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read(2) [redhat man page]

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

NAME
read - read from a file descriptor SYNOPSIS
#include <unistd.h> ssize_t read(int fd, void *buf, size_t count); DESCRIPTION
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. RETURN VALUE
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 avail- able 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. ERRORS
EINTR The call was interrupted by a signal before any data was read. EAGAIN Non-blocking I/O has been selected using O_NONBLOCK and no data was immediately available for reading. 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. EBADF fd is not a valid file descriptor or is not open for reading. EINVAL fd is attached to an object which is unsuitable for reading. EFAULT buf is outside your accessible address space. 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. CONFORMING TO
SVr4, SVID, AT&T, POSIX, X/OPEN, BSD 4.3 RESTRICTIONS
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 satis- fied 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. Many filesystems and disks were considered to be fast enough that the implementation of O_NONBLOCK was deemed unneccesary. So, O_NONBLOCK may not be available on files and/or disks. SEE ALSO
close(2), fcntl(2), ioctl(2), lseek(2), readdir(2), readlink(2), select(2), write(2), fread(3), readv(3) Linux 2.0.32 1997-07-12 READ(2)

Check Out this Related Man Page

WRITE(2)						     Linux Programmer's Manual							  WRITE(2)

NAME
write - write to a file descriptor SYNOPSIS
#include <unistd.h> ssize_t write(int fd, const void *buf, size_t count); DESCRIPTION
write() writes up to count bytes from the buffer pointed buf to the file referred to by the file descriptor fd. The number of bytes written may be less than count if, for example, there is insufficient space on the underlying physical medium, or the RLIMIT_FSIZE resource limit is encountered (see setrlimit(2)), or the call was interrupted by a signal handler after having written less than count bytes. (See also pipe(7).) For a seekable file (i.e., one to which lseek(2) may be applied, for example, a regular file) writing takes place at the current file off- set, and the file offset is incremented by the number of bytes actually written. If the file was open(2)ed with O_APPEND, the file offset is first set to the end of the file before writing. The adjustment of the file offset and the write operation are performed as an atomic step. POSIX requires that a read(2) which can be proved to occur after a write() has returned returns the new data. Note that not all file sys- tems are POSIX conforming. RETURN VALUE
On success, the number of bytes written is returned (zero indicates nothing was written). On error, -1 is returned, and errno is set appropriately. If count is zero and fd refers to a regular file, then write() may return a failure status if one of the errors below is detected. If no errors are detected, 0 will be returned without causing any other effect. If count is zero and fd refers to a file other than a regular file, the results are not specified. ERRORS
EAGAIN The file descriptor fd refers to a file other than a socket and has been marked nonblocking (O_NONBLOCK), and the write would block. EAGAIN or EWOULDBLOCK The file descriptor fd refers to a socket and has been marked nonblocking (O_NONBLOCK), and the write would block. POSIX.1-2001 allows either error to be returned for this case, and does not require these constants to have the same value, so a portable appli- cation should check for both possibilities. EBADF fd is not a valid file descriptor or is not open for writing. EFAULT buf is outside your accessible address space. EFBIG An attempt was made to write a file that exceeds the implementation-defined maximum file size or the process's file size limit, or to write at a position past the maximum allowed offset. EINTR The call was interrupted by a signal before any data was written; see signal(7). EINVAL fd is attached to an object which is unsuitable for writing; 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 A low-level I/O error occurred while modifying the inode. ENOSPC The device containing the file referred to by fd has no room for the data. EPIPE fd is connected to a pipe or socket whose reading end is closed. When this happens the writing process will also receive a SIGPIPE signal. (Thus, the write return value is seen only if the program catches, blocks or ignores this signal.) Other errors may occur, depending on the object connected to fd. CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001. Under SVr4 a write may be interrupted and return EINTR at any point, not just before any data is written. NOTES
A successful return from write() does not make any guarantee that data has been committed to disk. In fact, on some buggy implementations, it does not even guarantee that space has successfully been reserved for the data. The only way to be sure is to call fsync(2) after you are done writing all your data. If a write() is interrupted by a signal handler before any bytes are written, then the call fails with the error EINTR; if it is inter- rupted after at least one byte has been written, the call succeeds, and returns the number of bytes written. SEE ALSO
close(2), fcntl(2), fsync(2), ioctl(2), lseek(2), open(2), pwrite(2), read(2), select(2), writev(2), fwrite(3) COLOPHON
This page is part of release 3.25 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. Linux 2009-02-23 WRITE(2)

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