Msync Invalid Arg

Msync Invalid Arg

[darshan.ghumare]

Whne I try to execute following code then  get an erro message that "Invalid Argumemt" in msync. 


#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/errno.h>
#include <sys/time.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <errno.h>
#include <unistd.h>
#include <pthread.h>
#include <linux/types.h>

int main()
{
	int fd;
	char data[20];
	__u8 *start_addr,*end_addr;
	int PageSize;

	if ( (PageSize = sysconf(_SC_PAGE_SIZE)) < 0) {
	    perror("sysconf() Error=");
		    return -1;
	}
	printf("PageSize = %d\n",PageSize);

	if((fd = open("/dev/mem", O_RDWR | O_SYNC)) == -1)
	{ 
		printf("Error opening /dev/mem\n");
		return -1;
	}

	start_addr = mmap(0, PageSize, PROT_READ | PROT_WRITE, MAP_SHARED, fd,
										0);
	if(start_addr == MAP_FAILED)
	{
		printf("enc_status map failed %s\n", strerror(errno));
		return -1;
	}

	end_addr = start_addr + (4096 - 1);
	strcpy(data, "Test Msg");
	printf("\n%s:Writing NVRAM.......\n", __FUNCTION__);
	if(write(fd, data, strlen(data)) <= 0)
	{
		printf("%s: Not able to write on NVRAM. %s.\n", __FUNCTION__,
				strerror(errno));
		return -1;
	}
	printf("\n%s:Synching NVRAM.......\n", __FUNCTION__);
	if(msync(start_addr, PageSize, MS_SYNC) <= 0)
	{
		printf("%s: Not able to sync to NVRAM. %s.\n", __FUNCTION__,
				strerror(errno));
		return -1;
	}
	printf("\n%s:reading NVRAM.......\n", __FUNCTION__);
	strcpy(data, "");
	if(read(fd, data, sizeof(data)) <= 0)
	{
		printf("%s: Not able to read on NVRAM. %s.\n", __FUNCTION__,
				strerror(errno));
		return -1;
	}
	printf("%s: data  %s.\n", __FUNCTION__, data);
	return 0;
}


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Re: Msync Invalid Arg

[Michal Nazarewicz]

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darshan.ghumare@gmail.com writes:

> Whne I try to execute following code I get "Invalid Argumemt" error
> message in msync.

/* Code rewritten to be shorter */

> #include <fcntl.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
> #include <sys/mman.h>
> #include <unistd.h>
> 
> #define DIE_IF(cnd, msg) do{ if (cnd) { perror(msg); return 1; } }while(0)
> 
> int main(void)
> {
> 	char data[20];
> 	void *addr;
> 	int fd, ret;
> 
> 	fd = open("/dev/mem", O_RDWR | O_SYNC);
> 	DIE_IF(fd < 0, "open");
> 
> 	addr = mmap(0, 4096, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
> 	DIE_IF(addr == MAP_FAILED, "mmap");
> 
> 	strcpy(data, "Test Msg");
> 	ret = write(fd, data, strlen(data));
> 	DIE_IF(ret < 0, "write");
> 
> 	ret = msync(addr, 4096, MS_SYNC);
> 	DIE_IF(ret < 0, "msync");
> 
> 	ret = lseek(fd, 0, SEEK_SET);
> 	DIE_IF(ret < 0, "lseek");
> 
> 	memset(data, 0, sizeof data);
> 	ret = read(fd, data, sizeof data);
> 	DIE_IF(ret < 0, "read");
> 	printf("Read: %s.\n", data);
> 
> 	return 0;
> }


The msync(2) fails because /dev/mem driver (drivers/char/mem.c) does not
implement fsync file operation which is used by msync(2) system call.

This sort of makes sense since there is no need to sync the /dev/mem
file -- there is no backing storage on hard drive or anywhere, when you
write data there you write directly to the memory and same goes for
reading.

Anyhow, I'm Ccing this to linux-kernel as maybe we need a dummy fsync
operation for /dev/mem which would do nothing and return zero?

--
Best regards,                                         _     _
 .o. | Liege of Serenly Enlightened Majesty of      o' \,=./ `o
 ..o | Computer Science,  Michal "mina86" Nazarewicz   (o o)
 ooo +--<mina86*tlen.pl>--<jid:mina86*jabber.org>--ooO--(_)--Ooo--

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