1READ(2) Linux Programmer's Manual READ(2)
2
6 read - read from a file descriptor
7
9 #include <unistd.h>
10 ssize_t read(int fd, void *buf, size_t count);
12
14 read() attempts to read up to count bytes from file descriptor fd into
15 the buffer starting at buf.
16 If count is zero, read() returns zero and has no other results. If
18 count is greater than SSIZE_MAX, the result is unspecified.
19
21 On success, the number of bytes read is returned (zero indicates end of
22 file), and the file position is advanced by this number. It is not an
23 error if this number is smaller than the number of bytes requested;
24 this may happen for example because fewer bytes are actually available
25 right now (maybe because we were close to end-of-file, or because we
26 are reading from a pipe, or from a terminal), or because read() was
27 interrupted by a signal. On error, -1 is returned, and errno is set
28 appropriately. In this case it is left unspecified whether the file
29 position (if any) changes.
30
32 EAGAIN The file descriptor fd refers to a file other than a socket and
33 has been marked nonblocking (O_NONBLOCK), and the read would
34 block.
35 EAGAIN or EWOULDBLOCK
37 The file descriptor fd refers to a socket and has been marked
38 nonblocking (O_NONBLOCK), and the read would block.
39 POSIX.1-2001 allows either error to be returned for this case,
40 and does not require these constants to have the same value, so
41 a portable application should check for both possibilities.
42 EBADF fd is not a valid file descriptor or is not open for reading.
44 EFAULT buf is outside your accessible address space.
46 EINTR The call was interrupted by a signal before any data was read;
48 see signal(7).
49 EINVAL fd is attached to an object which is unsuitable for reading; or
51 the file was opened with the O_DIRECT flag, and either the
52 address specified in buf, the value specified in count, or the
53 current file offset is not suitably aligned.
54 EINVAL fd was created via a call to timerfd_create(2) and the wrong
56 size buffer was given to read(); see timerfd_create(2) for fur‐
57 ther information.
58 EIO I/O error. This will happen for example when the process is in
60 a background process group, tries to read from its controlling
61 tty, and either it is ignoring or blocking SIGTTIN or its
62 process group is orphaned. It may also occur when there is a
63 low-level I/O error while reading from a disk or tape.
64 EISDIR fd refers to a directory.
66 Other errors may occur, depending on the object connected to fd. POSIX
68 allows a read() that is interrupted after reading some data to return
69 -1 (with errno set to EINTR) or to return the number of bytes already
70 read.
71
73 SVr4, 4.3BSD, POSIX.1-2001.
74
76 On NFS file systems, reading small amounts of data will only update the
77 timestamp the first time, subsequent calls may not do so. This is
78 caused by client side attribute caching, because most if not all NFS
79 clients leave st_atime (last file access time) updates to the server
80 and client side reads satisfied from the client's cache will not cause
81 st_atime updates on the server as there are no server side reads. Unix
82 semantics can be obtained by disabling client side attribute caching,
83 but in most situations this will substantially increase server load and
84 decrease performance.
85 Many file systems and disks were considered to be fast enough that the
87 implementation of O_NONBLOCK was deemed unnecessary. So, O_NONBLOCK
88 may not be available on files and/or disks.
89
91 close(2), fcntl(2), ioctl(2), lseek(2), open(2), pread(2), readdir(2),
92 readlink(2), readv(2), select(2), write(2), fread(3)
93
95 This page is part of release 3.25 of the Linux man-pages project. A
96 description of the project, and information about reporting bugs, can
97 be found at http://www.kernel.org/doc/man-pages/.
98Linux 2009-02-23 READ(2)