import less(1)

[unleashed/tickless.git] / share / man / man2 / read.2

'\" te
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.TH READ 2 "Jan 15, 2015"
.SH NAME
read, readv, pread, preadv \- read from file
.SH SYNOPSIS
.LP
.nf
#include <unistd.h>

\fBssize_t\fR \fBread\fR(\fBint\fR \fIfildes\fR, \fBvoid *\fR\fIbuf\fR, \fBsize_t\fR \fInbyte\fR);
.fi

.LP
.nf
\fBssize_t\fR \fBpread\fR(\fBint\fR \fIfildes\fR, \fBvoid *\fR\fIbuf\fR, \fBsize_t\fR \fInbyte\fR, \fBoff_t\fR \fIoffset\fR);
.fi

.LP
.nf
#include <sys/uio.h>

\fBssize_t\fR \fBreadv\fR(\fBint\fR \fIfildes\fR, \fBconst struct iovec *\fR\fIiov\fR, \fBint\fR \fIiovcnt\fR);
.fi
.LP
.nf
\fBssize_t\fR \fBpreadv\fR(\fBint\fR \fIfildes\fR, \fBconst struct iovec *\fR\fIiov\fR, \fBint\fR \fIiovcnt\fR, \fBoff_t\fR \fIoffset\fR);
.fi


