read, readv, pread, preadv - read input
Standard C Library (libc, -lc)
#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 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.
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.
[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.
[EINVAL] The total length of the I/O is more than can be
expressed by the ssize_t return value.
[EAGAIN] The file was marked for non-blocking I/O, and no data
were ready to be read.
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] One of the iov_len values in the iov array was negative.
[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() and preadv() calls 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), poll(2), select(2), socket(2),
socketpair(2)
The read() function conforms to ISO/IEC 9945-1:1990 (``POSIX.1''). The
readv() and pread() functions conform to X/Open Portability Guide
Issue 4.2 (``XPG4.2'').
The preadv() function call appeared in NetBSD 1.4. 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.
BSD October 16, 2001 BSD
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