read(2) - NetBSD Manual Pages

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READ(2)                   NetBSD 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 specified 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.


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

     [EINVAL]      Iovcnt was less than or equal to 0, or greater than
                   {UIO_MAXIOV}.

     [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 ad-
                   dress space.


SEE ALSO

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


STANDARDS

     The read() function conforms to IEEE Std1003.1-1990 (``POSIX'').


HISTORY

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

4th Berkeley Distribution      February 26, 1994                             2