DragonFly On-Line Manual Pages

READ(2)                  DragonFly System Calls Manual                 READ(2)

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

LIBRARY
     Standard C Library (libc, -lc)

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);

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

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

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

     ssize_t
     extpreadv(int d, const struct iovec *iov, int iovcnt, int flags,
         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
     specified by the members of the iov array: iov[0], iov[1], ...,
     iov[iovcnt-1].  The pread(), preadv(), extpread(), and extpreadv() calls
     perform the same function, 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 {
                   char   *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.  Readv() and preadv() will always
     fill an area completely before proceeding to the next.

     The extpread() and extpreadv() allow a flags argument to also be passed
     in, controlling blocking/non-blocking and other features on a call-by-
     call basis, ignoring the related default for the descriptor.  Allowed
     flags are:

     O_FBLOCKING         Force the system call to operate in a blocking
                         fashion.

     O_FNONBLOCKING      Force the system call to operate in a non-blocking
                         fashion.

     O_FAPPEND           Force append mode for the operation.

     O_FOFFSET           Force offset mode for the operation.

     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
     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
     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
     reading 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(), preadv(), extpread() and extpreadv() 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
                        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.

     [EISDIR]           The file descriptor d refers to a directory and the
                        implementation does not allow the directory to be read
                        using read() or pread().  The readdir() function
                        should be used instead.

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

     [ENOBUFS]          A message was not delivered because it would have
                        overflowed the buffer.

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

     [EINVAL]           Iovcnt was less than or equal to 0, or greater than
                        16.

     [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(), preadv(), extpread() and extpreadv() 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), select(2), socket(2), socketpair(2),
     readdir(3)

STANDARDS
     The read() function call is expected to conform to IEEE Std 1003.1-1990
     ("POSIX.1").  The readv() and pread() functions are expected to conform
     to X/Open Portability Guide Issue 4, Version 2 ("XPG4.2").

     The extpread() and extpreadv() functions are DragonFly specific
     extensions.

HISTORY
     The preadv() function call was added in DragonFly 1.5.  The pread()
     function call appeared in AT&T System V Release 4 UNIX.  The readv()
     function call appeared in 4.2BSD.  A read() function call appeared in
     Version 6 AT&T UNIX.

DragonFly 5.9-DEVELOPMENT        June 8, 2020        DragonFly 5.9-DEVELOPMENT