Read, Write, & Compare Decimals Worksheet Answers

Important Terminology

What is the File Descriptor?
File descriptor is integer that uniquely identifies an open up file of the process.

File Descriptor table: File descriptor tabular array is the collection of integer assortment indices that are file descriptors in which elements are pointers to file table entries. One unique file descriptors table is provided in operating organization for each process.

File Table Entry: File table entries is a structure In-retention surrogate for an open file, which is created when procedure request to opens file and these entries maintains file position.

Standard File Descriptors: When any process starts, so that process file descriptors tabular array's fd(file descriptor) 0, 1, 2 open up automatically, (By default) each of these iii fd references file table entry for a file named /dev/tty

/dev/tty: In-memory 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 save to file named /dev/tty.
Write to stdout => write to fd i : Whenever nosotros run across 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 encounter any error to the video screen, information technology is too from that file write to stderr in screen through fd ii.

I/O Organisation calls

Basically there are total 5 types of I/O organisation calls:

one. Create: Used to Create a new empty file.

          Syntax in C language:                    int create(char *filename, mode_t mode)

Parameter:

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

Returns:

  • render first unused file descriptor (generally iii when first create employ in procedure considering 0, 1, 2 fd are reserved)
  • render -1 when error

How it piece of work in Os

  • Create new empty file on disk
  • Create file table entry
  • Ready first unused file descriptor to indicate to file tabular array entry
  • Return file descriptor used, -one upon failure

2. open: Used to Open 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 way ]);        

Parameters

  • Path: path to file which you want to use
    • use absolute path brainstorm with "/", when you are not work in same directory of file.
    • Employ relative path which is but file proper noun with extension, when yous are work in same directory of file.
  • flags : How you similar to utilise
    • O_RDONLY: read just, O_WRONLY: write just, O_RDWR: read and write, O_CREAT: create file if it doesn't exist, O_EXCL: prevent creation if information technology already exists

How information technology works in Bone

  • Find the existing file on disk
  • Create file table entry
  • Set beginning unused file descriptor to signal to file table entry
  • Return file descriptor used, -i upon failure

C

#include<stdio.h>

#include<fcntl.h>

#include<errno.h>

extern int errno ;

int principal()

{

int fd = open( "foo.txt" , O_RDONLY | O_CREAT);

printf ( "fd = %d/n" , fd);

if (fd ==-i)

{

printf ( "Fault Number % d\n" , errno );

perror ( "Program" );

}

return 0;

}

Output:

fd = 3

3. shut: Tells the operating organisation you are done with a file descriptor and Close the file which pointed past fd.

          Syntax in C linguistic communication          #include <fcntl.h> int close(int fd);        

Parameter:

  • fd :file descriptor

Return:

  • 0 on success.
  • -ane on mistake.

How it works in the Bone

  • Destroy file tabular array entry referenced by element fd of file descriptor table
    – Equally long every bit no other process is pointing to it!
  • Set element fd of file descriptor tabular array to NULL

C

#include<stdio.h>

#include <fcntl.h>

int primary()

{

int fd1 = open up( "foo.txt" , O_RDONLY);

if (fd1 < 0)

{

perror ( "c1" );

leave (1);

}

printf ( "opened the fd = % d\due north" , fd1);

if (shut(fd1) < 0)

{

perror ( "c1" );

exit (1);

}

printf ( "closed the fd.\n" );

}

Output:

opened the fd = 3 closed the fd.

C

#include<stdio.h>

#include<fcntl.h>

int main()

{

int fd1 = open up( "foo.txt" , O_RDONLY, 0);

shut(fd1);

int fd2 = open( "baz.txt" , O_RDONLY, 0);

printf ( "fd2 = % d\north" , fd2);

exit (0);

}

Output:

fd2 = 3

Here, In this code first open up() returns 3 because when chief procedure created, so fd 0, ane, 2 are already taken past stdin, stdout and stderr. Then first unused file descriptor is 3 in file descriptor tabular array. After that in shut() system call is costless it this 3 file descriptor and and so after set 3 file descriptor as null. Then when we called 2d open(), and so outset unused fd is also 3. So, output of this plan is 3.

iv. read: From the file indicated by the file descriptor fd, the read() office reads cnt bytes of input into the retentiveness area indicated by buf. A successful read() updates the admission time 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 end of file
  • return -1 on error
  • render -1 on indicate interrupt

Important points

  • buf needs to bespeak to a valid retention location with length not smaller than the specified size because of overflow.
  • fd should be a valid file descriptor returned from open() to perform read functioning because if fd is NULL then read should generate fault.
  • cnt is the requested number of bytes read, while the render value is the actual number of bytes read. Too, some times read system call should read less bytes than cnt.

