1read(2) System Calls read(2)
2
6 read, readv, pread - read from file
7
9 #include <unistd.h>
10 ssize_t read(int fildes, void *buf, size_t nbyte);
12 ssize_t pread(int fildes, void *buf, size_t nbyte, off_t offset);
15 #include <sys/uio.h>
18 ssize_t readv(int fildes, const struct iovec *iov, int iovcnt);
20
23 The read() function attempts to read nbyte bytes from the file associ‐
24 ated with the open file descriptor, fildes, into the buffer pointed to
25 by buf.
26 If nbyte is 0, read() returns 0 and has no other results.
29 On files that support seeking (for example, a regular file), the read()
32 starts at a position in the file given by the file offset associated
33 with fildes. The file offset is incremented by the number of bytes
34 actually read.
35 Files that do not support seeking (for example, terminals) always read
38 from the current position. The value of a file offset associated with
39 such a file is undefined.
40 If fildes refers to a socket, read() is equivalent to recv(3SOCKET)
43 with no flags set.
44 No data transfer will occur past the current end-of-file. If the
47 starting position is at or after the end-of-file, 0 will be returned.
48 If the file refers to a device special file, the result of subsequent
49 read() requests is implementation-dependent.
50 When attempting to read from a regular file with mandatory file/record
53 locking set (see chmod(2)), and there is a write lock owned by another
54 process on the segment of the file to be read:
55 o If O_NDELAY or O_NONBLOCK is set, read() returns −1 and sets
57 errno to EAGAIN.
58 o If O_NDELAY and O_NONBLOCK are clear, read() sleeps until
60 the blocking record lock is removed.
61 When attempting to read from an empty pipe (or FIFO):
64 o If no process has the pipe open for writing, read() returns
66 0 to indicate end-of-file.
67 o If some process has the pipe open for writing and O_NDELAY
69 is set, read() returns 0.
70 o If some process has the pipe open for writing and O_NONBLOCK
72 is set, read() returns −1 and sets errno to EAGAIN.
73 o If O_NDELAY and O_NONBLOCK are clear, read() blocks until
75 data is written to the pipe or the pipe is closed by all
76 processes that had opened the pipe for writing.
77 When attempting to read a file associated with a terminal that has no
80 data currently available:
81 o If O_NDELAY is set, read() returns 0.
83 o If O_NONBLOCK is set, read() returns −1 and sets errno to
85 EAGAIN.
86 o If O_NDELAY and O_NONBLOCK are clear, read() blocks until
88 data become available.
89 When attempting to read a file associated with a socket or a stream
92 that is not a pipe, a FIFO, or a terminal, and the file has no data
93 currently available:
94 o If O_NDELAY or O_NONBLOCK is set, read() returns −1 and sets
96 errno to EAGAIN.
97 o If O_NDELAY and O_NONBLOCK are clear, read() blocks until
99 data becomes available.
100 The read() function reads data previously written to a file. If any
103 portion of a regular file prior to the end-of-file has not been writ‐
104 ten, read() returns bytes with value 0. For example, lseek(2) allows
105 the file offset to be set beyond the end of existing data in the file.
106 If data is later written at this point, subsequent reads in the gap
107 between the previous end of data and the newly written data will return
108 bytes with value 0 until data is written into the gap.
109 For regular files, no data transfer will occur past the offset maximum
112 established in the open file description associated with fildes.
113 Upon successful completion, where nbyte is greater than 0, read() will
116 mark for update the st_atime field of the file, and return the number
117 of bytes read. This number will never be greater than nbyte. The value
118 returned may be less than nbyte if the number of bytes left in the file
119 is less than nbyte, if the read() request was interrupted by a signal,
120 or if the file is a pipe or FIFO or special file and has fewer than
121 nbyte bytes immediately available for reading. For example, a read()
122 from a file associated with a terminal may return one typed line of
123 data.
124 If a read() is interrupted by a signal before it reads any data, it
127 will return −1 with errno set to EINTR.
128 If a read() is interrupted by a signal after it has successfully read
131 some data, it will return the number of bytes read.
132 A read() from a streams file can read data in three different modes:
135 byte-stream mode, message-nondiscard mode, and message-discard mode.
136 The default is byte-stream mode. This can be changed using the
137 I_SRDOPT ioctl(2) request, and can be tested with the I_GRDOPT ioctl().
138 In byte-stream mode, read() retrieves data from the stream until as
139 many bytes as were requested are transferred, or until there is no more
140 data to be retrieved. Byte-stream mode ignores message boundaries.
141 In streams message-nondiscard mode, read() retrieves data until as many
144 bytes as were requested are transferred, or until a message boundary is
145 reached. If read() does not retrieve all the data in a message, the
146 remaining data is left on the stream, and can be retrieved by the next
147 read() call. Message-discard mode also retrieves data until as many
148 bytes as were requested are transferred, or a message boundary is
149 reached. However, unread data remaining in a message after the read()
150 returns is discarded, and is not available for a subsequent read(),
151 readv() or getmsg(2) call.
152 How read() handles zero-byte streams messages is determined by the cur‐
155 rent read mode setting. In byte-stream mode, read() accepts data until
156 it has read nbyte bytes, or until there is no more data to read, or
157 until a zero-byte message block is encountered. The read() function
158 then returns the number of bytes read, and places the zero-byte message
159 back on the stream to be retrieved by the next read(), readv() or
160 getmsg(2). In message-nondiscard mode or message-discard mode, a zero-
161 byte message returns 0 and the message is removed from the stream.
162 When a zero-byte message is read as the first message on a stream, the
163 message is removed from the stream and 0 is returned, regardless of the
164 read mode.