.SH DESCRIPTION
.LP
The \fBread()\fR function attempts to read \fInbyte\fR bytes from the file
associated with the open file descriptor, \fIfildes\fR, into the buffer pointed
to by \fIbuf\fR.
.sp
.LP
If \fInbyte\fR is 0, \fBread()\fR returns \fB0\fR and has no other results.
.sp
.LP
On files that support seeking (for example, a regular file), the \fBread()\fR
starts at a position in the file given by the file offset associated with
\fIfildes\fR. The file offset is incremented by the number of bytes actually
read.
.sp
.LP
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.
.sp
.LP
If \fIfildes\fR refers to a socket, \fBread()\fR is equivalent to
\fBrecv\fR(3SOCKET) with no flags set.
.sp
.LP
No data transfer will occur past the current end-of-file.  If the starting
position is at or after the end-of-file, \fB0\fR will be returned. If the file
refers to a device special file, the result of subsequent \fBread()\fR requests
is implementation-dependent.
.sp
.LP
When attempting to read from a regular file with mandatory file/record locking
set (see \fBchmod\fR(2)), and there is a write lock owned by another process on
the segment of the file to be read:
.RS +4
.TP
.ie t \(bu
.el o
If \fBO_NDELAY\fR or \fBO_NONBLOCK\fR is set, \fBread()\fR returns \fB\(mi1\fR
and sets \fBerrno\fR to \fBEAGAIN\fR.
.RE
.RS +4
.TP
.ie t \(bu
.el o
If \fBO_NDELAY\fR and \fBO_NONBLOCK\fR are clear, \fBread()\fR sleeps until the
blocking record lock is removed.
.RE
.sp
.LP
When attempting to read from an empty pipe (or FIFO):
.RS +4
.TP
.ie t \(bu
.el o
If no process has the pipe open for writing, \fBread()\fR returns \fB0\fR to
indicate end-of-file.
.RE
.RS +4
.TP
.ie t \(bu
.el o
If some process has the pipe open for writing and \fBO_NDELAY\fR is set,
\fBread()\fR returns \fB0\fR.
.RE
.RS +4
.TP
.ie t \(bu
.el o
If some process has the pipe open for writing and \fBO_NONBLOCK\fR is set,
\fBread()\fR returns \fB\(mi1\fR and sets \fBerrno\fR to \fBEAGAIN\fR.
.RE
.RS +4
.TP
.ie t \(bu
.el o
If \fBO_NDELAY\fR and \fBO_NONBLOCK\fR are clear, \fBread()\fR blocks until
data is written to the pipe or the pipe is closed by all processes that had
opened the pipe for writing.
.RE
.sp
.LP
When attempting to read a file associated with a terminal that has no data
currently available:
.RS +4
.TP
.ie t \(bu
.el o
If \fBO_NDELAY\fR is set, \fBread()\fR returns \fB0\fR.
.RE
.RS +4
.TP
.ie t \(bu
.el o
If \fBO_NONBLOCK\fR is set, \fBread()\fR returns \fB\(mi1\fR and sets
\fBerrno\fR to \fBEAGAIN\fR.
.RE
.RS +4
.TP
.ie t \(bu
.el o
If \fBO_NDELAY\fR and \fBO_NONBLOCK\fR are clear, \fBread()\fR blocks until
data become available.
.RE
.sp
.LP
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:
.RS +4
.TP
.ie t \(bu
.el o
If \fBO_NDELAY\fR or \fBO_NONBLOCK\fR is set, \fBread()\fR returns \fB\(mi1\fR
and sets \fBerrno\fR to \fBEAGAIN\fR.
.RE
.RS +4
.TP
.ie t \(bu
.el o
If \fBO_NDELAY\fR and \fBO_NONBLOCK\fR are clear, \fBread()\fR blocks until
data becomes available.
.RE
.sp
.LP
The \fBread()\fR 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,
\fBread()\fR returns bytes with value 0.  For example, \fBlseek\fR(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.
.sp
.LP
For regular files, no data transfer will occur past the offset maximum
established in the open file description associated with \fIfildes\fR.
.sp
.LP
Upon successful completion, where \fInbyte\fR is greater than 0, \fBread()\fR
will mark for update the \fBst_atime\fR field of the file, and return the
number of bytes read. This number will never be greater than \fInbyte\fR. The
value returned may be less than \fInbyte\fR if the number of bytes left in the
file is less than \fInbyte\fR, if the \fBread()\fR request was interrupted by a
signal, or if the file is a pipe or FIFO or special file and has fewer than
\fInbyte\fR bytes immediately available for reading.  For example, a
\fBread()\fR from a file associated with a terminal may return one typed line
of data.
.sp
.LP
If a \fBread()\fR is interrupted by a signal before it reads any data, it will
return \fB\(mi1\fR with \fBerrno\fR set to \fBEINTR\fR.
.sp
.LP
If a \fBread()\fR is interrupted by a signal after it has successfully read
some data, it will return the number of bytes read.
.sp
.LP
A \fBread()\fR from a streams file can read data in three different modes:
byte-stream mode, message-nondiscard mode, and message-discard mode.  The
default is byte-stream mode.  This can be changed using the \fBI_SRDOPT\fR
\fBioctl\fR(2) request, and can be tested with the \fBI_GRDOPT\fR
\fBioctl()\fR. In byte-stream mode, \fBread()\fR 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 message boundaries.
.sp
.LP
In streams message-nondiscard mode, \fBread()\fR retrieves data until as many
bytes as were requested are transferred, or until a message boundary is
reached.  If \fBread()\fR does not retrieve all the data in a message, the
remaining data is left on the stream, and can be retrieved by the next
\fBread()\fR 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  \fBread()\fR returns is
discarded, and is not available for a subsequent \fBread()\fR, \fBreadv()\fR or
\fBgetmsg\fR(2) call.
.sp
.LP
How \fBread()\fR handles zero-byte streams messages is determined by the
current read mode setting.  In byte-stream mode, \fBread()\fR accepts data
until it has read \fInbyte\fR bytes, or until there is no more data to read, or
until a zero-byte message block is encountered. The \fBread()\fR function then
returns the number of bytes read, and places the zero-byte message back on the
stream to be retrieved by the next \fBread()\fR, \fBreadv()\fR or
\fBgetmsg\fR(2). In message-nondiscard mode or message-discard mode, a
zero-byte message returns \fB0\fR 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 \fB0\fR is returned, regardless of the read
mode.
.sp
.LP
A \fBread()\fR 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.
.sp
.LP
By default, streams are in control-normal mode, in which a \fBread()\fR from a
streams file can only process messages that contain a data part but do not
contain a control part.  The \fBread()\fR 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 \fBI_SRDOPT\fR \fBioctl()\fR command.  In control-data mode,
\fBread()\fR 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, \fBread()\fR discards message control parts but returns
to the process any data part in the message.
.sp
.LP
In addition, \fBread()\fR and \fBreadv()\fR will fail if the stream head had
processed an asynchronous error before the call.  In this case, the value of
\fBerrno\fR does not reflect the result of \fBread()\fR or \fBreadv()\fR but
reflects the prior error. If a hangup occurs on the stream being read,
\fBread()\fR continues to operate normally until the stream head read queue is
empty. Thereafter, it returns \fB0\fR.
.SS "\fBreadv()\fR"
.LP
The \fBreadv()\fR function is equivalent to \fBread()\fR, but places the input
data into the \fIiovcnt\fR buffers specified by the members of the \fIiov\fR
array: \fIiov\fR[\fB0\fR], \fIiov\fR[\fB1\fR], \&.\|.\|.,
\fIiov\fR[\fIiovcnt\fR\(mi1]. The \fIiovcnt\fR argument is valid if greater
than 0 and less than or equal to {\fBIOV_MAX\fR}.
.sp
.LP
The \fBiovec\fR structure contains the following members:
.sp
.in +2
.nf
caddr_t   iov_base;
int       iov_len;
.fi
.in -2

