Ioctl

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Introduction

Sometimes some applications block, and strace show something like that:

ioctl(6, 0x400c4150

which means that the kernel blocks during that ioctl.

Howto

we will take the same example than before,with mplayer blocking.:

ioctl(6, 0x400c4150

there are two important information in it: 6 is the file descriptor,the corresponding file can be found with:

ls -l /proc/$(pidof mplayer)/fd/6

which gives

/dev/snd/pcmC0D0p

That correspond to alsa. Then we have the 0x400c4150 the important part is the last part: 41 is 0x41 which is often represented as a letter and can be decoded like this:

$ python
Python 2.6.5 (r265:79063, Apr 16 2010, 13:57:41) 
[GCC 4.4.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import binascii
>>> binascii.a2b_hex("41")
'A'

Or verified like this:

$ python
Python 2.6.5 (r265:79063, Apr 16 2010, 13:57:41) 
[GCC 4.4.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import binascii
>>> binascii.b2a_hex("A")
'41'

Then 0x50 remains. we don't need to decode it,it's fine like it is.

so with A and 50 and /dev/snd/pcmC0D0p file we can go to grep in include/sound of the kernel sources:

include/sound$ grep "0x50" *
ac97_codec.h:#define AC97_GPIO_STICKY	0x50	/* GPIO Pin Sticky, 0=not, 1=sticky */
ad1816a.h:#define AD1816A_SRC_MIC			0x50
ad1816a.h:#define AD1816A_SRC_MONO		0x50
asequencer.h:#define SNDRV_SEQ_IOCTL_GET_SUBSCRIPTION	_IOWR('S', 0x50, struct snd_seq_port_subscribe)
asoundef.h:#define MIDI_CTL_GENERAL_PURPOSE5     	0x50
asound.h:#define SNDRV_PCM_IOCTL_WRITEI_FRAMES	_IOW('A', 0x50, struct snd_xferi)
cs46xx.h:#define HDCR_SMS_256_DWORDS                     0x50000000
cs46xx.h:#define DMA_RQ_C1_SOURCE_MOD256                 0x50000000
emu10k1.h:#define CDCS			0x50		/* CD-ROM digital channel status register	*/
emu10k1.h:#define C_ffffffff	0x50
emu10k1.h:#define A_P16VIN(x)	(0x50 + (x))	/* x = 0x00 - 0x0f p16v ins (A2 only) "EMU32 inputs" */
gus.h:#define SNDRV_GF1_GB_DRAM_DMA_HIGH		0x50
sb.h:#define SB_ALS4000_3D_SND_FX	0x50
trident.h:#define SI_ASR0			    0x50

here we see:

asound.h:#define SNDRV_PCM_IOCTL_WRITEI_FRAMES	_IOW('A', 0x50, struct snd_xferi)

which is our ioctl.

The next step is to look where this ioctl is called: