I have a weird device that I'm using ioctl to talk to (on Linux 2.6). The code in C to do what I want is:

int f = open("/dev/plcm_drv", O_RDWR);
printf("%x\n", ioctl(f, 0xC, 0) );
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Simple and straightforward--I'm just sending ioctl # 0xC to the driver and printing out the response. Works every time. The equivalent code in Perl is (I believe):

sysopen(DEVICE, "/dev/plcm_drv", O_RDWR);
print sprintf("%x", ioctl(DEVICE, 0xC, 0)) . "\n";
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The driver, however, never receives the ioctl (I put some debug code in the driver that printk's whenever it gets an ioctl). The driver does receive the TCGETS ioctl from sysopen (with which it responds "Operation not supported" as expected). $! after the ioctl is "Invalid argument".

The strace tells a similar story:

Working (C, ioctl 0xC):

open("/dev/plcm_drv", O_RDWR)           = 3
ioctl(3, 0xc, 0)                        = 142

Working (Perl, ioctl 1):

open("/dev/plcm_drv", O_RDWR|O_LARGEFILE) = 3
ioctl(3, SNDCTL_TMR_TIMEBASE or TCGETS, 0xff86b540) = -1 EOPNOTSUPP (O
+peration not supported)
_llseek(3, 0, [0], SEEK_CUR)            = 0
fstat64(3, {st_mode=S_IFCHR|0644, st_rdev=makedev(241, 0), ...}) = 0
fcntl64(3, F_SETFD, FD_CLOEXEC)         = 0
ioctl(3, FIBMAP, 0)                     = 0
close(3)                                = 0

Not Working (Perl, ioctl 0xC):

open("/dev/plcm_drv", O_RDWR|O_LARGEFILE) = 3
ioctl(3, SNDCTL_TMR_TIMEBASE or TCGETS, 0xffe34310) = -1 EOPNOTSUPP (O
+peration not supported)
_llseek(3, 0, [0], SEEK_CUR)            = 0
fstat64(3, {st_mode=S_IFCHR|0644, st_rdev=makedev(241, 0), ...}) = 0
fcntl64(3, F_SETFD, FD_CLOEXEC)         = 0
ioctl(3, 0xc, 0)                        = -1 EINVAL (Invalid argument)
close(3)                                = 0
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So to sum up:

• C: Works fine with the handcoded constant.
• Perl: Works fine with other handcoded constants, just not 0xC. Behaves correctly with TCGETS (in that it errors). Does not appear to receive 0xC at all.

Any thoughts as to why 0xC is being blocked? I'd rather not have to have a trivial C helper app for this...