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READ(2)                   FreeBSD System Calls Manual                  READ(2)

NAME
     read, readv, pread - 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);

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() performs the same function, but reads from the
     specified position in the file without modifying the file pointer.

     For readv(), 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() 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(), and pread() 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(), and pread() 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.

     [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
                        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() call 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)

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'').

HISTORY
     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.

FreeBSD 11.0-PRERELEASE        February 26, 1994       FreeBSD 11.0-PRERELEASE

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | STANDARDS | HISTORY

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