.sp
.LP
Each \fBiovec\fR entry specifies the base address and length of an area in
memory where data should be placed.  The \fBreadv()\fR function always fills an
area completely before proceeding to the next.
.sp
.LP
Upon successful completion, \fBreadv()\fR marks for update the \fBst_atime\fR
field of the file.
.SS "\fBpread()\fR"
.LP
The \fBpread()\fR function performs the same action as \fBread()\fR, except
that it reads from a given position in the file without changing the file
pointer. The first three arguments to \fBpread()\fR are the same as
\fBread()\fR with the addition of a fourth argument \fIoffset\fR for the
desired position inside the file. \fBpread()\fR will read up to the maximum
offset value that can be represented in an \fBoff_t\fR for regular files. An
attempt to perform a \fBpread()\fR on a file that is incapable of seeking
results in an error.
.SS "\fBpreadv()\fR"
.LP
The \fBpreadv()\fR function performs the same action as \fBreadv()\fR except it
reads from a given position in the file without changing the file pointer. The
first three arguments are the same as \fBreadv()\fR with the addition of a
fourth argument \fIoffset\fR for the desired position inside the file.
\fBpreadv()\fR will read up to the maximum offset value that can be represented
in an \fBoff_t\fR for regular files. An attempt to perform a \fBpreadv()\fR on a
file that is incapable of seeking results in an error.

.SH RETURN VALUES
.LP
Upon successful completion, \fBread()\fR and \fBreadv()\fR return a
non-negative integer indicating the number of bytes actually read. Otherwise,
the functions return \fB\(mi1\fR and set \fBerrno\fR to indicate the error.
.SH ERRORS
.LP
The \fBread()\fR, \fBreadv()\fR, \fBpread()\fR, and \fBpreadv()\fR functions will fail if:
.sp
.ne 2
.na
\fB\fBEAGAIN\fR\fR
.ad
.RS 11n
Mandatory file/record locking was set, \fBO_NDELAY\fR or \fBO_NONBLOCK\fR 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 \fBO_NONBLOCK\fR
was set; or no message is waiting to be read on a stream and \fBO_NDELAY\fR or
\fBO_NONBLOCK\fR was set.
.RE

.sp
.ne 2
.na
\fB\fBEBADF\fR\fR
.ad
.RS 11n
The \fIfildes\fR argument is not a valid file descriptor open for reading.
.RE

.sp
.ne 2
.na
\fB\fBEBADMSG\fR\fR
.ad
.RS 11n
Message waiting to be read on a stream is not a data message.
.RE

.sp
.ne 2
.na
\fB\fBECONNRESET\fR\fR
.ad
.RS 11n
The \fIfiledes\fR argument refers to a connection oriented socket and the
connection was forcibly closed by the peer and is no longer valid.  I/O can no
longer be performed to \fIfiledes\fR.
.RE

