Close a file descriptor
#include <unistd.h> int close(int fd);
close() closes a file descriptor, so that it no longer refers to any file and may be reused. Any record locks (see fcntl(2)) held on the file it was associated with, and owned by the process, are removed (regardless of the file descriptor that was used to obtain the lock).
If fd is the last file descriptor referring to the underlying open file description (see open(2)), the resources associated with the open file description are freed; if the descriptor was the last reference to a file which has been removed using unlink(2), the file is deleted.
close() returns zero on success. On error, -1 is returned, and errno is set appropriately.
fd isn't a valid open file descriptor.
The close() call was interrupted by a signal; see signal(7).
An I/O error occurred.
SVr4, 4.3BSD, POSIX.1-2001.
Not checking the return value of close() is a common but nevertheless serious programming error. It is quite possible that errors on a previous write(2) operation are first reported at the final close(). Not checking the return value when closing the file may lead to silent loss of data. This can especially be observed with NFS and with disk quota. Note that the return value should only be used for diagnostics. In particular close() should not be retried after an EINTR since this may cause a reused descriptor from another thread to be closed.
A successful close does not guarantee that the data has been successfully saved to disk, as the kernel defers writes. It is not common for a filesystem to flush the buffers when the stream is closed. If you need to be sure that the data is physically stored, use fsync(2). (It will depend on the disk hardware at this point.)
It is probably unwise to close file descriptors while they may be in use by system calls in other threads in the same process. Since a file descriptor may be reused, there are some obscure race conditions that may cause unintended side effects.
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