--- name: "pwritev" section: "2" platform: "macos" package: "xnu" version: "12377.121.6" release: "macOS 26.5" platforms: ["linux", "freebsd", "macos"] url: "https://new.carta.tech/write/2" --- # pwritev(2) write output Part of [write(2)](/write/2.md). ## NAME **pwrite**, **write**, **pwritev**, **writev** – write output ## LIBRARY Standard C Library (libc, -lc) ## SYNOPSIS **#include \** *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 \** *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*); ## DESCRIPTION **write**() 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 **writev**() and **pwritev**(), 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. **writev**() and **pwritev**() 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 *fildes*, see [lseek(2)](/lseek/2.md). 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. **write**() and **pwrite**() will fail if the parameter *nbyte* exceeds INT\_MAX, and they do not attempt a partial write. ## 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: - \[EDQUOT\] The user's quota of disk blocks on the file system containing the file is exhausted. - \[EFAULT\] Part of *iov* or data to be written to the file points outside the process's allocated address space. - \[EINVAL\] The pointer associated with *fildes* is negative. The **write**() and **pwrite**() system calls will fail and the file pointer will remain unchanged if: - \[EAGAIN\] The file is marked for non-blocking I/O, and no data could be written immediately. - \[EBADF\] *fildes* is not a valid file descriptor open for writing. - \[ECONNRESET\] A write is attempted on a socket that is not connected. - \[EFBIG\] An attempt is made to write a file that exceeds the process's file size limit or the maximum file size. - \[EFBIG\] 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*. - \[EINTR\] A signal interrupts the write before it could be completed. - \[EINVAL\] The value provided for *nbyte* exceeds INT\_MAX. - \[EIO\] An I/O error occurs while reading from or writing to the file system. - \[ENETDOWN\] A write is attempted on a socket and the local network interface used to reach the destination is down. - \[ENETUNREACH\] A write is attempted on a socket and no route to the network is present. - \[ENOSPC\] There is no free space remaining on the file system containing the file. - \[ENXIO\] A request is made of a nonexistent device, or the request is outside the capabilities of the device. - \[EPIPE\] 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. - \[EDEADLK\] 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)](/getiopolicy_np/3.md)). The **write**() and **writev**() calls may also return the following errors: - \[EAGAIN\] See EWOULDBLOCK, below. - \[EWOULDBLOCK\] 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: - \[EDESTADDRREQ\] The destination is no longer available when writing to a UNIX domain datagram socket on which [connect(2)](/connect/2.md) or [connectx(2)](/connectx/2.md) had been used to set a destination address. - \[ENOBUFS\] The mbuf pool has been completely exhausted when writing to a socket. The **writev**() and **pwritev**() calls may also return the following errors: - \[EINVAL\] *Iovcnt* is less than or equal to 0, or greater than UIO\_MAXIOV. - \[EINVAL\] One of the *iov\_len* values in the *iov* array is negative. - \[EINVAL\] 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: - \[EINVAL\] The specified file offset is invalid. - \[ESPIPE\] The file descriptor is associated with a pipe, socket, or FIFO. ## LEGACY SYNOPSIS **#include \** **#include \** **#include \** These include files are needed for all three functions. ## SEE ALSO [fcntl(2)](/fcntl/2.md), [lseek(2)](/lseek/2.md), [open(2)](/open/2.md), [pipe(2)](/pipe/2.md), [select(2)](/select/2.md), [compat(5)](/compat/5.md) ## STANDARDS The **write**() function call is expected to conform to ISO/IEC 9945-1:1990 (“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. ## HISTORY 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.