Hi, how come reading memory from /dev/mem using pread(2) or mmap(2)
will give diffent results?
I run the little prog below 10 times and it always give the following
results
# for i in `seq 1 10`; do ./bar 0x4000000 ; done
04000000: 00000000
pread(04000000): 00cc7d83
04000000: 00000000
pread(04000000): 00cc7d83
04000000: 00000000
pread(04000000): 00cc7d83
04000000: 00000000
pread(04000000): 00cc7d83
04000000: 00000000
pread(04000000): 00cc7d83
04000000: 00000000
pread(04000000): 00cc7d83
04000000: 00000000
pread(04000000): 00cc7d83
04000000: 00000000
pread(04000000): 00cc7d83
04000000: 00000000
pread(04000000): 00cc7d83
04000000: 00000000
pread(04000000): 00cc7d83
thanx
----------------------------------- bar.c ---------------------------------
#include <unistd.h>
#include <stdio.h>
#include <inttypes.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <errno.h>
int
main(int argc, char** argv)
{
uint32_t *addr, item;
uint32_t offset, val;
int fd, update = 0;
if (argc < 2)
return 1;
offset = strtoul(argv[1], NULL, 0);
if (argc > 2) {
val = strtoul(argv[2], NULL, 0);
update = 1;
}
fd = open("/dev/mem", O_RDWR, 0666);
addr = mmap(NULL, 4096, PROT_READ|PROT_WRITE, 0x8080|MAP_SHARED, fd, offset);
if (addr)
{
printf("%8.8x: %8.8x", offset, *addr);
if (update) {
*addr = val;
msync(addr, 4096, MS_SYNC);
printf("-> %8.8x\n", *addr);
} else
printf("\n");
pread(fd, &item, sizeof(item), offset);
printf("pread(%8.8x): %8.8x\n", offset, item);
munmap(addr, 4096);
}
close(fd);
}
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