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

     read, readv, pread - 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);

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

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

     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

     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.

     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.

     Read(), readv(), and pread() will succeed unless:

     [EBADF]            D is not a valid file or socket descriptor open for

     [EFAULT]           Buf points outside the allocated address space.

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

     [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.

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

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

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

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

     [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 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.

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

     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 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


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