|
|
| |
READ(2) |
FreeBSD System Calls Manual |
READ(2) |
read , readv ,
pread , preadv —
read input
Standard C Library (libc, -lc)
#include <unistd.h>
ssize_t
read (int
fd, void *buf,
size_t nbytes);
ssize_t
pread (int
fd, void *buf,
size_t nbytes,
off_t offset);
#include
<sys/uio.h>
ssize_t
readv (int
fd, const struct iovec
*iov, int
iovcnt);
ssize_t
preadv (int
fd, const struct iovec
*iov, int iovcnt,
off_t offset);
The read () system call attempts to read
nbytes of data from the object referenced by the
descriptor fd into the buffer pointed to by
buf. The readv () system call
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]. The
pread () and preadv () system
calls 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; /* 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. The
readv () system call 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
fd (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.
In accordance with IEEE Std 1003.1-2004
(“POSIX.1”), both
read(2)
and
write(2)
syscalls are atomic with respect to each other in the effects on file
content, when they operate on regular files. If two threads each call one of
the
read(2)
or
write(2),
syscalls, each call will see either all of the changes of the other call, or
none of them. The FreeBSD kernel implements this
guarantee by locking the file ranges affected by the calls.
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.
The read (), readv (),
pread () and preadv () system
calls will succeed unless:
- [
EBADF ]
- The fd argument is not a valid file or socket
descriptor open for reading.
- [
ECONNRESET ]
- The fd argument refers to a socket, and the remote
socket end is forcibly closed.
- [
EFAULT ]
- The buf argument points outside the allocated
address space.
- [
EIO ]
- An I/O error occurred while reading from the file system.
- [
EINTEGRITY ]
- Corrupted data was detected while reading from the file system.
- [
EBUSY ]
- Failed to read from a file, e.g. /proc/<pid>/regs while <pid>
is not stopped
- [
EINTR ]
- A read from a slow device (i.e. one that might block for an arbitrary
amount of time) was interrupted by the delivery of a signal before any
data arrived.
- [
EINVAL ]
- The pointer associated with fd was negative.
- [
EAGAIN ]
- The file was marked for non-blocking I/O, and no data were ready to be
read.
- [
EISDIR ]
- The file descriptor is associated with a directory. Directories may only
be read directly by root if the filesystem supports it and the
security.bsd.allow_read_dir sysctl MIB is set to a
non-zero value. For most scenarios, the
readdir(3)
function should be used instead.
- [
EOPNOTSUPP ]
- The file descriptor is associated with a file system and file type that do
not allow regular read operations on it.
- [
EOVERFLOW ]
- The file descriptor is associated with a regular file,
nbytes is greater than 0,
offset is before the end-of-file, and
offset is greater than or equal to the offset
maximum established for this file system.
- [
EINVAL ]
- The value nbytes is greater than
INT_MAX .
In addition, readv () and
preadv () may return one of the following errors:
- [
EINVAL ]
- The iovcnt argument 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 array points outside the process's
allocated address space.
The pread () and
preadv () system calls may also return the following
errors:
- [
EINVAL ]
- The offset value was negative.
- [
ESPIPE ]
- The file descriptor is associated with a pipe, socket, or FIFO.
The read () system call is expected to conform to
IEEE Std 1003.1-1990 (“POSIX.1”). The
readv () and pread () system
calls are expected to conform to X/Open Portability Guide
Issue 4, Version 2 (“XPG4.2”).
The preadv () system call appeared in
FreeBSD 6.0. The pread ()
function appeared in AT&T System V
Release 4 UNIX. The readv () system call
appeared in 4.2BSD. The read ()
function appeared in Version 1 AT&T UNIX.
Visit the GSP FreeBSD Man Page Interface. Output converted with ManDoc. |