2:readv

From Linux Man Pages

      readv, writev - read or write data into multiple buffers
      

Contents 1 SYNOPSIS 2 DESCRIPTION 3 RETURN VALUE 4 ERRORS 5 CONFORMING TO 6 LINUX NOTES 7 BUGS 8 RELATED 9 CATEGORY

SYNOPSIS

      #include <sys/uio.h>
 
      ssize_t readv(int fd, const struct iovec *vector, int count);
 
      ssize_t writev(int fd, const struct iovec *vector, int count);

DESCRIPTION

      The  readv()  function  reads count blocks from the file associated with the file descriptor fd into the multiple
      buffers described by vector.
 
      The writev() function writes at most count blocks described by vector  to  the  file  associated  with  the  file
      descriptor fd.
 
      The pointer vector points to a struct iovec defined in <sys/uio.h> as
 
        struct iovec {
            void *iov_base;   /* Starting address */
            size_t iov_len;   /* Number of bytes */
        };
 
      Buffers are processed in the order specified.
 
      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.

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 count 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 count parameter, and int as return type for these functions.

LINUX NOTES

      POSIX.1-2001 allows an implementation to place a limit on the number of items that can be passed in  vector.   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 vector, passes that buffer in a call to read(), copies data
      from the buffer to the locations specified by the iov_base fields of the elements of vector, and then  frees  the
      buffer.   The  wrapper  function  for writev() performs the analogous task using a temporary buffer and a call to
      write().

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.

RELATED

      read(2), write(2)

CATEGORY