read



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


NAME

     read, readv, pread, preadv - read input


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);

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

     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
     specified 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 specified position offset 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
     memory where data should be placed.  readv() and preadv() 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
     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.

     Note that readv() and preadv() will fail if the value of iovcnt exceeds
     the constant IOV_MAX.


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(), and preadv() will succeed unless:

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

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

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

     [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.

     In addition, read() and readv() may return the following errors:

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

     [ENOTCONN]         The file is a socket associated with a connection-
                        oriented protocol and has not been connected.

     [EIO]              The process is a member of a background process
                        attempting to read from its controlling terminal, the
                        process is ignoring or blocking the SIGTTIN signal or
                        the process group is orphaned.

     read() and pread() may return the following error:

     [EINVAL]           nbytes was larger than SSIZE_MAX.

     pread() and preadv() may return the following error:

     [EINVAL]           offset was negative.

     [ESPIPE]           d is associated with a pipe, socket, FIFO, or tty.

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

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


SEE ALSO

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


STANDARDS

     The read(), readv(), and pread() functions conform to IEEE Std
     1003.1-2008 (``POSIX.1'').


HISTORY

     A read() system call first appeared in Version 1 AT&T UNIX; readv() in
     4.1cBSD; pread() in AT&T System V Release 4 UNIX; and preadv() in
     OpenBSD 2.7.


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)

OpenBSD 5.4                      July 17, 2013                     OpenBSD 5.4

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