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writev - Man Page

write a vector


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.


#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.

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.


Refer to write().

In addition, the writev() function shall fail if:


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:


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);

Application Usage



Refer to write().

Future Directions


See Also

readv(), write()

The Base Definitions volume of POSIX.1-2017, <limits.h>, <sys_uio.h>

Referenced By

readv(3p), sys_uio.h(0p), write(3p).

2017 IEEE/The Open Group POSIX Programmer's Manual