NAME

write, writev, pwrite, pwritev — write output

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <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);

#include <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.

ATOMICITY OF WRITES

When operating on regular files on local file systems, the effects of write() are atomic. As required by the POSIX standard, the read(), write(), and ftruncate() functions and their variations are atomic with respect to each other on the file data and metadata for regular files. See for instance IEEE Std 1003.1-2024 (“POSIX.1”) Volume 2, Section 2.9.7 for more information.

FreeBSD implements the requirement by taking a read/write range lock on the file byte range affected by the corresponding function.

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.
[EINVAL]: The value nbytes is greater than SSIZE_MAX (or greater than INT_MAX, if the sysctl debug.iosize_max_clamp is non-zero).
[EINVAL]: The file descriptor refers to a raw device, and the write offset or size is not a multiple of the device's block size.
[EINTEGRITY]: The backing store for fd detected corrupted data while reading. (For example, writing a partial filesystem block may require first reading the existing block which may trigger this error.)

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 is greater than SSIZE_MAX (or greater than INT_MAX, if the sysctl debug.iosize_max_clamp is non-zero).
[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 IEEE Std 1003.1-1990 (“POSIX.1”). The writev() and pwrite() system calls are expected to conform to X/Open Portability Guide Issue 4, Version 2 (“XPG4.2”).

HISTORY

The pwritev() system call appeared in FreeBSD 6.0. The pwrite() function appeared in AT&T System V Release 4 UNIX. The writev() system call appeared in 4.2BSD. The write() function appeared in Version 1 AT&T UNIX.

BUGS

The pwrite() system call appends the file without changing the file offset if O_APPEND is set, contrary to IEEE Std 1003.1-2008 (“POSIX.1”) where pwrite() writes into offset regardless of whether O_APPEND is set.