readv, writev, preadv, pwritev, preadv2, pwritev2 - read or write data into multiple buffers

#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)):

The readv() system call reads iovcnt buffers from the file associated with the file descriptor fd into the buffers described by iov ("scatter input").

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)).

preadv() and pwritev()

The preadv() system call combines the functionality of readv() and pread(2). It performs the same task as readv(), but adds a fourth argument, offset, which specifies the file offset at which the input operation is to be performed.

The pwritev() system call combines the functionality of writev() and pwrite(2). It performs the same task as writev(), but adds a fourth argument, offset, which specifies the file offset at which the output operation is to be performed.

The file offset is not changed by these system calls. The file referred to by fd must be capable of seeking.

preadv2() and pwritev2()

These system calls are similar to preadv() and pwritev() calls, but add a fifth argument, flags, which modifies the behavior on a per-call basis.

Unlike preadv() and pwritev(), if the offset argument is -1, then the current file offset is used and updated.

The flags argument contains a bitwise OR of zero or more of the following flags:

RWF_DSYNC (since Linux 4.7)

Provide a per-write equivalent of the O_DSYNC open(2) flag. This flag is meaningful only for pwritev2(), and its effect applies only to the data range written by the system call.

RWF_HIPRI (since Linux 4.6)

High priority read/write. Allows block-based filesystems to use polling of the device, which provides lower latency, but may use additional resources. (Currently, this feature is usable only on a file descriptor opened using the O_DIRECT flag.)

RWF_SYNC (since Linux 4.7)

Provide a per-write equivalent of the O_SYNC open(2) flag. This flag is meaningful only for pwritev2(), and its effect applies only to the data range written by the system call.

RWF_NOWAIT (since Linux 4.14)

Do not wait for data which is not immediately available. If this flag is specified, the preadv2() system call will return instantly if it would have to read data from the backing storage or wait for a lock. If some data was successfully read, it will return the number of bytes read. If no bytes were read, it will return -1 and set errno to EAGAIN. Currently, this flag is meaningful only for preadv2().

RWF_APPEND (since Linux 4.16)

Provide a per-write equivalent of the O_APPEND open(2) flag. This flag is meaningful only for pwritev2(), and its effect applies only to the data range written by the system call. The offset argument does not affect the write operation; the data is always appended to the end of the file. However, if the offset argument is -1, the current file offset is updated.

On success, readv(), preadv(), and preadv2() return the number of bytes read; writev(), pwritev(), and pwritev2() return the number of bytes written.

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.

The following code sample demonstrates the use of writev():

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);

pread(2), read(2), write(2)