man writev (2): write output

Other Alias

write, pwrite, pwritev

LIBRARY

Lb libc

SYNOPSIS

In sys/types.h In unistd.h Ft ssize_t Fn write int fd const void *buf size_t nbytes Ft ssize_t Fn pwrite int fd const void *buf size_t nbytes off_t offset In sys/uio.h Ft ssize_t Fn writev int fd const struct iovec *iov int iovcnt Ft ssize_t Fn pwritev int fd const struct iovec *iov int iovcnt off_t offset

DESCRIPTION

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

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

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

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

On objects capable of seeking, the Fn write starts at a position given by the pointer associated with Fa fd , see lseek(2). Upon return from Fn 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.

If the real user is not the super-user, then Fn write clears the set-user-id bit on a file. This prevents penetration of system security by a user who ``captures'' a writable set-user-id file owned by the super-user.

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

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.

ERRORS

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

Bq Er EBADF: The Fa fd argument is not a valid descriptor open for writing.
Bq Er EPIPE: An attempt is made to write to a pipe that is not open for reading by any process.
Bq Er EPIPE: An attempt is made to write to a socket of type SOCK_STREAM that is not connected to a peer socket.
Bq Er EFBIG: An attempt was made to write a file that exceeds the process's file size limit or the maximum file size.
Bq Er EFAULT: Part of Fa iov or data to be written to the file points outside the process's allocated address space.
Bq Er EINVAL: The pointer associated with Fa fd was negative.
Bq Er ENOSPC: There is no free space remaining on the file system containing the file.
Bq Er EDQUOT: The user's quota of disk blocks on the file system containing the file has been exhausted.
Bq Er EIO: An I/O error occurred while reading from or writing to the file system.
Bq Er EINTR: A signal interrupted the write before it could be completed.
Bq Er EAGAIN: The file was marked for non-blocking I/O, and no data could be written immediately.
Bq Er EROFS: An attempt was made to write over a disk label area at the beginning of a slice. Use disklabel(8) -W to enable writing on the disk label area.
Bq Er EINVAL: The value Fa nbytes is greater than INT_MAX

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

Bq Er EDESTADDRREQ: The destination is no longer available when writing to a UNIX domain datagram socket on which connect(2) had been used to set a destination address.
Bq Er EINVAL: The Fa iovcnt argument was less than or equal to 0, or greater than IOV_MAX
Bq Er EINVAL: One of the Fa iov_len values in the Fa iov array was negative.
Bq Er EINVAL: The sum of the Fa iov_len values in the Fa iov array overflowed a 32-bit integer.
Bq Er ENOBUFS: The mbuf pool has been completely exhausted when writing to a socket.

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

Bq Er EINVAL: The Fa offset value was negative.
Bq Er ESPIPE: The file descriptor is associated with a pipe, socket, or FIFO.

STANDARDS

The Fn write system call is expected to conform to St -p1003.1-90 . The Fn writev and Fn pwrite system calls are expected to conform to St -xpg4.2 .

HISTORY

The Fn pwritev system call appeared in Fx 6.0 . The Fn pwrite function appeared in AT&T System V.4 . The Fn writev system call appeared in BSD 4.2 The Fn write function appeared in AT&T System v6 .