Newbie question about usage of mmap

[justin8086]

Hi there,

I have a PCI device and I tried to write a short code to access it in user space with mmap.

first I "cat /proc/bus/pci/devices" and get the device's address: 0x00001800.

then I write the following code:

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

int main(void)
{
void *map;
int fd;
int src = 0;
ulong base;
ulong length;

base = 0x00001800;
length = 0xFF;

fd = open("/dev/mem", O_RDWR);
printf("%d\n",fd);
map = mmap(NULL, (size_t)length, (PROT_READ | PROT_WRITE), MAP_SHARED, fd,(off_t)base);
if (map == MAP_FAILED)
{
printf("map failed: %s\n",strerror(errno));
exit(1);
}

memcpy(map+0x85,&src, 1);

return 0;
}

I run the code and got the error says that "Invalid Argument", I guess that is because of some misuse of mmap, but where is the error?

TIA

justin

[lakerdonald]

Can you give an exact cut/paste of the error?

[justin8086]

as i described, the error message is

map failed: Invalid Argument

Thank you for reply

[lakerdonald]

According to the manpage for mmap(2), EINVAL is returned when the start, length, or offset is incorrect, so you might want to check on those values

[justin8086]

thanks again.
yeah, i know because it says the argument is invalid. what is more, since 'start' is always NULL, i guess this is not the problem. then the 'bad guy' should hide in length or offset, right?

if i make length very small (for instance, 1) and still get the same error, it must be the problem in offset, right?

Thank you for being patient and replying so fast...

[lakerdonald]

I suppose so.