write

Name

write, writev, pwrite
- write to a file descriptor

Library

libc.lib

Synopsis

#include <sys/types.h>

#include <sys/uio.h>

#include <unistd.h>

ssize_t write (int d, const void *buf, size_t nbytes);

ssize_t pwrite (int d, const void *buf, size_t nbytes, off_t offset);

ssize_t writev (int d, const struct iovec *iov, int iovcnt);

ssize_t pwritev (int d, const struct iovec *iov, int iovcnt, off_t offset);

Return values

Upon successful completion the number of bytes which were written is returned. Otherwise a -1 is returned and the global variable errno is set to indicate the error.

Detailed description

The write system call attempts to write nbytes of data to the object referenced by the descriptor d from the buffer pointed to by buf. The writev system call performs the same action, but gathers the output data from the iovcnt buffers specified by the members of the iov array: iov[0], iov[1], ..., iov[iovcnt-1]. The pwrite and pwritev system calls perform the same functions, but write to the specified position in the file without modifying the file pointer.

For writev and pwritev, the iovec structure is defined as:

struct iovec {
        void   *iov_base;  /* Base address. */
        size_t iov_len;    /* Length. */
};

Each iovec entry specifies the base address and length of an area in memory from which data should be written. The writev system call will always write a complete area before proceeding to the next.

On objects capable of seeking, the write starts at a position given by the pointer associated with d, see lseek. Upon return from write, the pointer is incremented by the number of bytes which were written.

Objects that are not capable of seeking always write from the current position. The value of the pointer associated with such an object is undefined.

When using non-blocking I/O on objects such as sockets that are subject to flow control, write and writev may write fewer bytes than requested; the return value must be noted, and the remainder of the operation should be retried when possible.

Examples

/*************************************************************************************
* Detailed description: This sample code creates an Example.txt file in the current working
* directory(if file existes then it is truncated) and writes "Hello World" string
* to the file.
*
* Preconditions: Example.txt if present, it should not be read-only.
***************************************************************************************/
int main()
{
 int fd = 0 ;
 char Buf[] = "Hello World" ;
 fd = open("Example.txt" , O_CREAT | O_TRUNC | O_RDWR  ,0666) ;
 if(fd < 0 )
 {
    printf("Failed to open file Example.txt") ;
    return -1 ;
 }
 if(write(fd , Buf , sizeof(Buf)) < sizeof(Buf))
  {
    printf("Failed to write string %s to file\n" , Buf) ;
    return -1 ;
  }
  printf("String %s written to file \n" , Buf) ;
  return 0 ;
 }

Output

String Hello World written to file

 /**
  * Detailed description  : Sample usage of readv system call
  *
  *
**/
#include <stdio.h>
#include <sys/types.h>
#include <sys/uio.h>
#include <fcntl.h>
int main()
{
  int fd = 0 ;
  struct iovec io_vec[1] ;
  char Buf[12] = "Hello world" ;
  io_vec[0].iov_base  = Buf ;
  io_vec[0].iov_len = 11 ;
  fd = open("Example.txt" , O_CREAT | O_RDWR , 0666 ) ;
  if(fd < 0 )  {
    printf("File open failed  \n") ;
    return -1 ;
  }
  if(writev(fd , io_vec , 1) <  11 )   {
    printf("Failed to read fron Example.txt file \n") ;
    return -1 ;
  }
  printf("writev succes %s written \n"  , io_vec[0].iov_base) ;
  return 0 ; }

Output

writev succes Hello world written

Errors

The write, writev, pwrite and pwritev system calls will fail and the file pointer will remain unchanged if:

[`EBADF`]
	The d argument is not a valid descriptor open for writing.
[`EPIPE`]
	An attempt is made to write to a pipe that is not open for reading by any process.
[`EPIPE`]
	An attempt is made to write to a socket of type `SOCK_STREAM` that is not connected to a peer socket.
[`EFBIG`]
	An attempt was made to write a file that exceeds the process’s file size limit or the maximum file size(Currently not supported).
[`EFAULT`]
	Part of iov or data to be written to the file points outside the process’s allocated address space (currently not supported).
[`EINVAL`]
	The pointer associated with d was negative.
[`ENOSPC`]
	There is no free space remaining on the file system containing the file(currently not supported).
[`EDQUOT`]
	The user’s quota of disk blocks on the file system containing the file has been exhausted(currently not supported)..
[`EIO`]	An I/O error occurred while reading from or writing to the file system(Not supported).
[`EINTR`]
	A signal interrupted the write before it could be completed(currently not supported)..
[`EAGAIN`]
	The file was marked for non-blocking I/O, and no data could be written immediately.
[`EROFS`]
	An attempt was made to write over a disk label area at the beginning of a slice. Use disklabel -W to enable writing on the disk label area(Not supported).
[`EINVAL`]
	The value nbytes is greater than `INT_MAX`.

In addition, writev and pwritev may return one of the following errors:

[`EDESTADDRREQ`]
	The destination is no longer available when writing to a Unix domain datagram socket on which connect had been used to set a destination address.
[`EINVAL`]
	The iovcnt argument was less than or equal to 0, or greater than `IOV_MAX`.
[`EINVAL`]
	One of the iov_len values in the iov array was negative.
[`EINVAL`]
	The sum of the iov_len values in the iov array overflowed a 32-bit integer.
[`ENOBUFS`]
	The mbuf pool has been completely exhausted when writing to a socket.

The pwrite and pwritev system calls may also return the following errors:

[`EINVAL`]
	The offset value was negative.
[`ESPIPE`]
	The file descriptor is associated with a pipe, socket, or FIFO.