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);
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14 read() attempts to read up to count bytes from file descriptor fd into
15 the buffer starting at buf.
16 On files that support seeking, the read operation commences at the cur‐
18 rent file offset, and the file offset is incremented by the number of
19 bytes read. If the current file offset is at or past the end of file,
20 no bytes are read, and read() returns zero.
21 If count is zero, read() may detect the errors described below. In the
23 absence of any errors, or if read() does not check for errors, a read()
24 with a count of 0 returns zero and has no other effects.
25 If count is greater than SSIZE_MAX, the result is unspecified.
27
29 On success, the number of bytes read is returned (zero indicates end of
30 file), and the file position is advanced by this number. It is not an
31 error if this number is smaller than the number of bytes requested;
32 this may happen for example because fewer bytes are actually available
33 right now (maybe because we were close to end-of-file, or because we
34 are reading from a pipe, or from a terminal), or because read() was
35 interrupted by a signal. On error, -1 is returned, and errno is set
36 appropriately. In this case it is left unspecified whether the file
37 position (if any) changes.
38
40 EAGAIN The file descriptor fd refers to a file other than a socket and
41 has been marked nonblocking (O_NONBLOCK), and the read would
42 block.
43 EAGAIN or EWOULDBLOCK
45 The file descriptor fd refers to a socket and has been marked
46 nonblocking (O_NONBLOCK), and the read would block.
47 POSIX.1-2001 allows either error to be returned for this case,
48 and does not require these constants to have the same value, so
49 a portable application should check for both possibilities.
50 EBADF fd is not a valid file descriptor or is not open for reading.
52 EFAULT buf is outside your accessible address space.
54 EINTR The call was interrupted by a signal before any data was read;
56 see signal(7).
57 EINVAL fd is attached to an object which is unsuitable for reading; or
59 the file was opened with the O_DIRECT flag, and either the
60 address specified in buf, the value specified in count, or the
61 current file offset is not suitably aligned.
62 EINVAL fd was created via a call to timerfd_create(2) and the wrong
64 size buffer was given to read(); see timerfd_create(2) for fur‐
65 ther information.
66 EIO I/O error. This will happen for example when the process is in
68 a background process group, tries to read from its controlling
69 terminal, and either it is ignoring or blocking SIGTTIN or its
70 process group is orphaned. It may also occur when there is a
71 low-level I/O error while reading from a disk or tape.
72 EISDIR fd refers to a directory.
74 Other errors may occur, depending on the object connected to fd. POSIX
76 allows a read() that is interrupted after reading some data to return
77 -1 (with errno set to EINTR) or to return the number of bytes already
78 read.
79
81 SVr4, 4.3BSD, POSIX.1-2001.
82
84 On NFS file systems, reading small amounts of data will update the
85 timestamp only the first time, subsequent calls may not do so. This is
86 caused by client side attribute caching, because most if not all NFS
87 clients leave st_atime (last file access time) updates to the server
88 and client side reads satisfied from the client's cache will not cause
89 st_atime updates on the server as there are no server side reads. UNIX
90 semantics can be obtained by disabling client side attribute caching,
91 but in most situations this will substantially increase server load and
92 decrease performance.
93
95 close(2), fcntl(2), ioctl(2), lseek(2), open(2), pread(2), readdir(2),
96 readlink(2), readv(2), select(2), write(2), fread(3)
97
99 This page is part of release 3.53 of the Linux man-pages project. A
100 description of the project, and information about reporting bugs, can
101 be found at http://www.kernel.org/doc/man-pages/.
102Linux 2013-02-12 READ(2)