165 A read() from a streams file returns the data in the message at the
168 front of the stream head read queue, regardless of the priority band of
169 the message.
170 By default, streams are in control-normal mode, in which a read() from
173 a streams file can only process messages that contain a data part but
174 do not contain a control part. The read() fails if a message contain‐
175 ing a control part is encountered at the stream head. This default
176 action can be changed by placing the stream in either control-data mode
177 or control-discard mode with the I_SRDOPT ioctl() command. In control-
178 data mode, read() converts any control part to data and passes it to
179 the application before passing any data part originally present in the
180 same message. In control-discard mode, read() discards message control
181 parts but returns to the process any data part in the message.
182 In addition, read() and readv() will fail if the stream head had pro‐
185 cessed an asynchronous error before the call. In this case, the value
186 of errno does not reflect the result of read() or readv() but reflects
187 the prior error. If a hangup occurs on the stream being read, read()
188 continues to operate normally until the stream head read queue is
189 empty. Thereafter, it returns 0.
190 readv()
192 The readv() function is equivalent to read(), but places the input data
193 into the iovcnt buffers specified by the members of the iov array:
194 iov[0], iov[1], ..., iov[iovcnt−1]. The iovcnt argument is valid if
195 greater than 0 and less than or equal to {IOV_MAX}.
196 The iovec structure contains the following members:
199 caddr_t iov_base;
201 int iov_len;
202 Each iovec entry specifies the base address and length of an area in
206 memory where data should be placed. The readv() function always fills
207 an area completely before proceeding to the next.
208 Upon successful completion, readv() marks for update the st_atime field
211 of the file.
212 pread()
214 The pread() function performs the same action as read(), except that it
215 reads from a given position in the file without changing the file
216 pointer. The first three arguments to pread() are the same as read()
217 with the addition of a fourth argument offset for the desired position
218 inside the file. pread() will read up to the maximum offset value that
219 can be represented in an off_t for regular files. An attempt to perform
220 a pread() on a file that is incapable of seeking results in an error.
221
223 Upon successful completion, read() and readv() return a non-negative
224 integer indicating the number of bytes actually read. Otherwise, the
225 functions return −1 and set errno to indicate the error.
226
228 The read(), readv(), and pread() functions will fail if:
229 EAGAIN Mandatory file/record locking was set, O_NDELAY or O_NON‐
231 BLOCK was set, and there was a blocking record lock; total
232 amount of system memory available when reading using raw I/O
233 is temporarily insufficient; no data is waiting to be read
234 on a file associated with a tty device and O_NONBLOCK was
235 set; or no message is waiting to be read on a stream and
236 O_NDELAY or O_NONBLOCK was set.
237 EBADF The fildes argument is not a valid file descriptor open for
240 reading.
241 EBADMSG Message waiting to be read on a stream is not a data mes‐
244 sage.
245 EDEADLK The read was going to go to sleep and cause a deadlock to
248 occur.
249 EINTR A signal was caught during the read operation and no data
252 was transferred.
253 EINVAL An attempt was made to read from a stream linked to a multi‐
256 plexor.
257 EIO A physical I/O error has occurred, or the process is in a
260 background process group and is attempting to read from its
261 controlling terminal, and either the process is ignoring or
262 blocking the SIGTTIN signal or the process group of the
263 process is orphaned.
264 EISDIR The fildes argument refers to a directory on a file system
267 type that does not support read operations on directories.
268 ENOLCK The system record lock table was full, so the read() or
271 readv() could not go to sleep until the blocking record lock
272 was removed.
273 ENOLINK The fildes argument is on a remote machine and the link to
276 that machine is no longer active.
277 ENXIO The device associated with fildes is a block special or
280 character special file and the value of the file pointer is
281 out of range.
282 The read() and pread() functions will fail if:
286 EFAULT The buf argument points to an illegal address.
288 EINVAL The nbyte argument overflowed an ssize_t.
291 The read() and readv() functions will fail if:
295 EOVERFLOW The file is a regular file, nbyte is greater than 0, the
297 starting position is before the end-of-file, and the
298 starting position is greater than or equal to the offset
299 maximum established in the open file description associ‐
300 ated with fildes.
301 The readv() function may fail if:
305 EFAULT The iov argument points outside the allocated address space.
307 EINVAL The iovcnt argument was less than or equal to 0 or greater
310 than {IOV_MAX}. See Intro(2) for a definition of {IOV_MAX}).
311 One of the iov_len values in the iov array was negative, or
313 the sum of the iov_len values in the iov array overflowed an
314 ssize_t.
315 The pread() function will fail and the file pointer remain unchanged
319 if:
320 ESPIPE The fildes argument is associated with a pipe or FIFO.
322
325 The pread() function has a transitional interface for 64-bit file off‐
326 sets. See lf64(5).
327
329 See attributes(5) for descriptions of the following attributes:
330 ┌─────────────────────────────┬─────────────────────────────┐
335 │ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
336 ├─────────────────────────────┼─────────────────────────────┤
337 │Interface Stability │Committed │
338 ├─────────────────────────────┼─────────────────────────────┤
339 │MT-Level │read() is Async-Signal-Safe │
340 ├─────────────────────────────┼─────────────────────────────┤
341 │Standard │See standards(5). │
342 └─────────────────────────────┴─────────────────────────────┘
343
345 Intro(2), chmod(2), creat(2), dup(2), fcntl(2), getmsg(2), ioctl(2),
346 lseek(2), open(2), pipe(2), recv(3SOCKET), attributes(5), lf64(5),
347 standards(5), streamio(7I), termio(7I)
348SunOS 5.11 13 Sep 2007 read(2)