System Calls read(2)
NAME
read, readv, pread - read from file
SYNOPSIS
#include
ssizet read(int fildes, void *buf, sizet nbyte);
ssizet pread(int fildes, void *buf, sizet nbyte, offt offset);
#include
ssizet readv(int fildes, const struct iovec *iov, int iovcnt);
DESCRIPTION
The read() function attempts to read nbyte bytes from the
file associated with the open file descriptor, fildes, into
the buffer pointed to by buf.
If nbyte is 0, read() returns 0 and has no other results.
On files that support seeking (for example, a regular file),
the read() starts at a position in the file given by the
file offset associated with fildes. The file offset is
incremented by the number of bytes actually read.
Files that do not support seeking (for example, terminals)
always read from the current position. The value of a file
offset associated with such a file is undefined.
If fildes refers to a socket, read() is equivalent to
recv(3SOCKET) with no flags set.
No data transfer will occur past the current end-of-file.
If the starting position is at or after the end-of-file, 0
will be returned. If the file refers to a device special
file, the result of subsequent read() requests is
implementation-dependent.
When attempting to read from a regular file with mandatory
file/record locking set (see chmod(2)), and there is a write
lock owned by another process on the segment of the file to
be read:
SunOS 5.11 Last change: 13 Sep 2007 1
System Calls read(2)
o If ONDELAY or ONONBLOCK is set, read() returns -1
and sets errno to EAGAIN.
o If ONDELAY and ONONBLOCK are clear, read() sleeps
until the blocking record lock is removed.
When attempting to read from an empty pipe (or FIFO):
o If no process has the pipe open for writing, read()
returns 0 to indicate end-of-file.
o If some process has the pipe open for writing and
ONDELAY is set, read() returns 0.
o If some process has the pipe open for writing and
ONONBLOCK is set, read() returns -1 and sets errno
to EAGAIN.
o If ONDELAY and ONONBLOCK are clear, read() blocks
until data is written to the pipe or the pipe is
closed by all processes that had opened the pipe
for writing.
When attempting to read a file associated with a terminal
that has no data currently available:
o If ONDELAY is set, read() returns 0.
o If ONONBLOCK is set, read() returns -1 and sets
errno to EAGAIN.
o If ONDELAY and ONONBLOCK are clear, read() blocks
until data become available.
When attempting to read a file associated with a socket or a
stream that is not a pipe, a FIFO, or a terminal, and the
file has no data currently available:
o If ONDELAY or ONONBLOCK is set, read() returns -1
and sets errno to EAGAIN.
o If ONDELAY and ONONBLOCK are clear, read() blocks
until data becomes available.
The read() function reads data previously written to a file.
If any portion of a regular file prior to the end-of-file
has not been written, read() returns bytes with value 0.
For example, lseek(2) allows the file offset to be set
SunOS 5.11 Last change: 13 Sep 2007 2
System Calls read(2)
beyond the end of existing data in the file. If data is
later written at this point, subsequent reads in the gap
between the previous end of data and the newly written data
will return bytes with value 0 until data is written into
the gap.
For regular files, no data transfer will occur past the
offset maximum established in the open file description
associated with fildes.
Upon successful completion, where nbyte is greater than 0,
read() will mark for update the statime field of the file,
and return the number of bytes read. This number will never
be greater than nbyte. The value returned may be less than
nbyte if the number of bytes left in the file is less than
nbyte, if the read() request was interrupted by a signal, or
if the file is a pipe or FIFO or special file and has fewer
than nbyte bytes immediately available for reading. For
example, a read() from a file associated with a terminal may
return one typed line of data.
If a read() is interrupted by a signal before it reads any
data, it will return -1 with errno set to EINTR.
If a read() is interrupted by a signal after it has success-
fully read some data, it will return the number of bytes
read.
A read() from a STREAMS file can read data in three dif-
ferent modes: byte-stream mode, message-nondiscard mode,
and message-discard mode. The default is byte-stream mode.
This can be changed using the ISRDOPT ioctl(2) request, and
can be tested with the IGRDOPT ioctl(). In byte-stream
mode, read() retrieves data from the STREAM until as many
bytes as were requested are transferred, or until there is
no more data to be retrieved. Byte-stream mode ignores mes-
sage boundaries.
In STREAMS message-nondiscard mode, read() retrieves data
until as many bytes as were requested are transferred, or
until a message boundary is reached. If read() does not
retrieve all the data in a message, the remaining data is
left on the STREAM, and can be retrieved by the next read()
call. Message-discard mode also retrieves data until as
many bytes as were requested are transferred, or a message
boundary is reached. However, unread data remaining in a
SunOS 5.11 Last change: 13 Sep 2007 3
System Calls read(2)
message after the read() returns is discarded, and is not
available for a subsequent read(), readv() or getmsg(2)
call.
How read() handles zero-byte STREAMS messages is determined
by the current read mode setting. In byte-stream mode,
read() accepts data until it has read nbyte bytes, or until
there is no more data to read, or until a zero-byte message
block is encountered. The read() function then returns the
number of bytes read, and places the zero-byte message back
on the STREAM to be retrieved by the next read(), readv() or
getmsg(2). In message-nondiscard mode or message-discard
mode, a zero-byte message returns 0 and the message is
removed from the STREAM. When a zero-byte message is read
as the first message on a STREAM, the message is removed
from the STREAM and 0 is returned, regardless of the read
mode.
