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@(#)write.2 8.5 (Berkeley) 4/2/94
$FreeBSD: releng/10.2/lib/libc/sys/write.2 255486 2013-09-12 00:53:38Z bdrewery $
.Dd September 11, 2013 .Dt WRITE 2 .Os .Sh NAME .Nm write , .Nm writev , .Nm pwrite , .Nm pwritev .Nd write output .Sh LIBRARY .Lb libc .Sh SYNOPSIS n sys/types.h n 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" n 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" .Sh 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.
p For .Fn writev and .Fn pwritev , the .Fa iovec structure is defined as:
p d -literal -offset indent -compact struct iovec { void *iov_base; /* Base address. */ size_t iov_len; /* Length. */ }; .Ed
p 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.
p On objects capable of seeking, the .Fn write starts at a position given by the pointer associated with .Fa fd , see .Xr lseek 2 . Upon return from .Fn write , the pointer is incremented by the number of bytes which were written.
p 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.
p 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 .Dq captures a writable set-user-id file owned by the super-user.
p 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. .Sh RETURN VALUES Upon successful completion the number of bytes which were written is returned. Otherwise a -1 is returned and the global variable .Va errno is set to indicate the error. .Sh ERRORS The .Fn write , .Fn writev , .Fn pwrite and .Fn pwritev system calls will fail and the file pointer will remain unchanged if: l -tag -width Er t Bq Er EBADF The .Fa fd argument is not a valid descriptor open for writing. t Bq Er EPIPE An attempt is made to write to a pipe that is not open for reading by any process. t Bq Er EPIPE An attempt is made to write to a socket of type .Dv SOCK_STREAM that is not connected to a peer socket. t Bq Er EFBIG An attempt was made to write a file that exceeds the process's file size limit or the maximum file size. t Bq Er EFAULT Part of .Fa iov or data to be written to the file points outside the process's allocated address space. t Bq Er EINVAL The pointer associated with .Fa fd was negative. t Bq Er ENOSPC There is no free space remaining on the file system containing the file. t Bq Er EDQUOT The user's quota of disk blocks on the file system containing the file has been exhausted. t Bq Er EIO An I/O error occurred while reading from or writing to the file system. t Bq Er EINTR A signal interrupted the write before it could be completed. t Bq Er EAGAIN The file was marked for non-blocking I/O, and no data could be written immediately. t Bq Er EROFS An attempt was made to write over a disk label area at the beginning of a slice. Use .Xr disklabel 8 .Fl W to enable writing on the disk label area. t Bq Er EINVAL The value .Fa nbytes is greater than .Dv INT_MAX . .El
p In addition, .Fn writev and .Fn pwritev may return one of the following errors: l -tag -width Er t Bq Er EDESTADDRREQ The destination is no longer available when writing to a x domain datagram socket on which .Xr connect 2 had been used to set a destination address. t Bq Er EINVAL The .Fa iovcnt argument was less than or equal to 0, or greater than .Dv IOV_MAX . t Bq Er EINVAL One of the .Fa iov_len values in the .Fa iov array was negative. t Bq Er EINVAL The sum of the .Fa iov_len values in the .Fa iov array overflowed a 32-bit integer. t Bq Er ENOBUFS The mbuf pool has been completely exhausted when writing to a socket. .El
p The .Fn pwrite and .Fn pwritev system calls may also return the following errors: l -tag -width Er t Bq Er EINVAL The .Fa offset value was negative. t Bq Er ESPIPE The file descriptor is associated with a pipe, socket, or FIFO. .El .Sh SEE ALSO .Xr fcntl 2 , .Xr lseek 2 , .Xr open 2 , .Xr pipe 2 , .Xr select 2 .Sh 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 . .Sh HISTORY The .Fn pwritev system call appeared in .Fx 6.0 . The .Fn pwrite function appeared in .At V.4 . The .Fn writev system call appeared in x 4.2 . The .Fn write function appeared in .At v6 .