read() - Unix, Linux System Call
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read() - Unix, Linux System Call

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read - read from a file descriptor


#include <unistd.h> 

ssize_t read(int fd, void *buf, size_t count);


read() attempts to read up to count bytes from file descriptor fd into the buffer starting at buf.

If count is zero, read() returns zero and has no other results. If count is greater than SSIZE_MAX, the result is unspecified.


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() 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.


EAGAIN Non-blocking I/O has been selected using O_NONBLOCK and no data was immediately available for reading.
EBADF fd is not a valid file descriptor or is not open for reading.
EFAULT buf is outside your accessible address space.
EINTR The call was interrupted by a signal before any data was read.
EINVAL fd is attached to an object which is unsuitable for reading; or the file was opened with the O_DIRECT flag, and either the address specified in buf, the value specified in count, or the current file offset is not suitably aligned.
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.
EISDIR fd refers to a directory.
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.


SVr4, 4.3BSD, POSIX.1-2001.


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 st_atime (last file access time) updates to the server and client side reads satisfied from the client’s cache will not cause st_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.

Many filesystems and disks were considered to be fast enough that the implementation of O_NONBLOCK was deemed unnecessary. So, O_NONBLOCK may not be available on files and/or disks.


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