【摘要】 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。
http://blog.csdn.net/hens007/article/details/7268447
原文链接：http://blog.csdn.net/xy010902100449/article/details/47028497
这个工具的原理也比较简单，就是应用程序通过mmap函数实现对/dev/mem驱动中mmap方法的使用，映射了设备的内存到用户空间，实现对这些物理地址的读写操作。
#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) { 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_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); 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); 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;}
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memdev：直接读写内存。可以在busybox的杂项中找到：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]]读取：在地址0x97000000读取32bit值(WIDTH默认等于32, 可选值为[8, 16, 32, 64])/dev 0x11111111读取：在地址0x97000000读取16bit值/dev 0x1111写入：在地址0x97000000写入32bit值0x7777ABCD/dev /dev 0x7777ABCD注意：如果/dev下没有mem这个node，会出现错误：/dev devmem: can't open '/dev/mem': No such file or directory这时可以在Host系统中手动创建一个（例如在NFS root filesystem模式）：host@host-laptop:~/embedded/tftpboot/nfsroot/dev$ sudo mknod mem -m666 c 1 1注意这里的权限是666，允许任何人任意读写，可以很好的配合程序debug。/dev 0x7777ABCD
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