READ(2) System Calls Manual READ(2)
NAME
pread, read, preadv, readv – read input
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/types.h>
#include <sys/uio.h>
#include <unistd.h>
ssize_t
pread(int d, void *buf, size_t nbyte, off_t offset);
ssize_t
read(int fildes, void *buf, size_t nbyte);
ssize_t
preadv(int d, const struct iovec *iov, int iovcnt, off_t offset);
ssize_t
readv(int d, const struct iovec *iov, int iovcnt);
DESCRIPTION
read() attempts to read nbyte bytes of data from the object referenced by
the descriptor fildes 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 {
char *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. readv() and preadv() 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 fildes (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.
read() and pread() will fail if the parameter nbyte exceeds INT_MAX, and
they do not attempt a partial read.
RETURN VALUES
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.
ERRORS
The read(), readv(), pread(), and preadv() calls will succeed unless:
[EAGAIN] The file was marked for non-blocking I/O, and no data
were ready to be read.
[EBADF] fildes is not a valid file or socket descriptor open
for reading.
[EFAULT] Buf points outside the allocated address space.
[EINTR] A read from a slow device was interrupted before any
data arrived by the delivery of a signal.
[EINVAL] The pointer associated with fildes was negative.
[EIO] An I/O error occurred while reading from the file
system.
[EIO] The process group is orphaned.
[EIO] The file is a regular file, nbyte is greater than 0,
the starting position is before the end-of-file, and
the starting position is greater than or equal to the
offset maximum established for the open file
descriptor associated with fildes.
[EISDIR] An attempt is made to read a directory.
[ENOBUFS] An attempt to allocate a memory buffer fails.
[ENOMEM] Insufficient memory is available.
[ENXIO] An action is requested of a device that does not
exist.
[ENXIO] A requested action cannot be performed by the device.
[ESTALE] An attempt to read a remote file through NFS that has
already been deleted in the server.
[ETIMEDOUT] The connection timed out while reading a remote file
from a soft mounted NFS volume (see mount_nfs(8)).
[ETIMEDOUT] The file is a “dataless” file that requires
materialization and materialization timed out or
encountered some other temporary failure.
[EDEADLK] The file is a “dataless” file that requires
materialization and the I/O policy of the current
thread or process disallows dataless file
materialization (see getiopolicy_np(3)).
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.
The read() call may also return the following errors:
[ECONNRESET] The connection is closed by the peer during a read
attempt on a socket.
[ENOTCONN] A read is attempted on an unconnected socket.
[ETIMEDOUT] A transmission timeout occurs during a read attempt on
a socket.
The read() and pread() call may also return the following error:
[EINVAL] The value provided for nbyte exceeds INT_MAX.
The readv() and preadv() calls may also return one of the following
errors:
[EFAULT] Part of the iov points outside the process's allocated
address space.
[EINVAL] Iovcnt was less than or equal to 0, or greater than
16.
[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.
LEGACY SYNOPSIS
#include <sys/types.h>
#include <sys/uio.h>
#include <unistd.h>
The include files <sys/types.h> and <sys/uio.h> are necessary for all
functions.
SEE ALSO
dup(2), fcntl(2), open(2), pipe(2), select(2), socket(2), socketpair(2),
compat(5)
STANDARDS
The read() function call is expected to conform to IEEE Std 1003.1-1990
(“POSIX.1”). The readv() and pread() functions are expected to conform
to X/Open Portability Guide Issue 4, Version 2 (“XPG4.2”). preadv() is
nonstandard.
HISTORY
The pread() function call appeared in AT&T System V Release 4 UNIX. The
readv() function call appeared in 4.2BSD. A read() function call
appeared in Version 6 AT&T UNIX.
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