lib/libsys/read.2

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.Dd June 4, 2020
.Dt READ 2
.Os
.Sh NAME
.Nm read ,
.Nm readv ,
.Nm pread ,
.Nm preadv
.Nd read input
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft ssize_t
.Fn read "int fd" "void *buf" "size_t nbytes"
.Ft ssize_t
.Fn pread "int fd" "void *buf" "size_t nbytes" "off_t offset"
.In sys/uio.h
.Ft ssize_t
.Fn readv "int fd" "const struct iovec *iov" "int iovcnt"
.Ft ssize_t
.Fn preadv "int fd" "const struct iovec *iov" "int iovcnt" "off_t offset"
.Sh DESCRIPTION
The
.Fn read
system call
attempts to read
.Fa nbytes
of data from the object referenced by the descriptor
.Fa fd
into the buffer pointed to by
.Fa buf .
The
.Fn readv
system call
performs the same action, but scatters the input data
into the
.Fa iovcnt
buffers specified by the members of the
.Fa iov
array: iov[0], iov[1], ..., iov[iovcnt\|\-\|1].
The
.Fn pread
and
.Fn preadv
system calls
perform the same functions, but read from the specified position in
the file without modifying the file pointer.
.Pp
For
.Fn readv
and
.Fn preadv ,
the
.Fa iovec
structure is defined as:
.Pp
.Bd -literal -offset indent -compact
struct iovec {
	void   *iov_base;  /* Base address. */
	size_t iov_len;    /* Length. */
};
.Ed
.Pp
Each
.Fa iovec
entry specifies the base address and length of an area
in memory where data should be placed.
The
.Fn readv
system call
will always fill an area completely before proceeding
to the next.
.Pp
On objects capable of seeking, the
.Fn read
starts at a position
given by the pointer associated with
.Fa fd
(see
.Xr lseek 2 ) .
Upon return from
.Fn read ,
the pointer is incremented by the number of bytes actually read.
.Pp
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.
.Pp
Upon successful completion,
.Fn read ,
.Fn readv ,
.Fn pread
and
.Fn 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.
.Pp
In accordance with
.St -p1003.1-2004 ,
both
.Xr read 2
and
.Xr 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
.Xr read 2
or
.Xr write 2 ,
syscalls, each call will see either all of the changes of the other call,
or none of them.
The
.Fx
kernel implements this guarantee by locking the file ranges affected by
the calls.
.Sh 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
.Va errno
is set to indicate the error.
.Sh ERRORS
The
.Fn read ,
.Fn readv ,
.Fn pread
and
.Fn preadv
system calls
will succeed unless:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa fd
argument
is not a valid file or socket descriptor open for reading.
.It Bq Er ECONNRESET
The
.Fa fd
argument refers to a socket, and the remote socket end is
forcibly closed.
.It Bq Er EFAULT
The
.Fa buf
argument
points outside the allocated address space.
.It Bq Er EIO
An I/O error occurred while reading from the file system.
.It Bq Er EINTEGRITY
Corrupted data was detected while reading from the file system.
.It Bq Er EBUSY
Failed to read from a file, e.g. /proc/<pid>/regs while <pid> is not stopped
.It Bq Er 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.
.It Bq Er EINVAL
The pointer associated with
.Fa fd
was negative.
.It Bq Er EAGAIN
The file was marked for non-blocking I/O,
and no data were ready to be read.
.It Bq Er EISDIR
The file descriptor is associated with a directory.
Directories may only be read directly by root if the filesystem supports it and
the
.Dv security.bsd.allow_read_dir
sysctl MIB is set to a non-zero value.
For most scenarios, the
.Xr readdir 3
function should be used instead.
.It Bq Er EOPNOTSUPP
The file descriptor is associated with a file system and file type that
do not allow regular read operations on it.
.It Bq Er EOVERFLOW
The file descriptor is associated with a regular file,
.Fa nbytes
is greater than 0,
.Fa offset
is before the end-of-file, and
.Fa offset
is greater than or equal to the offset maximum established
for this file system.
.It Bq Er EINVAL
The value
.Fa nbytes
is greater than
.Dv INT_MAX .
.El
.Pp
In addition,
.Fn readv
and
.Fn preadv
may return one of the following errors:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa iovcnt
argument
was less than or equal to 0, or greater than
.Dv IOV_MAX .
.It Bq Er EINVAL
One of the
.Fa iov_len
values in the
.Fa iov
array was negative.
.It Bq Er EINVAL
The sum of the
.Fa iov_len
values in the
.Fa iov
array overflowed a 32-bit integer.
.It Bq Er EFAULT
Part of the
.Fa iov
array points outside the process's allocated address space.
.El
.Pp
The
.Fn pread
and
.Fn preadv
system calls may also return the following errors:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa offset
value was negative.
.It Bq Er ESPIPE
The file descriptor is associated with a pipe, socket, or FIFO.
.El
.Sh SEE ALSO
.Xr dup 2 ,
.Xr fcntl 2 ,
.Xr getdirentries 2 ,
.Xr open 2 ,
.Xr pipe 2 ,
.Xr select 2 ,
.Xr socket 2 ,
.Xr socketpair 2 ,
.Xr fread 3 ,
.Xr readdir 3
.Sh STANDARDS
The
.Fn read
system call is expected to conform to
.St -p1003.1-90 .
The
.Fn readv
and
.Fn pread
system calls are expected to conform to
.St -xpg4.2 .
.Sh HISTORY
The
.Fn preadv
system call appeared in
.Fx 6.0 .
The
.Fn pread
function appeared in
.At V.4 .
The
.Fn readv
system call appeared in
.Bx 4.2 .
The
.Fn read
function appeared in
.At v1 .