read(2) - f7

1READ(2)                    Linux Programmer's Manual                   READ(2)
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NAME

6       read - read from a file descriptor
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SYNOPSIS

9       #include <unistd.h>
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11       ssize_t read(int fd, void *buf, size_t count);
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DESCRIPTION

14       read()  attempts to read up to count bytes from file descriptor fd into
15       the buffer starting at buf.
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17       If count is zero, read() returns zero and has  no  other  results.   If
18       count is greater than SSIZE_MAX, the result is unspecified.
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RETURN VALUE

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.
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ERRORS

32       EAGAIN Non-blocking  I/O has been selected using O_NONBLOCK and no data
33              was immediately available for reading.
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35       EBADF  fd is not a valid file descriptor or is not open for reading.
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37       EFAULT buf is outside your accessible address space.
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39       EINTR  The call was interrupted by a signal before any data was read.
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41       EINVAL fd is attached to an object which is unsuitable for reading;  or
42              the  file  was  opened  with  the  O_DIRECT flag, and either the
43              address specified in buf, the value specified in count,  or  the
44              current file offset is not suitably aligned.
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46       EIO    I/O error. This will happen for example when the process is in a
47              background process group, tries to  read  from  its  controlling
48              tty,  and  either  it  is  ignoring  or  blocking SIGTTIN or its
49              process group is orphaned.  It may also occur when  there  is  a
50              low-level I/O error while reading from a disk or tape.
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52       EISDIR fd refers to a directory.
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54       Other errors may occur, depending on the object connected to fd.  POSIX
55       allows a read() that is interrupted after reading some data  to  return
56       -1  (with  errno set to EINTR) or to return the number of bytes already
57       read.
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CONFORMING TO

60       SVr4, 4.3BSD, POSIX.1-2001.
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RESTRICTIONS

63       On NFS file systems, reading small amounts of data will only update the
64       time  stamp  the  first  time, subsequent calls may not do so.  This is
65       caused by client side attribute caching, because most if  not  all  NFS
66       clients  leave  st_atime  (last file access time) updates to the server
67       and client side reads satisfied from the client's cache will not  cause
68       st_atime updates on the server as there are no server side reads.  UNIX
69       semantics can be obtained by disabling client side  attribute  caching,
70       but in most situations this will substantially increase server load and
71       decrease performance.
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73       Many filesystems and disks were considered to be fast enough  that  the
74       implementation of O_NONBLOCK was deemed unnecessary. So, O_NONBLOCK may
75       not be available on files and/or disks.
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