READ(2)                   OpenBSD Programmer's Manual                  READ(2)

NAME
     read, readv - read input



SYNOPSIS
     #include <sys/types.h>
     #include <sys/uio.h>
     #include <unistd.h>

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

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

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].

     For readv(), 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 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 po-
     sition.  The value of the pointer associated with such an object is unde-
     fined.

     Upon successful completion, read() and readv() 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.

     Note that readv() will fail if the value of iovcnt exceedes the constant
     IOV_MAX.

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() and readv() will succeed unless:

     [EBADF]       d is not a valid file or socket descriptor open for read-
                   ing.

     [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.

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

     In addition, read() may return the following error:

     [EINVAL]      nbytes was larger than SSIZE_MAX.

     Also, readv() may return one of the following errors:

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

     [EINVAL]      The sum of the iov_len values in the iov array overflowed
                   an ssize_t.

     [EFAULT]      Part of the iov points outside the process's allocated ad-
                   dress space.

SEE ALSO
     dup(2),  fcntl(2),  open(2),  pipe(2),  poll(2),  select(2),  socket(2),
     socketpair(2)

STANDARDS
     The read() function is expected to conform to IEEE Std1003.1-1988
     (``POSIX'').

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)

HISTORY
     The readv() function call appeared in 4.2BSD. The read() function call
     appeared in Version 6 AT&T UNIX.

OpenBSD 2.6                      July 28, 1998                               2