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PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface
may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface
may not be implemented on Linux.
NAME
writev — write a vector
SYNOPSIS
#include <sys/uio.h>
ssize_t writev(int fildes, const struct iovec *iov, int iovcnt);
DESCRIPTION
The writev() function shall be equivalent to write(), except as described below. The writev() function
shall gather output data from the iovcnt buffers specified by the members of the iov array: iov[0],
iov[1], ..., iov[iovcnt-1]. The iovcnt argument is valid if greater than 0 and less than or equal to
{IOV_MAX}, as defined in <limits.h>.
Each iovec entry specifies the base address and length of an area in memory from which data should be
written. The writev() function shall always write a complete area before proceeding to the next.
If fildes refers to a regular file and all of the iov_len members in the array pointed to by iov are 0,
writev() shall return 0 and have no other effect. For other file types, the behavior is unspecified.
If the sum of the iov_len values is greater than {SSIZE_MAX}, the operation shall fail and no data shall
be transferred.
RETURN VALUE
Upon successful completion, writev() shall return the number of bytes actually written. Otherwise, it
shall return a value of -1, the file-pointer shall remain unchanged, and errno shall be set to indicate
an error.
ERRORS
Refer to write().
In addition, the writev() function shall fail if:
EINVAL The sum of the iov_len values in the iov array would overflow an ssize_t.
The writev() function may fail and set errno to:
EINVAL The iovcnt argument was less than or equal to 0, or greater than {IOV_MAX}.
The following sections are informative.
EXAMPLES
Writing Data from an Array
The following example writes data from the buffers specified by members of the iov array to the file
associated with the file descriptor fd.
#include <sys/types.h>
#include <sys/uio.h>
#include <unistd.h>
...
ssize_t bytes_written;
int fd;
char *buf0 = "short string\n";
char *buf1 = "This is a longer string\n";
char *buf2 = "This is the longest string in this example\n";
int iovcnt;
struct iovec iov[3];
iov[0].iov_base = buf0;
iov[0].iov_len = strlen(buf0);
iov[1].iov_base = buf1;
iov[1].iov_len = strlen(buf1);
iov[2].iov_base = buf2;
iov[2].iov_len = strlen(buf2);
...
iovcnt = sizeof(iov) / sizeof(struct iovec);
bytes_written = writev(fd, iov, iovcnt);
...
APPLICATION USAGE
None.
RATIONALE
Refer to write().
FUTURE DIRECTIONS
None.
SEE ALSO
readv(), write()
The Base Definitions volume of POSIX.1‐2017, <limits.h>, <sys_uio.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original
IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document.
The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page are most likely to have been introduced
during the conversion of the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .