Home > MOD and HACK > Android – Accessing Input device "msm_pcm_in" from User space

Android – Accessing Input device "msm_pcm_in" from User space

This article shows code snippet on how to access input device driver from user space.

Recording from input device (msm_pcm_in: a char Device Driver) follows the below events

Open device :

int ret = open("/dev/msm_pcm_in", O_RDWR); // open device

struct msm_audio_config confg;

ioctl(ret, AUDIO_GET_CONFIG, &confg) // this call fill the structure with default driver values.

cfg.channel_count = 1; // assign new number of channel(1= mono, 2 = stereo)
cfg.sample_rate = 8000;// assign sampling rate such as 8000 or 16000,.....
int val = ioctl(ret, AUDIO_SET_CONFIG, &confg);// set the new audio configuration

Before we being to start audio read/write operations, create a file to store buffers
const char *fn = /sdcard/tmp/file1;
int file = open(fn, O_CREAT | O_RDWR, 0666);

bool isrecording = true; // use this flag to stop recording (true default, false to disable reading)

ioctl(ret, AUDIO_START, 0);

fcntl(0, F_SETFL, O_NONBLOCK);// provides control of open file descriptors.

unsigned char buf[4092];

for (;;) {
while (!isrecording) {
// close all file descriptors and exit
}
if (read(ret, buf,confg.buffer_size ) != confg.buffer_size) {
// if fail to read
// close all file descriptors and exit
}

if (write(file, buf, confg.buffer_size) != confg.buffer_size) {
// if fail to read
// close all file descriptors and exit
}

}


Close device:

close(ret);
...
...
close (file);

Note: To run this as a recorder app, make sure you have root privileges.

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Categories: MOD and HACK
  1. March 10, 2011 at 10:44 pm

    Can this code be compiled in conjunction with the NDK or does this have to be included with the Android open source code?

  2. March 10, 2011 at 11:10 pm

    Yes, you can compile this as a lib or exe, not necessary to include in AOSP but only constraint is that you need to be root to access the device from user space. I have tested this on rooted device and worked like a charm.

  3. March 14, 2011 at 4:41 pm

    Would this be possible for msm_pcm_out instead of msm_pcm_in?

  4. March 14, 2011 at 4:51 pm

    So I got the program working but when I pull the file it says it is a text file. What do I need to do in order to play what I recorded?

  5. ama
    March 14, 2011 at 8:34 pm

    Recheck the recorded format and try playing through msm_pcm_out or any other player.

  6. May 3, 2011 at 4:42 pm

    This is useful because I´m on a tri I need to get this done. In this apartment in buenos aires I´m satying in I have a computer they gave me which works great but I wanted to know how to do this.Thanks!Kim

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