write(2)

NAME

write, writev − write on a file

SYNOPSIS

ssize_t write (fildes, buf, nbyte)
int fildes;
char ∗buf;
size_t nbyte;

SYNOPSIS (4.2BSD)

#include <sys/types.h>
#include <sys/uio.h>

int writev (fildes, iov, iovcnt)
int fildes;
struct iovec ∗iov;
int iovcnt;

DESCRIPTION

Fildes is a file descriptor obtained from a creat, open, dup, fcntl, or pipe system call.

write attempts to write nbyte bytes from the buffer pointed to by buf to the file associated with the fildes. Writev performs the same action, but gathers the output data from the iovcnt buffers specified by the members of the iovec array: iov[0], iov[1], ... , iov[iovcnt-1].

The iovec structure is defined as

struct iovec {
caddr_t iov_base;
intiov_len;
};

On devices capable of seeking, the actual writing of data proceeds from the position in the file indicated by the file pointer. Upon return from write, the file pointer is incremented by the number of bytes actually written.

On 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.

If the O_APPEND flag of the file status flags is set, the file pointer will be set to the end of the file prior to each write.

For regular disk files, if the O_DSYNC bit of the file status flags is set, all I/O operations on the file descriptor complete as defined by synchronized I/O data integrity completion. If the O_SYNC bit of the file status flags is set, all I/O operations on the file descriptor complete as defined by synchronized I/O file integrity completion (refer to the open(2) man page for the definitions of synchronized I/O data and file integrity completions). For files other than regular disk files the O_DSYNC flag will have no effect, for block special files the O_SYNC flag will force the data to be physically updated before returning.

If a write requests that more bytes be written than there is room for (e.g., the ulimit (see ulimit(2)) or the physical end of a medium), only as many bytes as there is room for will be written. For example, suppose there is space for 20 bytes more in a file before reaching a limit. A write of 512 bytes will return 20. The next write of a non-zero number of bytes will give a failure return (except as noted below).

If the file being written is a pipe (or FIFO) and the O_NDELAY or O_NONBLOCK flag of the file flag word is set, then write to a full pipe (or FIFO) will return a count of 0. Otherwise ( O_NDELAY and O_NONBLOCK clear), writes to a full pipe (or FIFO) will block until space becomes available.

When attempting to write to an ordinary file with enforced record locking enabled, and all or part of the file segment to be written has a read or write lock owned by another process (i.e., a blocking lock):

If O_NDELAY or O_NONBLOCK is set the function write will return −1 and errno will be set to EAGAIN .

If O_NDELAY and O_NONBLOCK are clear, the function write will sleep until all blocking locks are removed, or the function write is terminated by a signal.

For STREAMS [see intro(2)] files, the operation of write and writev is determined by the values of the minimum and maximum nbyte range ("packet size") accepted by the stream. These values are contained in the topmost stream module. Unless the user pushes [see I_PUSH in streamio(7)] the topmost module, these values can not be set or tested from user level. If nbyte falls within the packet size range, nbyte bytes will be written. If nbyte does not fall within the range and the minimum packet size value is zero, write will break the buffer into maximum packet size segments prior to sending the data downstream (the last segment may contain less than the maximum packet size). If nbyte does not fall within the range and the minimum value is non-zero, write will fail with errno set to ERANGE. Writing a zero-length buffer (nbyte is zero) sends zero bytes with zero returned.

For STREAMS files, if O_NDELAY is not set and the stream can not accept data (the stream write queue is full due to internal flow control conditions), write and writev will block until data can be accepted. O_NDELAY will prevent a process from blocking due to flow control conditions. If O_NDELAY is set and the stream can not accept data, write and writev will fail. If O_NDELAY is set and part of the buffer has been written when a condition in which the stream can not accept additional data occurs, write and writev will terminate and return the number of bytes written.

When attempting to write a file that has been opened in direct mode (i.e. O_DIRECT is set) the buffer pointed to by buf or iov->iov_base must be longword aligned. Additionally nbyte or iov->iov_len must be a multiple of the longword length (4 bytes) and less than the maximum physical transfer size (see CX/UX Programmer’s Guide for details). Lastly, the file pointer associated with fildes should be aligned to a multiple of the disk block size (1 Kbyte) before the write request is issued.

For pipes, FIFOs or character special devices, if the write is interrupted by a signal, the number of bytes transferred before the signal will be returned. If no bytes were transferred, -1 will be returned and errno will be set to EINTR.

For sockets, write has the same semantics as send(2) except that the options associated the flags parameter of send are not available for write.

write will fail and the file pointer will remain unchanged if one or more of the following are true:

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

[EAGAIN] Enforced record locking was enabled (see chmod(2)), O_NDELAY or O_NONBLOCK was set, and there were record locks on the file.

[EBADF] Fildes is not a valid file descriptor open for writing.

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

[EFAULT] Buf or iov or points outside the process’s allocated address space.

[EFAULT] The file is opened for direct mode and the buffer is not aligned on a longword (4-byte) boundary or the byte count is not a longword multiple.

[EFAULT] The file is opened for direct mode and the file pointer associated with fildes is not aligned to a multiple of the disk block size (1 Kbyte).

[EFBIG] An attempt was made to write a file that exceeds the process’s file size limit or the maximum file size. See ulimit(2).

[EINTR] A signal was caught during the system call. For a pipe, FIFO or character special device, no data will have been transferred.

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

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

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

[ENOSPC] There is no free space remaining on the device containing the file.

[ENXIO] The file is opened for direct mode and the byte count was greater than the maximum physical transfer size.

[EPIPE and SIGPIPE signal] An attempt is made to write to a pipe that is not open for reading by any process.

A write to a STREAMS file can fail if an error message has been received at the stream head. In this case, errno is set to the value included in the error message.

RETURN VALUE

Upon successful completion the number of bytes actually written is returned. Otherwise, −1 is returned and errno is set to indicate the error.

NOTE

The read function is defined in the 88open Binary and Object Compatibility Standards (BCS/OCS) for use in BCS/OCS compliant applications. Both functions are defined in the 88open BCS and OCS Networking Supplements (BCSNS/OCSNS) for use in BCS/OCS compliant networking applications. OCS/OCSNS-defined functions may be accessed by passing OCS options to cc(1) and ld(1).

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