.sp
.ne 2
.na
\fB\fBEDEADLK\fR\fR
.ad
.RS 11n
The read was going to go to sleep and cause a deadlock to occur.
.RE

.sp
.ne 2
.na
\fB\fBEINTR\fR\fR
.ad
.RS 11n
A signal was caught during the read operation and no data was transferred.
.RE

.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 11n
An attempt was made to read from a stream linked to a multiplexor.
.RE

.sp
.ne 2
.na
\fB\fBEIO\fR\fR
.ad
.RS 11n
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 \fBSIGTTIN\fR signal or the process group
of the process is orphaned.
.RE

.sp
.ne 2
.na
\fB\fBEISDIR\fR\fR
.ad
.RS 11n
The \fIfildes\fR argument refers to a directory on a file system type that does
not support read operations on directories.
.RE

.sp
.ne 2
.na
\fB\fBENOLCK\fR\fR
.ad
.RS 11n
The system record lock table was full, so the \fBread()\fR or \fBreadv()\fR
could not go to sleep until the blocking record lock was removed.
.RE

.sp
.ne 2
.na
\fB\fBENOLINK\fR\fR
.ad
.RS 11n
The \fIfildes\fR argument is on a remote machine and the link to that machine
is no longer active.
.RE

.sp
.ne 2
.na
\fB\fBENXIO\fR\fR
.ad
.RS 11n
The device associated with \fIfildes\fR is a block special or character special
file and the value of the file pointer is out of range.
.RE

.sp
.LP
The \fBread()\fR and \fBpread()\fR functions will fail if:
.sp
.ne 2
.na
\fB\fBEFAULT\fR\fR
.ad
.RS 10n
The \fIbuf\fR argument points to an illegal address.
.RE

.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 10n
The \fInbyte\fR argument overflowed an \fBssize_t\fR.
.RE

.sp
.LP
The \fBread()\fR and \fBreadv()\fR functions will fail if:
.sp
.ne 2
.na
\fB\fBEOVERFLOW\fR\fR
.ad
.RS 13n
The file is a regular file, \fInbyte\fR 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 \fIfildes\fR.
.RE

.sp
.LP
The \fBreadv()\fR and \fBpreadv()\fR functions may fail if:
.sp
.ne 2
.na
\fB\fBEFAULT\fR\fR
.ad
.RS 10n
The \fIiov\fR argument points outside the allocated address space.
.RE

.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 10n
The \fIiovcnt\fR argument was less than or equal to \fB0\fR or greater than
{\fBIOV_MAX\fR}. See \fBIntro\fR(2) for a definition of {\fBIOV_MAX\fR}).
.sp
One of the \fBiov_len\fR values in the \fIiov\fR array was negative, or the sum
of the \fBiov_len\fR values in the \fIiov\fR array overflowed an \fBssize_t\fR.
.RE

.sp
.LP
The \fBpread()\fR and \fBpreadv()\fR functions will fail and the file pointer
remain unchanged if:
.sp
.ne 2
.na
\fB\fBESPIPE\fR\fR
.ad
.RS 10n
The \fIfildes\fR argument is associated with a pipe or FIFO.
.RE

.SH USAGE
.LP
The \fBpread()\fR function has a transitional interface for 64-bit file
offsets.  See \fBlf64\fR(5).
.SH ATTRIBUTES
.LP
See \fBattributes\fR(5) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE  ATTRIBUTE VALUE
_
Interface Stability     Committed
_
MT-Level        \fBread()\fR is Async-Signal-Safe
_
Standard        See \fBstandards\fR(5).
.TE

.SH SEE ALSO
.LP
\fBIntro\fR(2), \fBchmod\fR(2), \fBcreat\fR(2), \fBdup\fR(2), \fBfcntl\fR(2),
\fBgetmsg\fR(2), \fBioctl\fR(2), \fBlseek\fR(2), \fBopen\fR(2), \fBpipe\fR(2),
\fBrecv\fR(3SOCKET), \fBattributes\fR(5), \fBlf64\fR(5), \fBstandards\fR(5),
\fBstreamio\fR(7I), \fBtermio\fR(7I)