I have been trying to read() audio data into an mmap()ed region (from a file
just created and empty before the mmap call. I know that read() normally
puts the data in a buffer and not a file, but with mmap() you get back a
caddr_t pointer which I am hoping read() can use. Here is the relevant chunk
of code (full listing at bottom):
/* create and memory map the target region */
printf("0: I'm still here sir!");
status = target = open("targetfile", O_CREAT | O_RDWR, 777);
if (status < 0)
perror("error opening target file");
printf("1: I'm still here sir!");
region = mmap(NULL, (off_t) 512, PROT_WRITE, MAP_SHARED, target, 0);
printf("2: I'm still here sir!");
if (region == (caddr_t) (-1))
perror("mmap error");
/* record sound into mmaped region */
printf("3: I'm still here sir!");
status = read(audio, (void *) region, 512);
if (status != sizeof(region))
perror("read screwed one way or another");
msync(region, 512, MS_SYNC);
with the read(audio, (void *) region, 512);
line not commented out, when I run this program I get:
Bus error
and a file in the current directory:
-r----x--t 1 gandalf gandalf 0 Feb 4 17:04 targetfile
None of the little I'm still here sir! message print with the read in there,
so I assume I'm doing something drastic. My questions:
1. Why does the file have those strange permissions? I thought I specified
mode 777.
2. Is it possible to use read() into an mmap() this way? Is there some other
function I should be using instead of read()?
3. Even if it is possible, is it a completely hairbrained approach?
The full source of the short program in question:
/*
* rawrec.c Program to test recording principle of multiple mmaps,
* forked into the background before syncing.
*/
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <stdlib.h>
#include <stdio.h>
#include <linux/soundcard.h>
#define LENGTH 600 /* how many seconds of the CD to store */
#define RATE 44100 /* the sampling rate */
#define SIZE 16 /* sample size: 8 or 16 bits */
#define CHANNELS 2 /* 1 = mono 2 = stereo */
int main()
{
int audio; /* sound device file descriptor */
int arg; /* argumrnt for ioctl calls */
int status; /* return status of system calls */
int target; /* file descriptor to write to */
void * region; /* for the pointer to the mapped file */
/* open sound device */
audio = open("/dev/dsp", O_RDWR);
if (audio < 0) {
perror("open of /dev/dsp failed");
exit(1);
}
/* set sampling parameters */
arg = SIZE; /* sample size */
status = ioctl(audio, SOUND_PCM_WRITE_BITS, &arg);
if (status == -1)
perror("SOUND_PCM_WRITE_BITS ioctl fialed");
if (arg != SIZE)
perror("unable to set sample size");
arg = CHANNELS; /* mono or stereo */
status = ioctl(audio, SOUND_PCM_WRITE_CHANNELS, &arg);
if (status == -1)
perror("SOUND_PCM_WRITE_CHANNELS ioctl failed");
if (arg != CHANNELS)
perror("unable to set number of channels");
arg = RATE; /* sampling rate */
status = ioctl(audio, SOUND_PCM_WRITE_RATE, &arg);
if (status == -1)
perror("SOUND_PCM_WRITE_RATE ioctl failed");
/* create and memory map the target region */
printf("0: I'm still here sir!");
status = target = open("targetfile", O_CREAT | O_RDWR, 777);
if (status < 0)
perror("error opening target file");
printf("1: I'm still here sir!");
region = mmap(NULL, (off_t) 512, PROT_WRITE, MAP_SHARED, target, 0);
printf("2: I'm still here sir!");
if (region == (caddr_t) (-1))
perror("mmap error");
/* record sound into mmaped region */
printf("3: I'm still here sir!");
status = read(audio, (void *) region, 512);
if (status != sizeof(region))
perror("read screwed one way or another");
msync(region, 512, MS_SYNC);
}
