WRITE(2) FreeBSD System Calls Manual WRITE(2)
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.
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).
[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.
SEE ALSO
fcntl(2), lseek(2), open(2), pipe(2), select(2)
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.
FreeBSD 13.1-RELEASE-p6 February 11, 2021 FreeBSD 13.1-RELEASE-p6
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