READV(2)

NAME
readv, writev - read or write data into multiple buffers

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);

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

The writev() function 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() function works just like read(2) except that multiple buffers are filled.

The writev() function 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
On success, the readv() function returns the number of bytes read; the writev() function returns the number of bytes written. On error, -1 is returned, and errno is set appropriately.

ERRORS
The errors are as given for read(2) and write(2). Additionally the following error is defined:

EINVAL
    The sum of the iov_len values overflows an ssize_t value. Or, the vector count iovcnt is less than zero or greater than the permitted maximum.

CONFORMING TO
4.4BSD (the readv() and writev() functions first appeared in 4.2BSD), POSIX.1-2001. Linux libc5 used size_t as the type of the iovcnt parameter, and int as return type for these functions.

NOTES

  Linux Notes
POSIX.1-2001 allows an implementation to place a limit on the number of items that can be passed in iov. An implementation can advertise its limit by defining IOV_MAX in <limits.h> or at run time via the return value from sysconf(_SC_IOV_MAX). On Linux, the limit advertised by these mechanisms is 1024, which is the true kernel limit. However, the glibc wrapper functions do some extra work if they detect that the underlying kernel system call failed because this limit was exceeded. In the case of readv() the wrapper function allocates a temporary buffer large enough for all of the items specified by iov, passes that buffer in a call to read(2), copies data from the buffer to the locations specified by the iov_base fields of the elements of iov, and then frees the buffer. The wrapper function for writev() performs the analogous task using a temporary buffer and a call to write(2).

BUGS
It is not advisable to mix calls to functions like readv() or writev(), which operate on file descriptors, with the functions from the stdio library; the results will be undefined and probably not what you want.

EXAMPLE
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);

SEE ALSO
read(2), write(2)