c++ problem setting "const char *str" from function returning a string

ocularb0b · 03-04-2008, 07:43 AM

i hope my post title is not totally stupid...

so my program needs to read a main config file that will specify the name of another config file.
i have done this:

Code:

const char *sysConfig = "main.conf";

int read_sys_config()
{
	cfg_opt_t sys_opts[] =
	{
		CFG_STR("mechConf", "defconfig.conf", CFGF_NONE),
		CFG_END()
	};
	cfg_t *sys_cfg;
	sys_cfg = cfg_init(sys_opts, CFGF_NONE);
	if(cfg_parse(sys_cfg, sysConfig) == CFG_PARSE_ERROR)
		return 1;
	const char *mechConfig = cfg_getstr(sys_cfg, "mechConf");
	cfg_free(sys_cfg);
	cfg_opt_t mech_opts[] =
	{
		CFG_INT("thing", 0, CFGF_NONE),       
		CFG_END()
	};
	cfg_t *mech_cfg;
	mech_cfg = cfg_init(mech_opts, CFGF_NONE);
	if(cfg_parse(mech_cfg, mechConfig) == CFG_PARSE_ERROR)
		return 1;
	thing = cfg_getint(mech_cfg, "thing");
	cfg_free(mech_cfg);
	return 0;
}

and the config file has:

Code:

mechConf = "other.conf"

i get no compile errors and other integer values from main.conf(not shown in posted code) get set correctly but it doesn't seem to put the string from main.conf into *mechConfig.
what am i missing?
thanks in advance

osor · 03-04-2008, 01:10 PM

Does cfg_getstr allocate its own memory? I think the problem lies in that function.

ocularb0b · 03-04-2008, 01:23 PM

i have no idea or how to find out exactly. it is part of libconfuse which i have used for a while and seems reasonably solid.
for what its worth this works fine.

printf("mechConf = %s\n", cfg_getstr(sys_cfg, "mechConf"));

osor · 03-04-2008, 09:36 PM

Well, I think I see the problem now (and it does have to do with memory management). All char arrays for the first config file are allocated in “sys_cfg = cfg_init(sys_opts, CFGF_NONE)” and “cfg_parse(sys_cfg, sysConfig)” (and the call to cfg_getstr(sys_cfg, "mechConf") gets you a pointer to the interally-allocated string). All this data will be deallocated by the call to cfg_free(sys_cfg). But you try to use mechConfig after this call, in which case you have a pointer to freed data.

The solution would be to move down the call to cfg_free(sys_cfg) to a line after you are done with using mechConfig.

E.g.,

Code:

const char *sysConfig = "main.conf";

int read_sys_config()
{
	cfg_opt_t sys_opts[] =
	{
		CFG_STR("mechConf", "defconfig.conf", CFGF_NONE),
		CFG_END()
	};
	cfg_t *sys_cfg;
	sys_cfg = cfg_init(sys_opts, CFGF_NONE);
	if(cfg_parse(sys_cfg, sysConfig) == CFG_PARSE_ERROR)
		return 1;
	const char *mechConfig = cfg_getstr(sys_cfg, "mechConf");
	//cfg_free(sys_cfg);
	cfg_opt_t mech_opts[] =
	{
		CFG_INT("thing", 0, CFGF_NONE),       
		CFG_END()
	};
	cfg_t *mech_cfg;
	mech_cfg = cfg_init(mech_opts, CFGF_NONE);
	if(cfg_parse(mech_cfg, mechConfig) == CFG_PARSE_ERROR)
		return 1;
	thing = cfg_getint(mech_cfg, "thing");
	cfg_free(mech_cfg);
	cfg_free(sys_cfg);
	return 0;
}

See if that works.