14.4 readv and writev Functions

These two functions are similar to read and write, but readv and writev let us read into or write from one or more buffers with a single function call. These operations are called scatter read (since the input data is scattered into multiple application buffers) and gather write (since multiple buffers are gathered for a single output operation).

#include <sys/uio.h>
ssize_t readv(int filedes, const struct iovec *iov, int iovcnt);
ssize_t writev(int filedes, const struct iovec *iov, int iovcnt);
Both return: number of bytes read or written, –1 on error

The second argument to both functions is a pointer to an array of iovec structures, which is defined by including the <sys/uio.h> header.

struct iovec {
  void   *iov_base;   /* starting address of buffer */
  size_t  iov_len;    /* size of buffer */
};

The datatypes shown for the members of the iovec structure are those specified by POSIX. You may encounter implementations that define iov_base to be a char *, and iov_len to be an int.

There is some limit to the number of elements in the array of iovec structures that an implementation allows. Linux, for example, allows up to 1,024, while HP-UX has a limit of 2,100. POSIX requires that the constant IOV_MAX be defined by including the <sys/uio.h> header and that its value be at least 16.

The readv and writev functions can be used with any descriptor, not just sockets. Also, writev is an atomic operation. For a record-based protocol such as UDP, one call to writev generates a single UDP datagram.

We mentioned one use of writev with the TCP_NODELAY socket option in Section 7.9. We said that a write of 4 bytes followed by a write of 396 bytes could invoke the Nagle algorithm and a preferred solution is to call writev for the two buffers.