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System Calls read(2)
NAME
read, readv, pread - read from file
SYNOPSIS
#include <unistd.h>
ssize_t read(int fildes, void *buf, size_t nbyte);
ssize_t pread(int fildes, void *buf, size_t nbyte, off_t
offset);
#include <sys/uio.h>
ssize_t 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:
o If O_NDELAY or O_NONBLOCK is set, read() returns -1 and
sets errno to EAGAIN.
o If O_NDELAY and O_NONBLOCK are clear, read() sleeps
until the blocking record lock is removed.
SunOS 5.10 Last change: 7 May 2001 1
System Calls read(2)
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
O_NDELAY is set, read() returns 0.
o If some process has the pipe open for writing and
O_NONBLOCK is set, read() returns -1 and sets errno to
EAGAIN.
o If O_NDELAY and O_NONBLOCK 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 O_NDELAY is set, read() returns 0.
o If O_NONBLOCK is set, read() returns -1 and sets errno
to EAGAIN.
o If O_NDELAY and O_NONBLOCK 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 O_NDELAY or O_NONBLOCK is set, read() returns -1 and
sets errno to EAGAIN.
o If O_NDELAY and O_NONBLOCK 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
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
SunOS 5.10 Last change: 7 May 2001 2
System Calls read(2)
associated with fildes.
Upon successful completion, where nbyte is greater than 0,
read() will mark for update the st_atime 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 I_SRDOPT ioctl(2) request, and
can be tested with the I_GRDOPT 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
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
SunOS 5.10 Last change: 7 May 2001 3
System Calls read(2)
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 I_SRDOPT 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: iov0, iov1, ..., iov[iovcnt-1]. The iovcnt
argument is valid if greater than 0 and less than or equal
to IOV_MAX.
The iovec structure contains the following members:
caddr_t iov_base;
int iov_len;
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
st_atime field of the file.
SunOS 5.10 Last change: 7 May 2001 4
System Calls read(2)
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 off_t 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.
ERRORS
The read(), readv(), and pread() functions will fail if:
EAGAIN Mandatory file/record locking was set,
O_NDELAY or O_NONBLOCK was set, and there
was a blocking record lock; total amount of
system memory available 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 O_NONBLOCK was set; or
no message is waiting to be read on a stream
and O_NDELAY or O_NONBLOCK was set.
EBADF The fildes argument is not a valid file
descriptor open for reading.
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 opera-
tion and no data was transferred.
SunOS 5.10 Last change: 7 May 2001 5
System Calls read(2)
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 attempting to read from its controlling
terminal, and either the process is ignoring
or blocking the SIGTTIN signal or the pro-
cess group of the process 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
special 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 ssize_t.
The read() and readv() functions will fail if:
SunOS 5.10 Last change: 7 May 2001 6
System Calls read(2)
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
associated with fildes.
The readv() function may fail if:
EFAULT The iov argument points outside the allo-
cated address space.
EINVAL The iovcnt argument was less than or equal
to 0 or greater than IOV_MAX. See intro(2)
for a definition of IOV_MAX).
One of the iov_len values in the iov array
was negative, or the sum of the iov_len
values in the iov array overflowed an
ssize_t.
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).
ATTRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
____________________________________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|_____________________________|_____________________________|
| Interface Stability | Standard |
|_____________________________|_____________________________|
| MT-Level | read() is Async-Signal-Safe |
|_____________________________|_____________________________|
SunOS 5.10 Last change: 7 May 2001 7
System Calls read(2)
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 5.10 Last change: 7 May 2001 8
Man(1) output converted with
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This page was generated on Wed Sep 12 21:37:21 GMT 2007
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