hi NeilBrown:
2014-07-10 8:47 GMT+08:00 NeilBrown <neilb@xxxxxxx>:
> On Wed, 9 Jul 2014 22:13:28 +0800 loody <miloody@xxxxxxxxx> wrote:
>
>> hi NeilBrown:
>> we use below c source but there is no read command firing from usb
>> host to device.
>> except O_DIRECT, is there any flag we need to use?
>> appreciate all your kind help,
>>
>> #include <stdio.h>
>> #include <stdlib.h>
>> #include <sys/types.h>
>> #include <sys/stat.h>
>> #include <fcntl.h>
>>
>> char message[] = "/mnt/usb/4854344154343452/
>> test.txt";
>> int main()
>> {
>> int fd;
>> char buffer[5];
>> int count = 0;
>> char *buf="1234567890";
>> if((fd=open(message,O_CREAT|O_TRUNC|O_RDWR|O_DIRECT, 0777))<0)
>
> I said "Open the device with O_DIRECT". You are opening a file in the
> filesystem which is mounted from the device. That is a different thing.
sorry for misunderstanding your explanation.
>
>> {
>> perror("open");
>> return -1;
>> }
>> printf("fd=%d\n", fd);
>> write(fd, buf, strlen(buf));
>> while(1){
>> lseek(fd,0,SEEK_SET);
>> sleep(3);
>> count = read(fd, buffer, 3);
>
> Did you do any research to understand how O_DIRECT works?
> I recommend the man page for "open(2)".
>
> You need to read thoroughly, but towards the end it says:
>
> Under Linux 2.4, transfer sizes, and the alignment of the user buffer and the
> file offset must all be multiples of the logical block size of the file sys-
> tem. Under Linux 2.6, alignment to 512-byte boundaries suffices.
>
> Neither your buffer nor your IO size is 512-byte aligned.
I follow your suggestion and try to read /dev/sda or /dev/sda1 for
512Bytes like below.
But the read back count is -1, that mean the read is not successful.
I try to open both with "O_DIRECT" or "O_DIRECT|O_RDONLY" but all of
them get read back count are -1.
Does that mean block device node not support system read command?
appreciate your help,
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
char message[] = "/dev/sda1";
int main()
{
int fd;
char buffer[1024];
int count;
//if((fd=open(message,O_DIRECT))<0)
if((fd=open(message,O_DIRECT|O_RDONLY))<0)
{
perror("open");
return -1;
}
printf("fd=%d\n", fd);
while(1){
sleep(3);
count = read(fd, buffer, 512); //read back fail
printf("count=%d,%x,%x,%x\n", count,buffer[0],buffer[1],buffer[2]);
}
close(fd);
}
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