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Linux开发--mmap映射/dev/mem内存

2019-07-13 08:35发布

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    /dev/mem是linux下的一个字符设备, 源文件是kernel/drivers/char/mem.c, 这个设备文件是专门用来读写物理地址用的。里面的内容是所有物理内存的地址以及内容信息。通常只有root用户对其有读写权限。
     利用mmap和/dev/mem建立起直接读写系统物理内存的渠道。利用/dev/mem和mmap导出系统物理地址，免去了用户虚拟地址到内核逻辑地址的繁琐拷贝，提升效率。
    mmap及munmap函数原型如下:
 #include void *mmap(void *start, size_t length, int prot, int flags, int fd, off_t offset); int munmap(void *start, size_t length);
    mmap详细用法不在此展开, 特别注意参数start(一般赋值为NULL)和offset是页(page, 一般默认大小为4096bytes)对齐的，而且一定要判断mmap函数的返回值。
    下面是利用mmap对/dev/mem进行映射的例子:
#include #include #include #include #include #include #include #include #include #include #include #define FATAL do { fprintf(stderr, "Error at line %d, file %s (%d) [%s] ", __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, " Usage: %s { address } [ type [ data ] ] " " address : memory address to act upon " " type : access operation type : [b]yte, [h]alfword, [w]ord " " data : data to be written ", 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. "); fflush(stdout); /* Map one page */ 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. ", 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'. ", access_type); exit(2); } printf("Value at address 0x%X (%p): 0x%X ", 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 ", writeval, read_result); fflush(stdout); } if(munmap(map_base, MAP_SIZE) == -1) { FATAL; } close(fd); return 0; }

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