read, readv, pread, preadv - read input
#include <sys/types.h>
#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/types.h>
#include <sys/uio.h>
#include <unistd.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 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(), 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.
Note that readv() and preadv() will fail if the value of
iovcnt exceeds
the constant IOV_MAX.
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(), 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] An I/O error occurred while reading from the
file system,
or the process is a member of a background
process attempting
to read from its controlling terminal, the
process is
ignoring or blocking the SIGTTIN signal or the
process
group is orphaned.
[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, read() and pread() may return the following error:
[EINVAL] nbytes was larger than SSIZE_MAX.
Also, 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] The sum of the iov_len values in the iov array
overflowed
an ssize_t.
[EFAULT] Part of the iov points outside the process's
allocated address
space.
dup(2), fcntl(2), open(2), pipe(2), poll(2), select(2),
socket(2),
socketpair(2)
The read() function conforms to IEEE Std 1003.1-1990
(``POSIX''). The
readv() and pread() functions conform to X/Open Portability
Guide Issue
4.2 (``XPG4.2'').
The preadv() function first appeared in OpenBSD 2.7. The
pread() function
appeared in AT&T System V.4 UNIX. The readv() function
call appeared
in 4.2BSD. The read() function call appeared in Version 2 AT&T
UNIX.
Error checks should explicitly test for -1. Code such as
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)
OpenBSD 3.6 July 28, 1998
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