WRITE

Section: System Calls (2)
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BSD mandoc
BSD 4

NAME

write writev - write output

SYNOPSIS

Fd #include <sys/types.h> Fd #include <sys/uio.h> Fd #include <unistd.h> Ft ssize_t Fn write int d const void *buf size_t nbytes Ft ssize_t Fn writev int d const struct iovec *iov int iovcnt

DESCRIPTION

Fn Write attempts to write Fa nbytes of data to the object referenced by the descriptor Fa d from the buffer pointed to by Fa buf . Fn Writev 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].

For Fn writev , the Fa iovec structure is defined as:

struct iovec {
        void *iov_base;
        size_t iov_len;
};

Each Fa iovec entry specifies the base address and length of an area in memory from which data should be written. Fn Writev will always write a complete area before proceeding to the next.

On objects capable of seeking, the Fn write starts at a position given by the pointer associated with Fa d (see lseek(2)). Upon return from Fn 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 Fn 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, 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.

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

Fn Write and Fn writev will fail and the file pointer will remain unchanged if:

Bq Er EBADF: Fa D is not a valid descriptor open for writing.
Bq Er EPIPE: An attempt is made to write to a pipe that is not open for reading by any process.
Bq Er EPIPE: An attempt is made to write to a socket of type that is not connected to a peer socket.
Bq Er EFBIG: An attempt was made to write a file that exceeds the process's file size limit or the maximum file size.
Bq Er EFAULT: Part of Fa iov or data to be written to the file points outside the process's allocated address space.
Bq Er EINVAL: The pointer associated with Fa d was negative.
Bq Er ENOSPC: There is no free space remaining on the file system containing the file.
Bq Er EDQUOT: The user's quota of disk blocks on the file system containing the file has been exhausted.
Bq Er EIO: An I/O error occurred while reading from or writing to the file system.
Bq Er EAGAIN: The file was marked for non-blocking I/O, and no data could be written immediately.

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

Bq Er EINVAL: Fa Iovcnt was less than or equal to 0, or greater than {UIO_MAXIOV}
Bq Er EINVAL: One of the Fa iov_len values in the Fa iov array was negative.
Bq Er EINVAL: The sum of the Fa iov_len values in the Fa iov array overflowed a 32-bit integer.

STANDARDS

The Fn write function is expected to conform to St -p1003.1-88 .

HISTORY

The Fn writev function call appeared in BSD 4.2 The Fn write function call appeared in AT&T System v6 .

This document was created by man2html, using the manual pages.
Time: 16:28:59 GMT, April 18, 2022