Linux 對內存的直接讀寫（devmem）

【摘要】 在Linux開發中着實用到的調試工具並非不少。devmem的方式是提供給驅動開發人員，在應用層可以偵測內存地址中的數據變化，以此來檢測驅動中對內存或者相關配置的正確性驗證。

http://blog.csdn.net/hens007/article/details/7268447

這個工具的原理也比較簡單，就是應用程序經過mmap函數實現對/dev/mem驅動中mmap方法的使用，映射了設備的內存到用戶空間，實現對這些物理地址的讀寫操做。

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <signal.h>
#include <fcntl.h>
#include <ctype.h>
#include <termios.h>
#include <sys/types.h>
#include <sys/mman.h>

#define FATAL do { fprintf(stderr, "Error at line %d, file %s (%d) [%s]\n", \
  __LINE__, __FILE__, errno, strerror(errno)); exit(1); } while(0)

#define MAP_SIZE 4096UL
#define MAP_MASK (MAP_SIZE - 1)

int main(int argc, char **argv) {
    int fd;
    void *map_base, *virt_addr; 
    unsigned long read_result, writeval;
    off_t target;
    int access_type = 'w';

    if(argc < 2) {//若參數個數少於兩個則打印此工具的使用方法
        fprintf(stderr, "\nUsage:\t%s { address } [ type [ data ] ]\n"
            "\taddress : memory address to act upon\n"
            "\ttype    : access operation type : [b]yte, [h]alfword, [w]ord\n"
            "\tdata    : data to be written\n\n",
            argv[0]);
        exit(1);
    }
    target = strtoul(argv[1], 0, 0);

    if(argc > 2)
        access_type = tolower(argv[2][0]);


    if((fd = open("/dev/mem", O_RDWR | O_SYNC)) == -1) FATAL;
    printf("/dev/mem opened.\n"); 
    fflush(stdout);

    /* Map one page */ //將內核空間映射到用戶空間
    map_base = mmap(0, MAP_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, target & ~MAP_MASK);
    if(map_base == (void *) -1) FATAL;
    printf("Memory mapped at address %p.\n", map_base); 
    fflush(stdout);

    virt_addr = map_base + (target & MAP_MASK);
    //針對不一樣的參數獲取不一樣類型內存數據
&nbsp;   switch(access_type) {
        case 'b':
            read_result = *((unsigned char *) virt_addr);
            break;
        case 'h':
            read_result = *((unsigned short *) virt_addr);
            break;
        case 'w':
            read_result = *((unsigned long *) virt_addr);
            break;
        default:
            fprintf(stderr, "Illegal data type '%c'.\n", access_type);
            exit(2);
    }
    printf("Value at address 0x%X (%p): 0x%X\n", target, virt_addr, read_result); 
    fflush(stdout);
        //若參數大於3個，則說明爲寫入操做，針對不一樣參數寫入不一樣類型的數據
    if(argc > 3) {
        writeval = strtoul(argv[3], 0, 0);
        switch(access_type) {
            case 'b':
                *((unsigned char *) virt_addr) = writeval;
                read_result = *((unsigned char *) virt_addr);
                break;
            case 'h':
                *((unsigned short *) virt_addr) = writeval;
                read_result = *((unsigned short *) virt_addr);
                break;
            case 'w':
                *((unsigned long *) virt_addr) = writeval;
                read_result = *((unsigned long *) virt_addr);
                break;
        }
        printf("Written 0x%X; readback 0x%X\n", writeval, read_result); 
        fflush(stdout);
    }

    if(munmap(map_base, MAP_SIZE) == -1) FATAL;
    close(fd);
    return 0;
}

memdev：直接讀寫內存。
能夠在busybox的雜項中找到：

CONFIG_USER_BUSYBOX_DEVMEM:                                       

devmem is a small program that reads and writes from physical     
memory using /dev/mem.                                           

Symbol: USER_BUSYBOX_DEVMEM [=y]                                  
Prompt: devmem                                                    
  Defined at ../user/busybox/busybox-1.14.3/miscutils/Kconfig:216 
  Depends on: USER_BUSYBOX_BUSYBOX                                
  Location:                                                       
    -> BusyBox (USER_BUSYBOX_BUSYBOX [=y])                        
      -> Miscellaneous Utilities                                  

[用法]
Usage: devmem ADDRESS [WIDTH [VALUE]]
讀取：在地址0x97000000讀取32bit值(WIDTH默認等於32, 可選值爲[8, 16, 32, 64])
/dev # devmem 0x97000000 
0x11111111
讀取：在地址0x97000000讀取16bit值
/dev # devmem 0x97000000 16
0x1111

寫入：在地址0x97000000寫入32bit值0x7777ABCD
/dev # devmem 0x97000000 32 0x7777ABCD
/dev # devmem 0x97000000              
0x7777ABCD

注意：若是/dev下沒有mem這個node，會出現錯誤：
/dev # devmem 0x97000000 
devmem: can't open '/dev/mem': No such file or directory

這時能夠在Host系統中手動建立一個（例如在NFS root filesystem模式）：
host@host-laptop:~/embedded/tftpboot/nfsroot/dev$ sudo mknod mem -m666 c 1 1
注意這裏的權限是666，容許任何人任意讀寫，能夠很好的配合程序debug。

/dev # devmem 0x97000000
0x7777ABCD