ssize_t
read(
int d
, void *buf
, size_t nbytes
)
ssize_t
pread(
int d
, void *buf
, size_t nbytes
, off_t offset
)
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
)
)
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.
),
readv(
),
pread(
),
and
preadv(
)
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
]
EINTR
]
EINVAL
]
d
was negative.
EINVAL
]
EAGAIN
]
In addition,
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
]
iov_len
values in the
iov
array was negative.
EINVAL
]
iov_len
values in the
iov
array overflowed a 32-bit integer.
EFAULT
]
iov
points outside the process's allocated address space.
The
pread()
and
preadv(
)
calls may also return the following errors:
EINVAL
]
ESPIPE
]
)
function conforms to
ISO/IEC 9945-1:1990 (``POSIX.1'') .
The
readv(
)
and
pread(
)
functions conform to
X/Open Portability Guide Issue 4, Version 2 (``XPG4.2'') .
)
function call
appeared in
NetBSD1.4.
The
pread(
)
function call
appeared in
AT&T
System V.4 UNIX
.
The
readv(
)
function call
appeared in
4.2BSD.
The
read(
)
function call appeared in
Version 6 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)