, c, ten);
printf ( "called read(% d, c, x). returned that"
" %d bytes were read.\n" , fd, sz);
c[sz] = '\0' ;
printf ( "Those bytes are as follows: % southward\northward" , c);
}
Output:
called read(iii, 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". So what is the output of the following plan?
C
#include<stdio.h>
#include<unistd.h>
#include<fcntl.h>
#include<stdlib.h>
int main()
{
char c;
int fd1 = open up( "sample.txt" , O_RDONLY, 0);
int fd2 = open( "sample.txt" , O_RDONLY, 0);
read(fd1, &c, one);
read(fd2, &c, 1);
printf ( "c = %c\north" , c);
exit (0);
}
Output:
c = f
The descriptors fd1 and fd2 each accept their ain open file table entry, so each descriptor has its own file position for foobar.txt . Thus, the read from fd2 reads the start byte of foobar.txt , and the output is c = f, not c = o.
5. write: Writes cnt bytes from buf to the file or socket associated with fd. cnt should non be greater than INT_MAX (defined in the limits.h header file). If cnt is zero, write() simply returns 0 without attempting whatsoever other activeness.
#include <fcntl.h> size_t write (int fd, void* buf, size_t cnt);
Parameters:
- fd: file descriptor
- buf: buffer to write data to
- cnt: length of buffer
Returns: How many bytes were really written
- return Number of bytes written on success
- render 0 on reaching end of file
- return -1 on fault
- return -one on betoken interrupt
Important points
- The file needs to be opened for write operations
- buf needs to be at to the lowest degree every bit long equally specified past cnt because if buf size less than the cnt and so buf will lead to the overflow condition.
- cnt is the requested number of bytes to write, while the return value is the actual number of bytes written. This happens when fd take a less number of bytes to write than cnt.
- If write() is interrupted by a signal, the effect is ane of the following:
-If write() has non written any data notwithstanding, information technology returns -i and sets errno to EINTR.
-If write() has successfully written some data, information technology returns the number of bytes it wrote earlier it was interrupted.
C
#include<stdio.h>
#include <fcntl.h>
master()
{
int sz;
int fd = open up( "foo.txt" , O_WRONLY | O_CREAT | O_TRUNC, 0644);
if (fd < 0)
{
perror ( "r1" );
exit (1);
}
sz = write(fd, "hello geeks\north" , strlen ( "hello geeks\n" ));
printf ( "chosen write(% d, \"howdy geeks\\n\", %d)."
" Information technology returned %d\n" , fd, strlen ( "hello geeks\due north" ), sz);
close(fd);
}
Output:
called write(3, "hello geeks\n", 12). information technology returned 11
Here, when you see in the file foo.txt afterward running the code, y'all get a "hello geeks". If foo.txt file already have some content in information technology then write system call overwrite the content and all previous content are deleted and only "hullo geeks" content volition have in the file.
Print "hello globe" from the plan without utilize whatever printf or cout function.
C
#include<stdio.h>
#include<cord.h>
#include<unistd.h>
#include<fcntl.h>
int primary ( void )
{
int fd[2];
char buf1[12] = "howdy world" ;
char buf2[12];
fd[0] = open( "foobar.txt" , O_RDWR);
fd[1] = open( "foobar.txt" , O_RDWR);
write(fd[0], buf1, strlen (buf1));
write(1, buf2, read(fd[i], buf2, 12));
close(fd[0]);
close(fd[one]);
return 0;
}
Output:
how-do-you-do globe
In this lawmaking, buf1 array's string "howdy world" is showtime write in to stdin fd[0] and so after that this string write into stdin to buf2 array. After that write into buf2 assortment to the stdout and print output " hi world ".
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