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WRITEV

Section: POSIX Programmer's Manual (3P)
Updated: 2013
Index Return to Main Contents
 

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.1-2008, <limits.h>, <sys_uio.h>  

COPYRIGHT

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/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 .


 

Index

PROLOG
NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
EXAMPLES
Writing Data from an Array
APPLICATION USAGE
RATIONALE
FUTURE DIRECTIONS
SEE ALSO
COPYRIGHT


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