【FatFS】之逻辑盘符与物理盘符

FatFS源代码中的函数逻辑关系

第一　　调用函数 f_mount()

result = f_mount(&fs, FS_VOLUME_NAND, 0);    /* Mount a logical drive */

FS_VOLUME_NAND 代表 “路径”

第二　　进入函数 f_mount()内

/*-----------------------------------------------------------------------*/
/* Mount/Unmount a Logical Drive                                         */
/*-----------------------------------------------------------------------*/

FRESULT f_mount (
    FATFS* fs,            /* Pointer to the file system object (NULL:unmount)*/
    const TCHAR* path,    /* Logical drive number to be mounted/unmounted */
    BYTE opt              /* 0:Do not mount (delayed mount), 1:Mount immediately */
)
{
    FATFS *cfs;
    int vol;
    FRESULT res;
    const TCHAR *rp = path;


    vol = get_ldnumber(&rp);
    if (vol < 0) return FR_INVALID_DRIVE;
    cfs = FatFs[vol];                    /* Pointer to fs object */

    if (cfs) {
#if _FS_LOCK
        clear_lock(cfs);
#endif
#if _FS_REENTRANT                        /* Discard sync object of the current volume */
        if (!ff_del_syncobj(cfs->sobj)) return FR_INT_ERR;
#endif
        cfs->fs_type = 0;                /* Clear old fs object */
    }

    if (fs) {
        fs->fs_type = 0;                 /* Clear new fs object */
#if _FS_REENTRANT                        /* Create sync object for the new volume */
        if (!ff_cre_syncobj((BYTE)vol, &fs->sobj)) return FR_INT_ERR;
#endif
    }
    FatFs[vol] = fs;                     /* Register new fs object */

    if (!fs || opt != 1) return FR_OK;   /* Do not mount now, it will be mounted later */

    res = find_volume(&fs, &path, 0);    /* Force mounted the volume */
    LEAVE_FF(fs, res);
}

代码中第17行 vol = get_ldnumber(&rp) 将路径转换为 logical drive number

第三　　进入函数 get_ldnumber()

/*-----------------------------------------------------------------------*/
/* Get logical drive number from path name                               */
/*-----------------------------------------------------------------------*/

static
int get_ldnumber (        /* Returns logical drive number (-1:invalid drive) */
    const TCHAR** path    /* Pointer to pointer to the path name */
)
{
    const TCHAR *tp, *tt;
    UINT i;
    int vol = -1;


    if (*path) {    /* If the pointer is not a null */
        for (tt = *path; (UINT)*tt >= (_USE_LFN ? ' ' : '!') && *tt != ':'; tt++) ;    /* Find ':' in the path */
        if (*tt == ':') {    /* If a ':' is exist in the path name */
            tp = *path;
            i = *tp++ - '0'; 
            if (i < 10 && tp == tt) {    /* Is there a numeric drive id? */
                if (i < _VOLUMES) {      /* If a drive id is found, get the value and strip it */
                    vol = (int)i;
                    *path = ++tt;
                }
            } else {      /* No numeric drive number */
#if _STR_VOLUME_ID        /* Find string drive id */
                static const char* const str[] = {_VOLUME_STRS};
                const char *sp;
                char c;
                TCHAR tc;

                i = 0; tt++;
                do {
                    sp = str[i]; tp = *path;
                    do {    /* Compare a string drive id with path name */
                        c = *sp++; tc = *tp++;
                        if (IsLower(tc)) tc -= 0x20;
                    } while (c && (TCHAR)c == tc);
                } while ((c || tp != tt) && ++i < _VOLUMES);    /* Repeat for each id until pattern match */
                if (i < _VOLUMES) {    /* If a drive id is found, get the value and strip it */
                    vol = (int)i;
                    *path = tt;
                }
#endif
            }
            return vol;
        }
#if _FS_RPATH && _VOLUMES >= 2
        vol = CurrVol;    /* Current drive */
#else
        vol = 0;          /* Drive 0 */
#endif
    }
    return vol;
}

本函数获取 path 后解析 path 得到 logical drive number

代码中

第15行 判断 path 的内容是否为空字符 NULL

第16行 检测冒号 ‘：’，for循环的条件为 路径名中的字符 >= (_USE_LFN ? ' '： '!') 且 != ‘：’，只要_USE_LFN>0则第一个条件恒满足，找到冒号‘：’后则第二个条件失败，即跳出循环

第17行 再次确认当前路径字符为冒号‘：’

第18行 暂存*path

第19行 得到路径中的首字符那个数字字符对应的数字

第20行 判断该数字在0~9之中，且路径中的第二个字符为冒号‘：’

第21行 判断该数字在定义的范围内

第22行 将该数字赋值给vol

第46行 最终将vol值返回

 

帮助理解的代码示例

示例一 指针与字符串

#include "stdio.h"

void ptf(char **p);

int main(void)
{
    char *pstr = "0:wojiushiyixia";
    
    ptf(&pstr);
    
    return 0;
}

void ptf(char **p)
{
    if(*p)
    {
        printf("*p = %d\n", *p);
        printf("*p = %s\n", *p);
    }
    else
    {
        printf("*p = NULL\n");
    }
    
}

运行环境 C-Free5.0

运行结果

 示例二 多参数宏定义

#include "stdio.h"

#define _VOLUME_STRS    "RAM","NAND","CF","SD1","SD2","USB1","USB2","USB3"


int main(void)
{
    int i;    
    static const char* const str[] = {_VOLUME_STRS};
    
    for(i = 0; i < 8; i++)
    {
        printf("str[%d] = %s\n", i, str[i]);
    }
    
    return 0;
}

运行环境 C-Free5.0

运行结果