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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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WRITEV(3P) POSIX Programmer's Manual WRITEV(3P)
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.
writev — write a vector
#include <sys/uio.h>
ssize_t writev(int fildes, const struct iovec *iov, int iovcnt);
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.
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.
Refer to write(3p).
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.
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);
...
None.
Refer to write(3p).
None.
readv(3p), write(3p)
The Base Definitions volume of POSIX.1‐2017, limits.h(0p),
sys_uio.h(0p)
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 .
IEEE/The Open Group 2017 WRITEV(3P)
Pages that refer to this page: sys_uio.h(0p), readv(3p), write(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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