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  man pages->NetBSD man pages -> readv (2)              



NAME    [Toc]    [Back]

     read, readv, pread, preadv - read input

LIBRARY    [Toc]    [Back]

     Standard C Library (libc, -lc)

SYNOPSIS    [Toc]    [Back]

     #include <unistd.h>

     read(int d, void *buf, size_t nbytes);

     pread(int d, void *buf, size_t nbytes, off_t offset);

     #include <sys/uio.h>

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

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

DESCRIPTION    [Toc]    [Back]

     read() attempts to read nbytes of data from the object referenced by the
     descriptor d into the buffer pointed to by buf.  readv() performs the
     same action, but scatters the input data into the iovcnt buffers specified
 by the members of the iov array: iov[0], iov[1], ..., iov[iovcnt-1].
     pread() and preadv() perform the same functions, but read from the specified
 position in the file without modifying the file pointer.

     For readv() and preadv(), the iovec structure is defined as:

           struct iovec {
                   void *iov_base;
                   size_t iov_len;

     Each iovec entry specifies the base address and length of an area in memory
 where data should be placed.  readv() will always fill an area completely
 before proceeding to the next.

     On objects capable of seeking, the read() starts at a position given by
     the pointer associated with d (see lseek(2)).  Upon return from read(),
     the pointer is incremented by the number of bytes actually read.

     Objects that are not capable of seeking always read from the current
     position.  The value of the pointer associated with such an object is

     Upon successful completion, read(), readv(), pread(), and preadv() return
     the number of bytes actually read and placed in the buffer.  The system
     guarantees to read the number of bytes requested if the descriptor references
 a normal file that has that many bytes left before the end-of-file,
     but in no other case.

RETURN VALUES    [Toc]    [Back]

     If successful, the number of bytes actually read is returned. Upon reading
 end-of-file, zero is returned.  Otherwise, a -1 is returned and the
     global variable errno is set to indicate the error.

ERRORS    [Toc]    [Back]

     read(), readv(), pread(), and preadv() will succeed unless:

     [EBADF]            d is not a valid file or socket descriptor open for

     [EFAULT]           buf points outside the allocated address space.

     [EIO]              An I/O error occurred while reading from the file system.

     [EINTR]            A read from a slow device was interrupted before any
                        data arrived by the delivery of a signal.

     [EINVAL]           The pointer associated with d was negative.

     [EINVAL]           The total length of the I/O is more than can be
                        expressed by the ssize_t return value.

     [EAGAIN]           The file was marked for non-blocking I/O, and no data
                        were ready to be read.

     In addition, readv() and preadv() may return one of the following errors:

     [EINVAL]           iovcnt was less than or equal to 0, or greater than

     [EINVAL]           One of the iov_len values in the iov array was negative.

     [EINVAL]           The sum of the iov_len values in the iov array overflowed
 a 32-bit integer.

     [EFAULT]           Part of the iov points outside the process's allocated
                        address space.

     The pread() and preadv() calls may also return the following errors:

     [EINVAL]           The specified file offset is invalid.

     [ESPIPE]           The file descriptor is associated with a pipe, socket,
                        or FIFO.

SEE ALSO    [Toc]    [Back]

     dup(2), fcntl(2), open(2), pipe(2), poll(2), select(2), socket(2),

STANDARDS    [Toc]    [Back]

     The read() function conforms to ISO/IEC 9945-1:1990 (``POSIX.1'').  The
     readv() and pread() functions conform to X/Open Portability Guide
     Issue 4.2 (``XPG4.2'').

HISTORY    [Toc]    [Back]

     The preadv() function call appeared in NetBSD 1.4.  The pread() function
     call appeared in AT&T System V.4 UNIX.  The readv() function call
     appeared in 4.2BSD.  The read() function call appeared in Version 6 AT&T

BSD                            October 16, 2001                            BSD
[ Back ]
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