I have two cooperating programs. One writes to a file and one reads from the file. The file writes will be very infrequent. The reads will be more frequent.
The file has json data in it and I am using jsoncpp library to parse the file. It takes a stream and everything is working fine:
It parses the file just fine into wu_access. But on the rare chance that the reader and the writer need to access the file at the same time I want to have some sort of locking. Seems that Linux's flock() is a reasonable choice for two cooperating processes.
The flock command needs the OS file descriptor being used. It doesn't take a stream or a file path.
My problem is I can't seem to get the OS file descriptor that the ifstream uses. I know it is in there somewehre as you can't make a read or write call to any file without it.
I have seen "int fd = wu_init_config.rdbuf()->fd();" recommended but I get an error that fd() isn't defined. I also saw this "int fd = fileno(wu_init_config.rdbuf()->file())". That complained about file() not defined.
I guess it's an issue of ifstream wanting to be standard across various operating systems and the Linux file descriptor concept doesn't apply to other OS's. So, apparently it is being hidden in order to be more standard across multiple OS's.
How can I use flock() on an open ifstream() or ofstream() ?
Thanks
Chris
The file has json data in it and I am using jsoncpp library to parse the file. It takes a stream and everything is working fine:
Code:
const string weather_station_config = "<path to configuration_file>" Json::Reader wu_config; Json::Value wu_access; ifstream wu_config_file(weather_station_config); if (wu_config_file.is_open() == false) { sd_journal_print(LOG_ERR, "Failed to open the config file."); exit(1); } if (wu_config.parse(wu_config_file, wu_access) == false) { sd_journal_print(LOG_ERR, "Failed to parse config file."); exit(1); } wu_config_file.close();The flock command needs the OS file descriptor being used. It doesn't take a stream or a file path.
Code:
int flock(int fd, int op);DESCRIPTION Apply or remove an advisory lock on the open file specified by fd. The argument op is one of the following: LOCK_SH Place a shared lock. More than one process may hold a shared lock for a given file at a given time. LOCK_EX Place an exclusive lock. Only one process may hold an exclusive lock for a given file at a given time. LOCK_UN Remove an existing lock held by this process.I have seen "int fd = wu_init_config.rdbuf()->fd();" recommended but I get an error that fd() isn't defined. I also saw this "int fd = fileno(wu_init_config.rdbuf()->file())". That complained about file() not defined.
I guess it's an issue of ifstream wanting to be standard across various operating systems and the Linux file descriptor concept doesn't apply to other OS's. So, apparently it is being hidden in order to be more standard across multiple OS's.
How can I use flock() on an open ifstream() or ofstream() ?
Thanks
Chris
Statistics: Posted by chriskot870 — Mon Dec 30, 2024 3:10 am