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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);

     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() system call 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 {
                   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(), 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.

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

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

     [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 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]           The iovcnt argument was less than or equal to 0, or
                        greater than 16.

     [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() system 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), 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 IEEE Std 1003.1-1990
     (``POSIX.1'').  The readv() and pread() system calls are expected to
     conform to X/Open Portability Guide Issue 4, Version 2 (``XPG4.2'').

     The 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.

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


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