End of Data Reached Before Content Length Was Read

Important Terminology

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

File Descriptor tabular array: File descriptor table is the collection of integer array indices that are file descriptors in which elements are pointers to file table entries. 1 unique file descriptors table is provided in operating system for each process.

File Table Entry: File table entries is a construction In-memory 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, and so that procedure file descriptors tabular array'south 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
Final: 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 1 : Whenever we see whatever 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 besides from that file write to stderr in screen through fd two.

I/O Organisation calls

Basically there are full v types of I/O organization calls:

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

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

Parameter:

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

Returns:

• render first unused file descriptor (generally 3 when first create use in procedure because 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 tabular array entry
• Set up first unused file descriptor to point to file table entry
• Render file descriptor used, -1 upon failure

ii. open: Used to Open up the file for reading, writing or both.

          Syntax in C language                    #include<sys/types.h> #include<sys/stat.h> #include <fcntl.h>   int open (const char* Path, int flags [, int style ]);        

Parameters

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

How information technology works in OS

• Find the existing file on deejay
• Create file table entry
• Ready beginning unused file descriptor to point to file tabular array entry
• Return file descriptor used, -i upon failure

C

#include<stdio.h>

#include<fcntl.h>

#include<errno.h>

extern int errno ;

int primary()

{

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

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

if (fd ==-ane)

{

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

perror ( "Program" );

}

return 0;

}

Output:

fd = 3

3. close: Tells the operating system you are washed with a file descriptor and Close the file which pointed by fd.

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

Parameter:

• fd :file descriptor

Render:

• 0 on success.
• -ane on error.

How it works in the OS

• Destroy file tabular array entry referenced by chemical element fd of file descriptor tabular array
  – As long as no other procedure is pointing to information technology!
• Set up element fd of file descriptor table to NULL

C

#include<stdio.h>

#include <fcntl.h>

int chief()

{

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

if (fd1 < 0)

{

perror ( "c1" );

exit (1);

}

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

if (close(fd1) < 0)

{

perror ( "c1" );

exit (one);

}

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

}

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\n" , fd2);

exit (0);

}

Output:

fd2 = 3

Here, In this lawmaking first open() returns 3 considering when main process created, and so fd 0, ane, 2 are already taken by stdin, stdout and stderr. And then starting time unused file descriptor is 3 in file descriptor table. After that in close() system phone call is costless it this 3 file descriptor and so after set 3 file descriptor as null. So when we chosen 2nd open(), then first unused fd is also 3. And then, output of this program is 3.

4. read: From the file indicated past the file descriptor fd, the read() part reads cnt bytes of input into the memory area indicated by buf. A successful read() updates the access 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 terminate of file
• return -1 on error
• return -1 on signal interrupt

Important points

• buf needs to point to a valid retention location with length not smaller than the specified size considering of overflow.
• fd should be a valid file descriptor returned from open up() to perform read operation because if fd is NULL and then read should generate error.
• cnt is the requested number of bytes read, while the return value is the actual number of bytes read. Likewise, some times read system call should read less bytes than cnt.

C

#include<stdio.h>

#include <fcntl.h>

int chief()

{

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, x). returned that"

" %d bytes were read.\n" , fd, sz);

c[sz] = '\0' ;

printf ( "Those bytes are equally follows: % s\due north" , c);

}

Output:

chosen 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". Then what is the output of the following program?

C

#include<stdio.h>

#include<unistd.h>

#include<fcntl.h>

#include<stdlib.h>

int chief()

{

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, ane);

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

exit (0);

}

Output:

c = f

The descriptors fd1 and fd2 each take their ain open file table entry, and so each descriptor has its own file position for foobar.txt . Thus, the read from fd2 reads the showtime byte of foobar.txt , and the output is c = f, not c = o.

five. write: Writes cnt bytes from buf to the file or socket associated with fd. cnt should non be greater than INT_MAX (divers in the limits.h header file). If cnt is aught, write() only returns 0 without attempting whatever 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
• return -1 on error
• return -1 on betoken interrupt

Important points

• The file needs to be opened for write operations
• buf needs to be at least as long as specified by cnt because if buf size less than the cnt so buf will lead to the overflow status.
• cnt is the requested number of bytes to write, while the return 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 by a signal, the effect is one of the following:
  -If write() has not written any data still, 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>

main()

{

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, "howdy geeks\n" , strlen ( "howdy geeks\n" ));

printf ( "called write(% d, \"hello geeks\\n\", %d)."

" It returned %d\northward" , fd, strlen ( "hello geeks\north" ), sz);

shut(fd);

}

Output:

called write(3, "how-do-you-do geeks\n", 12).  it returned 11

Here, when you see in the file foo.txt after running the code, you get a "hello geeks". If foo.txt file already take some content in it then write system telephone call overwrite the content and all previous content are deleted and simply "how-do-you-do geeks" content will accept in the file.

Print "hullo world" from the plan without utilise any printf or cout function.

C

#include<stdio.h>

#include<cord.h>

#include<unistd.h>

#include<fcntl.h>

int main ( void )

{

int fd[2];

char buf1[12] = "hello world" ;

char buf2[12];

fd[0] = open up( "foobar.txt" , O_RDWR);

fd[ane] = open( "foobar.txt" , O_RDWR);

write(fd[0], buf1, strlen (buf1));

write(1, buf2, read(fd[1], buf2, 12));

shut(fd[0]);

shut(fd[1]);

render 0;

}

Output:

hello globe

In this lawmaking, buf1 array'due south string "hello world" is kickoff write in to stdin fd[0] so after that this cord write into stdin to buf2 assortment. After that write into buf2 array to the stdout and print output " howdy world ".
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End of Data Reached Before Content Length Was Read

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

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