Examples of file Locking in GCC and Python
Beej's guide for file locking in GCC
GCC: putting a pair of floats into a file
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
int main()
{
int fd;
int ret_status;
char buffy[128];
float ra_deg = 292.4344;
float dec_deg = 28.0004;
sprintf(buffy, "%8.4f %8.4f \n", ra_deg, dec_deg);
struct flock fl;
fl.l_type = F_WRLCK; /* F_RDLCK, F_WRLCK, F_UNLCK */
fl.l_whence = SEEK_SET; /* SEEK_SET, SEEK_CUR, SEEK_END */
fl.l_start = 0; /* Offset from l_whence */
fl.l_len = 0; /* length, 0 = to EOF */
fl.l_pid = getpid(); /* our PID */
if (access("tel_position", F_OK) == 0) {
fd = open("tel_position",O_RDWR|O_TRUNC);}
else {
fd = creat("tel_position",S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP);}
ret_status = fcntl(fd, F_SETLKW, &fl);
write(fd,buffy,strlen(buffy));
// sleep(5);
fl.l_type = F_UNLCK;
fcntl(fd, F_SETLK, &fl); /* set the region to unlocked */
close(fd);
}
GCC: getting a pair of floats from a file
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
int main()
{
float ra_deg;
float de_deg;
int fd;
char buffy[32];
int nchar =32;
struct flock fl;
fl.l_type = F_RDLCK; /* F_RDLCK, F_WRLCK, F_UNLCK */
fl.l_whence = SEEK_SET; /* SEEK_SET, SEEK_CUR, SEEK_END */
fl.l_start = 0; /* Offset from l_whence */
fl.l_len = 0; /* length, 0 = to EOF */
fl.l_pid = getpid(); /* our PID */
fd = open("tel_position",O_RDONLY); /* get the file descriptor */
fcntl(fd, F_SETLKW, &fl);
read(fd, buffy, 32);
fl.l_type = F_UNLCK; /* tell it to unlock the region */
fcntl(fd, F_SETLK, &fl); /* set the region to unlocked */
close(fd);
sscanf (buffy, "%f %f", &ra_deg, &de_deg);
printf("telescope position: %f %f \n",ra_deg, de_deg);
}
python: putting a pair of floats into a file
import time
import fcntl, struct
ra_deg = '192.0'
dec_deg = '29.0'
tel_pos_out = open('tel_position','w')
lockdata = struct.pack('hhllhh', fcntl.F_WRLCK, 0, 0, 0, 0, 0)
while 1:
try:
fcntl.fcntl(tel_pos_out.fileno(),fcntl.F_SETLKW, lockdata)
tel_pos_out.write(ra_deg+' '+dec_deg +"\n")
# time.sleep(15)
tel_pos_out.close()
break
except:
time.sleep(0.01)
python: getting a pair of floats from a file
import time
import fcntl, struct
tel_pos_out = open('tel_position','r')
lockdata = struct.pack('hhllhh', fcntl.F_RDLCK, 0, 0, 0, 0, 0)
while 1:
try:
fcntl.fcntl(tel_pos_out.fileno(),fcntl.F_SETLKW, lockdata)
line = tel_pos_out.readline()
tel_pos_out.close()
print line.split()
break
except:
time.sleep(0.01)
