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     read, readv, pread, preadv — read input


     Standard C Library (libc, -lc)


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

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

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

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

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


     The read() system call attempts to read nbytes of data from the object
     referenced by the descriptor d into the buffer pointed to by buf.  The
     readv() system call 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].  The pread() and preadv() system
     calls 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;  /* Base address. */
                   size_t iov_len;    /* Length. */

     Each iovec entry specifies the base address and length of an area in
     memory where data should be placed.  The readv() system call 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.


     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.


     The read(), readv(), pread() and preadv() system calls will succeed

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

     [ECONNRESET]       The d argument refers to a socket, and the remote
                        socket end is forcibly closed.

     [EFAULT]           The buf argument points outside the allocated address

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

     [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.

     [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.

     [EISDIR]           The file descriptor is associated with a directory
                        residing on a file system that does not allow regular
                        read operations on directories (e.g. NFS).

     [EOPNOTSUPP]       The file descriptor is associated with a file system
                        and file type that do not allow regular read
                        operations on it.

     [EOVERFLOW]        The file descriptor is associated with a regular file,
                        nbytes is greater than 0, offset is before the end-of-
                        file, and offset is greater than or equal to the
                        offset maximum established for this file system.

     [EINVAL]           The value nbytes is greater than INT_MAX.

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

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

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

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

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

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

     [EINVAL]           The offset value was negative.

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


     dup(2), fcntl(2), getdirentries(2), open(2), pipe(2), select(2),
     socket(2), socketpair(2), fread(3), readdir(3)


     The read() system call is expected to conform to ISO/IEC 9945-1:1990
     (“POSIX.1”).  The readv() and pread() system calls are expected to
     conform to X/Open Portability Guide Issue 4, Version 2 (“XPG4.2”).


     The preadv() system call appeared in FreeBSD 6.0.  The pread() function
     appeared in AT&T System V Release 4 UNIX.  The readv() system call
     appeared in 4.2BSD.  The read() function appeared in Version 6 AT&T UNIX.