READ(2)                   NetBSD System Calls Manual                   READ(2)

NAME
     read, readv, pread, preadv -- read input

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <unistd.h>

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

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

     #include <sys/uio.h>

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

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

DESCRIPTION
     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 speci-
     fied 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 speci-
     fied 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 mem-
     ory where data should be placed.  readv() will always fill an area com-
     pletely before proceeding to the next.

     On objects capable of seeking, the read() starts at a position given by
     the file pointer associated with d (see lseek(2)).  Upon return from
     read(), the file 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 file pointer associated with such an object
     is undefined.

     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 refer-
     ences a normal file that has that many bytes left before the end-of-file,
     but in no other case.

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

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

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

     [EFAULT]           buf points outside the allocated address space.

     [EIO]              An I/O error occurred while reading from the file sys-
                        tem.

     [EINTR]            A read from a slow device (i.e. one that might block
                        for an arbitrary amount of time) was interrupted by
                        the delivery of a signal before any data arrived.  See
                        sigaction(2) for more information on the interaction
                        between signals and system calls.

     [EINVAL]           The file 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
                        {IOV_MAX}.

     [EINVAL]           One of the iov_len values in the iov array was nega-
                        tive.

     [EINVAL]           The sum of the iov_len values in the iov array over-
                        flowed 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
     dup(2), fcntl(2), open(2), pipe(2), poll(2), select(2), sigaction(2),
     socket(2), socketpair(2)

STANDARDS
     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, Version 2 (``XPG4.2'').

HISTORY
     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
     UNIX.

CAVEATS
     Error checks should explicitly test for -1.  Code such as

             while ((nr = read(fd, buf, sizeof(buf))) > 0)

     is not maximally portable, as some platforms allow for nbytes to range
     between SSIZE_MAX and SIZE_MAX - 2, in which case the return value of an
     error-free read() may appear as a negative number distinct from -1.
     Proper loops should use

             while ((nr = read(fd, buf, sizeof(buf))) != -1 && nr != 0)

NetBSD 5.0_STABLE               August 2, 2007               NetBSD 5.0_STABLE
