NAME¶

SYNOPSIS¶

#include <sys/uio.h>

ssize_t readv(int fd, const struct iovec *iov, int iovcnt);

ssize_t writev(int fd, const struct iovec *iov, int iovcnt);

ssize_t preadv(int fd, const struct iovec *iov, int iovcnt,
               off_t offset);

ssize_t pwritev(int fd, const struct iovec *iov, int iovcnt,
                off_t offset);

ssize_t preadv2(int fd, const struct iovec *iov, int iovcnt,
                off_t offset, int flags);

ssize_t pwritev2(int fd, const struct iovec *iov, int iovcnt,
                 off_t offset, int flags);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

preadv(), pwritev(): Since glibc 2.19: _DEFAULT_SOURCE Glibc 2.19 and earlier: _BSD_SOURCE

DESCRIPTION¶

The writev() system call writes iovcnt buffers of data described by iov to the file associated with the file descriptor fd ("gather output").

The pointer iov points to an array of iovec structures, defined in <sys/uio.h> as:

struct iovec {
    void  *iov_base;    /* Starting address */
    size_t iov_len;     /* Number of bytes to transfer */
};

The readv() system call works just like read(2) except that multiple buffers are filled.

The writev() system call works just like write(2) except that multiple buffers are written out.

Buffers are processed in array order. This means that readv() completely fills iov[0] before proceeding to iov[1], and so on. (If there is insufficient data, then not all buffers pointed to by iov may be filled.) Similarly, writev() writes out the entire contents of iov[0] before proceeding to iov[1], and so on.

The data transfers performed by readv() and writev() are atomic: the data written by writev() is written as a single block that is not intermingled with output from writes in other processes (but see pipe(7) for an exception); analogously, readv() is guaranteed to read a contiguous block of data from the file, regardless of read operations performed in other threads or processes that have file descriptors referring to the same open file description (see open(2)).

RETURN VALUE¶

Note that it is not an error for a successful call to transfer fewer bytes than requested (see read(2) and write(2)).

On error, -1 is returned, and errno is set appropriately.

ERRORS¶

EINVAL

The sum of the iov_len values overflows an ssize_t value.

EINVAL

The vector count, iovcnt, is less than zero or greater than the permitted maximum.

EOPNOTSUPP

An unknown flag is specified in flags.

VERSIONS¶

preadv2() and pwritev2() first appeared in Linux 4.6. Library support was added in glibc 2.26.

CONFORMING TO¶

preadv(), pwritev(): nonstandard, but present also on the modern BSDs.

preadv2(), pwritev2(): nonstandard Linux extension.

NOTES¶

EXAMPLE¶

char *str0 = "hello ";
char *str1 = "world\n";
struct iovec iov[2];
ssize_t nwritten;
iov[0].iov_base = str0;
iov[0].iov_len = strlen(str0);
iov[1].iov_base = str1;
iov[1].iov_len = strlen(str1);
nwritten = writev(STDOUT_FILENO, iov, 2);

SEE ALSO¶

COLOPHON¶