write — write to a file descriptor
#include <unistd.h>
ssize_t write( |
int | fd, |
const void * | buf, | |
size_t | count) ; |
write
() writes up to
count
bytes to the
file referenced by the file descriptor fd
from the buffer starting at
buf
. 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 systems
are POSIX conforming.
On success, the number of bytes written are 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.
Non-blocking I/O has been selected using
O_NONBLOCK
and the write
would block.
fd
is not a
valid file descriptor or is not open for writing.
buf
is
outside your accessible address space.
An attempt was made to write a file that exceeds the implementation-defined maximum file size or the process' file size limit, or to write at a position past the maximum allowed offset.
The call was interrupted by a signal before any data was written.
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.
A low-level I/O error occurred while modifying the inode.
The device containing the file referred to by
fd
has no room
for the data.
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
.
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.
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.
close(2), fcntl(2), fsync(2), ioctl(2), lseek(2), open(2), pwrite(2), read(2), select(2), writev(2), fwrite(3)
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