Programming

There are many processes writing to log files at random times (they are all programmed in different manners and can not be altered). I am writing a new c program which should read from the log files whenever they are updated. At the moment it is just spooling, waiting for a change in file size, but I really want to block until an update is made on one of the files.
Can this be done?
I thought select or poll might work but apparently they arent much use when your using regular files.
Also looked at using fifo's but seeing as I cant alter the original code it seems impossible.

Any ideas?

thanks

There are two types of locking mechanisms: mandatory and advisory. Mandatory systems will actually prevent read()'s and write()'s to file. Several Unix systems support them.

To make an advisory lock in POSIX-systems fcntl must be used:

Code:

#include <fcntl.h>
...

struct flock fl;
int fd;

fl.l_type   = F_RDLCK;  /* read lock */
fl.l_whence = SEEK_SET; /* beginning of file */
fl.l_start  = 0;        /* offset from l_whence */
fl.l_len    = 0;        /* length, 0 = to EOF */
fl.l_pid    = getpid(); /* PID */

fd = open("filename", O_RDONLY);

fcntl(fd, F_SETLKW, &fl); /* set lock */

...


fl.l_type   = F_UNLCK;
fcntl(fd, F_SETLK, &fl); /* unset lock */

Note that there are no guarantees with advisory locks, mandatory locks are not supported by all systems however

Thanks very much, I'll have a mess around and see if I can get it going.

thanks again