Penguin
Annotated edit history of read(2) version 2, including all changes. View license author blame.
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1 perry 1 !!NAME
2 PerryLorier 2 read - read from a file descriptor
1 perry 3
4 !!SYNOPSIS
2 PerryLorier 5 __#include <unistd.h>__
6 __ssize_t read(int __ ''fd''__, void *__''buf''__, size_t__ ''count''__);__
1 perry 7
8 !!DESCRIPTION
2 PerryLorier 9 __read()__ attempts to read up to ''count'' bytes from file descriptor ''fd'' into the buffer starting at ''buf''.
1 perry 10
2 PerryLorier 11 If ''count'' is zero, __read()__ returns zero and has no other results. If ''count'' is greater than SSIZE_MAX, the result is unspecified.
1 perry 12
13 !!RETURN VALUE
2 PerryLorier 14 On success, the number of bytes read is returned (zero indicates end of file), and the file position is advanced by this number. It is not an error if this number is smaller than the number of bytes requested; this may happen for example because fewer bytes are actually available right now (maybe because we were close to end-of-file, or because we are reading from a pipe, or from a terminal), or because read(2) was interrupted by a signal. On error, -1 is returned, and ''errno'' is set appropriately. In this case it is left unspecified whether the file position (if any) changes.
1 perry 15 !!ERRORS
2 PerryLorier 16 ;[EINTR]: The call was interrupted by a signal before any data was read.
17 ;[EAGAIN]: Non-blocking I/O has been selected using __O_NONBLOCK__ and no data was immediately available for reading.
18 ;[EIO]: I/O error. This will happen for example when the process is in a background process group, tries to read from its controlling tty, and either it is ignoring or blocking [SIGTTIN] or its process group is orphaned. It may also occur when there is a low-level I/O error while reading from a disk or tape.
19 ;[EISDIR]: ''fd'' refers to a directory.
20 ;[EBADF]: ''fd'' is not a valid file descriptor or is not open for reading.
21 ;[EINVAL]: ''fd'' is attached to an object which is unsuitable for reading.
22 ;[EFAULT]: ''buf'' is outside your accessible address space.
1 perry 23
2 PerryLorier 24 Other errors may occur, depending on the object connected to ''fd''. [POSIX] allows a __read__ that is interrupted after reading some data to return -1 (with ''errno'' set to [EINTR]) or to return the number of bytes already read.
1 perry 25
26 !!CONFORMING TO
2 PerryLorier 27 SVr4, SVID, AT
1 perry 28
29 !!RESTRICTIONS
2 PerryLorier 30 On NFS file systems, reading small amounts of data will only update the time stamp the first time, subsequent calls may not do so. This is caused by client side attribute caching, because most if not all NFS clients leave atime updates to the server and client side reads satisfied from the client's cache will not cause atime updates on the server as there are no server side reads. UNIX semantics can be obtained by disabling client side attribute caching, but in most situations this will substantially increase server load and decrease performance.
1 perry 31
32 !!SEE ALSO
2 PerryLorier 33 close(2), fcntl(2), ioctl(2), lseek(2), readdir(2), readlink(2), select(2), write(2), fread(3)
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