OSF

NAME

write, writev − Writes to a file

SYNOPSIS

int write(
int filedes ,
const char ∗buffer,
unsigned int nbytes ); #include <sys/types.h>
#include <sys/uio.h> int writev(
int filedes ,
struct iovec ∗iov,
int iov_count );

PARAMETERS

filedesIdentifies the object to which the data is to be written. 

bufferIdentifies the buffer containing the data to be written. 

nbytesSpecifies the number of bytes to write. 

iovPoints to an array of iovec structures, which identifies the buffers containing the data to be written. The iovec structure is defined in the sys/uio.h header file and contains the following members: caddr_t  iov_base;
int      iov_len;

iov_countSpecifies the number of iovec structures pointed to by the iov parameter. 

DESCRIPTION

The write() function attempts to write nbytes of data to the file associated with the filedes parameter from the buffer pointed to by the buffer parameter. 

If the nbyte parameter is 0 (zero), the write() function returns 0 (zero) and has no other results if the file is a regular file. 

The writev() function performs the same action as the write() function, but gathers the output data from the iov_count buffers specified by the array of iovec structures pointed to by the iov parameter. Each iovec entry specifies the base address and length of an area in memory from which data should be written. The writev() function always writes a complete area before proceeding to the next. 

The write() and writev() functions, which suspend the calling process until the request is completed, are redefined so that only the calling thread is suspended. 

With regular files and devices capable of seeking, the actual writing of data proceeds from the position in the file indicated by the file pointer.  If this incremented file pointer is greater than the length of the file, the length of the file is set to this file offset.  Upon return from the write() function, the file pointer increments by the number of bytes actually written. 

With devices incapable of seeking, writing always takes place starting at the current position. The value of a file pointer associated with such a device is undefined. 

Fewer bytes than requested can be written if there is not enough room to satisfy the request. In this case the number of bytes written is returned. The next attempt to write a nonzero number of bytes fails (except as noted in the following text). The limit reached can be either the ulimit() or the end of the physical medium.  For example, suppose there is space for 20 bytes more in a file before reaching a limit.  A write of 512 bytes returns 20.  The next write of a nonzero number of bytes will give a failure return (except as noted below). 

Upon successful completion, the write() function returns the number of bytes actually written to the file associated with the fildes parameter.  This number is never be greater than the nbyte parameter. 

If the O_APPEND flag of the file status is set, the file offset is set to the end of the file prior to each write. 

If the O_SYNC flag of the file status flags is set and the fildes parameter refers to a regular file, a successful write() function does not return until the data is delivered to the underlying hardware (as described in the open() function). 

Write requests to a pipe (or FIFO) are handled the same as a regular file with the following exceptions:

       •There is no file offset associated with a pipe; hence each write() request appends to the end of the pipe. 

       •If the size of the write() request is less than or equal to the value of the PIPE_BUF system variable, the write() function is guaranteed to be atomic. The data is not interleaved with data from other processes doing writes on the same pipe.  Writes of greater than PIPE_BUF bytes can have data interleaved, on arbitrary boundaries, with writes by other processes, whether or not O_NONBLOCK or O_NDELAY are set. 

       •If neither O_NONBLOCK nor O_NDELAY are set, a write() request to a full pipe  causes the process to block until enough space becomes available to handle the entire request. 

       •If the O_NONBLOCK or O_NDELAY flag is set, write() requests are handled differently in the following ways: the write() function does block the process; write() requests for PIPE_BUF or fewer bytes either succeed completely and return nbyte, or return −1 and set errno to [EAGAIN]. A write() request for greater than PIPE_BUF bytes either transfers what it can and returns the number of bytes written, or transfers no data and returns −1 with errno set to [EAGAIN].  Also, if a request is greater than PIPE_BUF bytes and all data previously written to the pipe has been read, write() transfers at least PIPE_BUF bytes. 

When attempting to write to a regular file with enforcement mode record locking enabled, and all or part of the region to be written is currently locked by another process:

       •If O_NDELAY and O_NONBLOCK are clear (the default), the calling process blocks until all the blocking locks are removed, or the write() function is terminated by a signal. 

       •If O_NDELAY or O_NONBLOCK is set, then the write() function returns −1 and sets errno to [EAGAIN].

Upon successful completion, the write() function marks the st_ctime and st_mtime fields of the file for update, and clears its set-user ID and set-group ID attributes if the file is a regular file. 

The fcntl() function provides more information about record locks. 

The behavior of an interrupted write() function depends on how the handler for the arriving signal was installed:

       •If a write() function is interrupted by a signal before it writes any data, it returns −1 with errno set to [EINTR]. 

       •If a write() function is interrupted by a signal after it successfully writes some data, it returns the number of bytes written.  A write() request to a pipe or FIFO never returns with errno set to [EINTR] if it has transferred any data and nbyte is less than or equal to PIPE_BUF.

       •If the handler was installed with an indication that functions should not be restarted, the write() function returns a value of -1 and sets errno to [EINTR] (even if some data was already written). 

       •If the handler was installed with an indication that functions should be restarted:

       •If no data was written when the interrupt was handled, the write() function does not return a value (it is restarted). 

       •If data was written when the interrupt was handled, the write() function returns the amount of data already written. 

NOTES

AES Support Level:
Full use (write())

RETURN VALUES

Upon successful completion, the write() and writev() functions return the number of bytes that were actually written. Otherwise, -1 is returned and errno is set to indicate the error. 

ERRORS

If the write() or writev() function fails, errno may be set to one of the following values:

[EBADF]The filedes  parameter does not specify a valid file descriptor open for writing. 

[EINVAL]The file position pointer associated with the filedes parameter was negative. 

[EINVAL]The iov_count parameter value was not between 1 and 16, inclusive. 

[EINVAL]One of the iov_len values in the iov array was negative or the sum overflowed a 32-bit integer. 

[EINVAL]Attempt to write to a stream linked below a multiplexor. 

[EFAULT]The buffer parameter or part of the iov parameter points to a location outside of the allocated address space of the process. 

[EPIPE]An attempt was made to write to a pipe or FIFO that is not opened for reading by any process.  A SIGPIPE signal is sent to the process. 

[EPERM]An attempt was made to write to a socket or type SOCK_STREAM that is not connected to a peer socket. 

[EAGAIN]The O_NONBLOCK flag is set on this file and the process would be delayed in the write operation. 

[EAGAIN]An enforcement mode record lock is outstanding in the portion of the file that is to be written, and O_NDELAY or O_NONBLOCK is set. 

[EAGAIN]Attempt to write to a stream that can not accept data with either the O_NDELAY or O_NONBLOCK flag set. 

[ENOLCK]Enforced record locking is enabled and LOCK_MAX regions are already locked in the system. 

[EDEADLK]Enforced record locking is enabled, O_NDELAY is clear, and a deadlock condition is detected. 

[EFBIG]An attempt was made to write a file that exceeds the maximum file size. 

[ENOSPC]No free space is left on the file system containing the file. 

[EINTR]A signal was caught during the write() operation, and the signal handler was installed with an indication that functions are not to be restarted. 

[EIO]The process is a member of a background process group attempting to write to its controlling terminal, TOSTOP is set, the process is neither ignoring nor blocking SIGTTOU, and the process group of the process is orphaned. 

RELATED INFORMATION

Functions: open(2), fcntl(2), fcntl(2), lseek(2), open(2), pipe(2), poll(2), select(2), ulimit(3)