WRITE(2) System Calls Manual WRITE(2)

pwrite, write, pwritev, writev — write output

Standard C Library (libc, -lc)

#include <unistd.h>

ssize_t
pwrite(int fildes, const void *buf, size_t nbyte, off_t offset);

ssize_t
write(int fildes, const void *buf, size_t nbyte);

#include <sys/uio.h>

ssize_t
writev(int fildes, const struct iovec *iov, int iovcnt);

ssize_t
pwritev(int fildes, const struct iovec *iov, int iovcnt, off_t offset);

() attempts to write nbyte of data to the object referenced by the descriptor fildes from the buffer pointed to by buf. writev() 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]. pwrite() and pwritev() perform the same functions, but write to the specified position in the file without modifying the file pointer.

For () and (), the iovec structure is defined as:

struct iovec {
	char   *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. () and () will always write a complete area before proceeding to the next.

On objects capable of seeking, the () starts at a position given by the pointer associated with fildes, 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 () 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, () 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.

() and () will fail if the parameter nbyte exceeds INT_MAX, and they do not attempt a partial write.

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.

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

[]
The user's quota of disk blocks on the file system containing the file is exhausted.
[]
Part of iov or data to be written to the file points outside the process's allocated address space.
[]
The pointer associated with fildes is negative.

The write() and pwrite() system calls will fail and the file pointer will remain unchanged if:

[]
The file is marked for non-blocking I/O, and no data could be written immediately.
[]
fildes is not a valid file descriptor open for writing.
[]
A write is attempted on a socket that is not connected.
[]
An attempt is made to write a file that exceeds the process's file size limit or the maximum file size.
[]
The file is a regular file, nbyte is greater than 0, and the starting position is greater than or equal to the offset maximum established in the open file description associated with fildes.
[]
A signal interrupts the write before it could be completed.
[]
The value provided for nbyte exceeds INT_MAX.
[]
An I/O error occurs while reading from or writing to the file system.
[]
A write is attempted on a socket and the local network interface used to reach the destination is down.
[]
A write is attempted on a socket and no route to the network is present.
[]
There is no free space remaining on the file system containing the file.
[]
A request is made of a nonexistent device, or the request is outside the capabilities of the device.
[]
An attempt is made to write to a pipe that is not open for reading by any process.
[]
An attempt is made to write to a socket of type SOCK_STREAM that is not connected to a peer socket.
[]
The file is a “dataless” file that requires materialization and the I/O policy of the current thread or process disallows dataless file materialization (see getiopolicy_np(3)).

The write() and writev() calls may also return the following errors:

[]
See EWOULDBLOCK, below.
[]
The file descriptor is for a socket, is marked O_NONBLOCK, and write would block. The exact error code depends on the protocol, but EWOULDBLOCK is more common.

In addition, writev() may return one of the following errors:

[]
The destination is no longer available when writing to a UNIX domain datagram socket on which connect(2) or connectx(2) had been used to set a destination address.
[]
The mbuf pool has been completely exhausted when writing to a socket.

The writev() and pwritev() calls may also return the following errors:

[]
Iovcnt is less than or equal to 0, or greater than UIO_MAXIOV.
[]
One of the iov_len values in the iov array is negative.
[]
The sum of the iov_len values in the iov array overflows a 32-bit integer.

The pwrite() and pwritev() calls may also return the following errors:

[]
The specified file offset is invalid.
[]
The file descriptor is associated with a pipe, socket, or FIFO.

#include <sys/types.h> #include <sys/uio.h> #include <unistd.h>

These include files are needed for all three functions.

fcntl(2), lseek(2), open(2), pipe(2), select(2), compat(5)

The write() function call is expected to conform to IEEE Std 1003.1-1990 (“POSIX.1”). The writev() and pwrite() functions are expected to conform to X/Open Portability Guide Issue 4, Version 2 (“XPG4.2”). pwritev() is nonstandard.

The pwrite() function call appeared in AT&T System V Release 4 UNIX. The writev() function call appeared in 4.2BSD. A write() function call appeared in Version 6 AT&T UNIX.

June 3, 2021 macOS 15.2