December 1, 2017
freebsd-manpages
Manual pages for a GNU/kFreeBSD system
NAME
write, writev, pwrite, pwritev - write output
LIBRARY
.Lb libc
SYNOPSIS
.In unistd.h ssize_t
write int fd const void *buf size_t nbytes
ssize_t pwrite int fd const void *buf size_t nbytes off_t offset
.In sys/uio.h ssize_t
writev int fd const struct iovec *iov int iovcnt
ssize_t pwritev int fd const struct iovec *iov int iovcnt off_t offset
DESCRIPTION
The
write
system call attempts to write nbytes of data to the object referenced by the descriptor fd 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 fd, see lseek(2). 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.
If the real user is not the super-user, then
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,
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.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
write
, writev
, pwrite
and pwritev
system calls will fail and the file pointer will remain unchanged if:
[EBADF ] |
|
The fd 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. | |
[EFAULT ] |
|
Part of iov or data to be written to the file points outside the process’s allocated address space. | |
[EINVAL ] |
|
The pointer associated with fd was negative. | |
[ENOSPC ] |
|
There is no free space remaining on the file system containing the file. | |
[EDQUOT ] |
|
The user’s quota of disk blocks on the file system containing the file has been exhausted. | |
[EIO ] |
An I/O error occurred while reading from or writing to the file system. |
[EINTR ] |
|
A signal interrupted the write before it could be completed. | |
[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(8) -W to enable writing on the disk label area. | |
[EINVAL ] |
|
The value nbytes is greater than SSIZE_MAX (or greater than INT_MAX , if the sysctl debug.iosize_max_clamp is non-zero). |
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(2) 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. |
STANDARDS
The
write
system call is expected to conform to -p1003.1-90. The writev
and pwrite
system calls are expected to conform to -xpg4.2.HISTORY
The
.Fx 6.0 . The
pwritev
system call appeared in.Fx 6.0 . The
pwrite
function appeared in AT&T V.4 . The writev
system call appeared in BSD 4.2 . The write
function appeared in AT&T v1 .