devmem调试内核驱动

在Linux开发中着实用到的调试工具并不是很多。devmem的方式是提供给驱动开发人员，在应用层能够侦测内存地址中的数据变化，以此来检测驱动中对内存或者相关配置的正确性验证。

    devmem2使用方法：    devmem2 { address } [ type [ data ] ]            address : 物理地址            type :要访问的数据类型 : [b]yte, [h]alfword, [w]ord            data :想要写入的数据，若为读取操作则省略此参数，若为写入，则必须含有此参数。

如下就贴出devmem2的源代码一探究竟。

01.#include <stdio.h>  02.#include <stdlib.h>  03.#include <unistd.h>  04.#include <string.h>  05.#include <errno.h>  06.#include <signal.h>  07.#include <fcntl.h>  08.#include <ctype.h>  09.#include <termios.h>  10.#include <sys/types.h>  11.#include <sys/mman.h>  12.    13.#define FATAL do { fprintf(stderr, "Error at line %d, file %s (%d) [%s]\n", \  14.  __LINE__, __FILE__, errno, strerror(errno)); exit(1); } while(0)  15.   16.#define MAP_SIZE 4096UL  17.#define MAP_MASK (MAP_SIZE - 1)  18.  19.int main(int argc, char **argv) {  20.    int fd;  21.    void *map_base, *virt_addr;   22.    unsigned long read_result, writeval;  23.    off_t target;  24.    int access_type = 'w';  25.      26.    if(argc < 2) {//若参数个数少于两个则打印此工具的使用方法  27.        fprintf(stderr, "\nUsage:\t%s { address } [ type [ data ] ]\n"  28.            "\taddress : memory address to act upon\n"  29.            "\ttype    : access operation type : [b]yte, [h]alfword, [w]ord\n"  30.            "\tdata    : data to be written\n\n",  31.            argv[0]);  32.        exit(1);  33.    }  34.    target = strtoul(argv[1], 0, 0);  35.  36.    if(argc > 2)  37.        access_type = tolower(argv[2][0]);  38.  39.  40.    if((fd = open("/dev/mem", O_RDWR | O_SYNC)) == -1) FATAL;  41.    printf("/dev/mem opened.\n");   42.    fflush(stdout);  43.      44.    /* Map one page */ //将内核空间映射到用户空间  45.    map_base = mmap(0, MAP_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, target & ~MAP_MASK);  46.    if(map_base == (void *) -1) FATAL;  47.    printf("Memory mapped at address %p.\n", map_base);   48.    fflush(stdout);  49.      50.    virt_addr = map_base + (target & MAP_MASK);  51.    //针对不同的参数获取不同类型内存数据  52.    switch(access_type) {  53.        case 'b':  54.            read_result = *((unsigned char *) virt_addr);  55.            break;  56.        case 'h':  57.            read_result = *((unsigned short *) virt_addr);  58.            break;  59.        case 'w':  60.            read_result = *((unsigned long *) virt_addr);  61.            break;  62.        default:  63.            fprintf(stderr, "Illegal data type '%c'.\n", access_type);  64.            exit(2);  65.    }  66.    printf("Value at address 0x%X (%p): 0x%X\n", target, virt_addr, read_result);   67.    fflush(stdout);  68.        //若参数大于3个，则说明为写入操作，针对不同参数写入不同类型的数据  69.    if(argc > 3) {  70.        writeval = strtoul(argv[3], 0, 0);  71.        switch(access_type) {  72.            case 'b':  73.                *((unsigned char *) virt_addr) = writeval;  74.                read_result = *((unsigned char *) virt_addr);  75.                break;  76.            case 'h':  77.                *((unsigned short *) virt_addr) = writeval;  78.                read_result = *((unsigned short *) virt_addr);  79.                break;  80.            case 'w':  81.                *((unsigned long *) virt_addr) = writeval;  82.                read_result = *((unsigned long *) virt_addr);  83.                break;  84.        }  85.        printf("Written 0x%X; readback 0x%X\n", writeval, read_result);   86.        fflush(stdout);  87.    }  88.      89.    if(munmap(map_base, MAP_SIZE) == -1) FATAL;  90.    close(fd);  91.    return 0;  92.}