| READ(2) | System Calls Manual | READ(2) | 
read, readv,
  pread, preadv —
#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);
ssize_t
  
  preadv(int
    d, const struct iovec
    *iov, int iovcnt,
    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() and
  preadv() perform the same functions, 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 {
	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() 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 file pointer associated with
    d (see
    lseek(2)). Upon return from
    read(), the file 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 file 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.
read(), readv(),
  pread(), and preadv() will
  succeed unless:
EAGAIN]EBADF]EFAULT]EINTR]EINVAL]EIO]EISDIR]read() or
      pread(). The readdir()
      function should be used instead.In addition, readv() and
    preadv() may return one of the following errors:
EFAULT]EINVAL]IOV_MAX}; or one of the
      iov_len values in the iov
      array was negative; or the sum of the iov_len values
      in the iov array overflowed a 32-bit integer.The pread() and
    preadv() calls may also return the following
  errors:
read() function conforms to IEEE
  Std 1003.1-1990 (“POSIX.1”). The
  readv() and pread() functions
  conform to X/Open Portability Guide Issue 4,
  Version 2 (“XPG4.2”).
preadv() function call appeared in
  NetBSD 1.4. The pread()
  function call appeared in AT&T System V
  Release 4 UNIX. The readv() function
  call appeared in 4.2BSD. The
  read() function call appeared in
  Version 1 AT&T UNIX.
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)
| September 2, 2019 | NetBSD 9.3 |