A read() from a STREAMS file returns the data in the message
at the front of the STREAM head read queue, regardless of
the priority band of the message.
By default, STREAMs are in control-normal mode, in which a
read() from a STREAMS file can only process messages that
contain a data part but do not contain a control part. The
read() fails if a message containing a control part is
encountered at the STREAM head. This default action can be
changed by placing the STREAM in either control-data mode or
control-discard mode with the ISRDOPT ioctl() command. In
control-data mode, read() converts any control part to data
and passes it to the application before passing any data
part originally present in the same message. In control-
discard mode, read() discards message control parts but
returns to the process any data part in the message.
In addition, read() and readv() will fail if the STREAM head
had processed an asynchronous error before the call. In
this case, the value of errno does not reflect the result of
read() or readv() but reflects the prior error. If a hangup
occurs on the STREAM being read, read() continues to operate
normally until the STREAM head read queue is empty.
Thereafter, it returns 0.
readv()
The readv() function is equivalent to read(), but places the
input data into the iovcnt buffers specified by the members
of the iov array: iov[0], iov[1], ..., iov[iovcnt-1]. The
iovcnt argument is valid if greater than 0 and less than or
SunOS 5.11 Last change: 13 Sep 2007 4
System Calls read(2)
equal to {IOVMAX}.
The iovec structure contains the following members:
caddrt iovbase;
int iovlen;
Each iovec entry specifies the base address and length of an
area in memory where data should be placed. The readv()
function always fills an area completely before proceeding
to the next.
Upon successful completion, readv() marks for update the
statime field of the file.
pread()
The pread() function performs the same action as read(),
except that it reads from a given position in the file
without changing the file pointer. The first three arguments
to pread() are the same as read() with the addition of a
fourth argument offset for the desired position inside the
file. pread() will read up to the maximum offset value that
can be represented in an offt for regular files. An attempt
to perform a pread() on a file that is incapable of seeking
results in an error.
RETURN VALUES
Upon successful completion, read() and readv() return a
non-negative integer indicating the number of bytes actually
read. Otherwise, the functions return -1 and set errno to
indicate the error.
ERORS
The read(), readv(), and pread() functions will fail if:
EAGAIN Mandatory file/record locking was set, ONDELAY
or ONONBLOCK was set, and there was a blocking
record lock; total amount of system memory avail-
able when reading using raw I/O is temporarily
insufficient; no data is waiting to be read on a
file associated with a tty device and ONONBLOCK
was set; or no message is waiting to be read on a
stream and ONDELAY or ONONBLOCK was set.
EBADF The fildes argument is not a valid file descrip-
tor open for reading.
SunOS 5.11 Last change: 13 Sep 2007 5
System Calls read(2)
EBADMSG Message waiting to be read on a stream is not a
data message.
EDEADLK The read was going to go to sleep and cause a
deadlock to occur.
EINTR A signal was caught during the read operation and
no data was transferred.
EINVAL An attempt was made to read from a stream linked
to a multiplexor.
EIO A physical I/O error has occurred, or the process
is in a background process group and is attempt-
ing to read from its controlling terminal, and
either the process is ignoring or blocking the
SIGTIN signal or the process group of the pro-
cess is orphaned.
EISDIR The fildes argument refers to a directory on a
file system type that does not support read
operations on directories.
ENOLCK The system record lock table was full, so the
read() or readv() could not go to sleep until the
blocking record lock was removed.
ENOLINK The fildes argument is on a remote machine and
the link to that machine is no longer active.
ENXIO The device associated with fildes is a block spe-
cial or character special file and the value of
the file pointer is out of range.
The read() and pread() functions will fail if:
EFAULT The buf argument points to an illegal address.
EINVAL The nbyte argument overflowed an ssizet.
SunOS 5.11 Last change: 13 Sep 2007 6
System Calls read(2)
The read() and readv() functions will fail if:
EOVERFLOW 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 in the open file description asso-
ciated with fildes.
The readv() function may fail if:
EFAULT The iov argument points outside the allocated
address space.
EINVAL The iovcnt argument was less than or equal to 0 or
greater than {IOVMAX}. See Intro(2) for a defini-
tion of {IOVMAX}).
One of the iovlen values in the iov array was
negative, or the sum of the iovlen values in the
iov array overflowed an ssizet.
The pread() function will fail and the file pointer remain
unchanged if:
ESPIPE The fildes argument is associated with a pipe or
FIFO.
USAGE
The pread() function has a transitional interface for 64-bit
file offsets. See lf64(5).
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
SunOS 5.11 Last change: 13 Sep 2007 7
System Calls read(2)
ATRIBUTE TYPE ATRIBUTE VALUE
Interface Stability Committed
MT-Level read() is Async-Signal-Safe
Standard See standards(5).
SEE ALSO
Intro(2), chmod(2), creat(2), dup(2), fcntl(2), getmsg(2),
ioctl(2), lseek(2), open(2), pipe(2), recv(3SOCKET), attri-
butes(5), lf64(5), standards(5), streamio(7I), termio(7I)
SunOS 5.11 Last change: 13 Sep 2007 8
|
