gd_open_limit(3)

NAME

gd_open_limit—set a Dirfile's allowed number of open files

SYNOPSIS

#include <getdata.h>

long gd_open_limit(DIRFILE *dirfile, long limit);

DESCRIPTION

The gd_open_limit() function can be used to limit the number of RAW file descriptors which GetData simultaneously holds open in the open dirfile(5) database specified by dirfile. This function can also be used to report the current number of open files, or the current limit. When limiting is active, to stay below the limit, GetData will automatically close the least recently accessed RAW fields (as if gd_raw_close(3) were called), when needed.

The default value for the limit is zero, indicating that limiting is disabled. In this case, GetData will never automatically close open RAW fields. To enable limiting call this function with a positive value for limit. If limited, the minimum limit is two; if this function is passed a limit of one, it will behave as if the value 2 were passed instead.

When setting a limit, this function will immediately close enough RAW fields to remain below the specified limit.

The argument limit may alternately be one of the following special values:

GD_OLIMIT_NONE (=0): Disable limiting, if it was previously active.
GD_OLIMIT_CURRENT: Do nothing other than report the current limit. This is equivalent to calling this function with limit equal to the current limit.
GD_OLIMIT_COUNT: Instead of returning the current limit, return the current number filed descriptors for this dirfile. The current limit is not changed.

RETURN VALUE

If limit is GD_OLIMIT_COUNT, then the current number of file descriptors in use by this dirfile is returned, or zero if limiting is disabled (i.e. the limit is zero).

For other values of limit, if successful, this function returns the open field limit, after possibly modifying it. This limit is never negative. It will be zero if limiting is disabled.

If limit is one of the special values GD_OLIMIT_COUNT, GD_OLIMIT_CURRENT, or GD_OLIMIT_NONE (=0), then this function always succeeds. Otherwise, on error, gd_open_limit() returns a negative-valued error code. Possible error codes are:

GD_E_ALLOC: The library was unable to allocate memory.
GD_E_BAD_DIRFILE: The supplied dirfile was invalid.
GD_E_IO: An I/O error occurred while trying to write modified data to disk as a result in a reduction of the open field limit.

The error code is also stored in the DIRFILE object and may be retrieved after this function returns by calling gd_error(3). A descriptive error string for the error may be obtained by calling gd_error_string(3).

NOTES

The file descriptors tracked by this functionality are only those associated with RAW fields which are held open across function calls. During the course of any library call, GetData may temporarily exceed a file descriptor limit set with this function without triggering an automatic close. Some headroom is always a good idea.

Furthermore, this limit is specific to the specified dirfile. It is not a global limit for the whole GetData library. If a process has multiple DIRFILEs open, each of them has its own limit (or is unlimited), even though they all draw from the same limited pool of file descriptors.

In some cases, usually when writing to a compressed RAW field, two file descriptors may be held open for a single field.

HISTORY

The gd_open_limit() function appeared in GetData-0.11.0.