C

#include<stdio.h>

#include <fcntl.h>

int master()

{

int fd, sz;

char *c = ( char *) calloc (100, sizeof ( char ));

fd = open( "foo.txt" , O_RDONLY);

if (fd < 0) { perror ( "r1" ); exit (ane); }

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);

}

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

#include<stdio.h>

#include<unistd.h>

#include<fcntl.h>

#include<stdlib.h>

int main()

{

char c;

int fd1 = open( "sample.txt" , O_RDONLY, 0);

int fd2 = open( "sample.txt" , O_RDONLY, 0);

read(fd1, &c, ane);

read(fd2, &c, ane);

printf ( "c = %c\n" , c);

exit (0);

}

Output:

c = f

The descriptors fd1 and fd2 each have their ain 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.

5. write: Writes cnt bytes from buf to the file or socket associated with fd. cnt should not be greater than INT_MAX (defined in the limits.h header file). If cnt is zero, write() but returns 0 without attempting any other action.

#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 actually written

  • return Number of bytes written on success
  • return 0 on reaching end of file
  • render -one on error
  • return -1 on signal interrupt

Important points

  • The file needs to exist opened for write operations
  • buf needs to be at least as long equally specified by cnt because if buf size less than the cnt then buf will lead to the overflow condition.
  • cnt is the requested number of bytes to write, while the render value is the actual number of bytes written. This happens when fd have a less number of bytes to write than cnt.
  • If write() is interrupted past a signal, the event is i of the following:
    -If write() has not written any information even so, it returns -1 and sets errno to EINTR.
    -If write() has successfully written some information, it returns the number of bytes information technology wrote earlier information technology was interrupted.

C

#include<stdio.h>

#include <fcntl.h>

main()

{

int sz;

int fd = open( "foo.txt" , O_WRONLY | O_CREAT | O_TRUNC, 0644);

if (fd < 0)

{

perror ( "r1" );

get out (1);

}

sz = write(fd, "hello geeks\n" , strlen ( "hullo geeks\north" ));

printf ( "chosen write(% d, \"how-do-you-do geeks\\n\", %d)."

" It returned %d\due north" , fd, strlen ( "hi geeks\north" ), sz);

shut(fd);

}

Output:

called write(iii, "hullo geeks\n", 12).  information technology returned xi

Here, when y'all see in the file foo.txt later running the code, you go a "hello geeks". If foo.txt file already have some content in it then write organisation phone call overwrite the content and all previous content are deleted and only "hello geeks" content will have in the file.

Print "hello world" from the program without use whatever printf or cout function.

C

#include<stdio.h>

#include<string.h>

#include<unistd.h>

#include<fcntl.h>

int main ( void )

{

int fd[two];

char buf1[12] = "hullo 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(i, buf2, read(fd[ane], buf2, 12));

close(fd[0]);

close(fd[1]);

render 0;

}

Output:

hello world

In this code, buf1 array's cord "hello world" is outset write in to stdin fd[0] then after that this string write into stdin to buf2 array. After that write into buf2 assortment to the stdout and impress output " howdy earth ".
This commodity is contributed by Kadam Patel. If you like GeeksforGeeks and would like to contribute, y'all can also write an article using write.geeksforgeeks.org or mail your commodity to review-squad@geeksforgeeks.org. See your commodity actualization on the GeeksforGeeks main page and help other Geeks.
Please write comments if you find annihilation wrong, or you want to share more information nigh the topic discussed higher up.


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Source: https://www.geeksforgeeks.org/input-output-system-calls-c-create-open-close-read-write/

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