通过/dev/mem操作物理内存

　　/dev/mem设备可以用来访问物理内存。下面一段应用程序的代码，实现了通过/dev/mem对物理内存空间中SRAM1的访问。

#include <stdio.h>
#include <stdint.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mman.h>

#define SRAM1_ADDR  0x10000000

struct test_data {
        int32_t val;
};

int main(void)
{
        struct test_data *data;
        int ret = 0;
        void *base;
        int fd;

        fd = open("/dev/mem", O_RDWR | O_SYNC);
        if (fd == -1) {
                printf("failed to open /dev/mem!");
                return -1;
        }

        base = mmap(NULL, sizeof(struct test_data), PROT_READ | PROT_WRITE,
                    MAP_SHARED, fd, SRAM1_ADDR);
        if (base == MAP_FAILED) {
                printf("failed to mmap /dev/mem!");
                ret = -1;
                goto err_mmap;
        }

        data = (struct test_data *)base;

        printf("data value: %x\n", data->val);
        data->val = 0x12345678;
        printf("write: %x\n", data->val);

        if (munmap(base, sizeof(struct test_data)) == -1) {
                printf("failed to munmap /dev/mem!");
                ret = -1;
        }

err_mmap:
        close(fd);
        return ret;
}

　　可以通过devmem命令验证该程序。通过devmem预先把data设置成0xffffffff，然后用该程序把0x12345678写入data，最后用devmem再读取验证。

# devmem2 0x10000000 w 0xffffffff
/dev/mem opened.
Memory mapped at address 0xb6f36000.
Value at address 0x10000000 (0xb6f36000): 0x11111111
Written 0xFFFFFFFF; readback 0xFFFFFFFF

# ./a.out
data value: ffffffff
write: 12345678

# devmem2 0x10000000
/dev/mem opened.
Memory mapped at address 0xb6f15000.
Value at address 0x10000000 (0xb6f15000): 0x12345678