ptr[-1]的作用和用法

今天在看ＡＰＵＥ中４.７遍历目录程序的时候，发现一个语句　ptr[-1]=0; 这个用法很巧妙。

ptr[-1]其实表示的是ptr指针当前指向的上一个元素。指针名可当作数组名来用，在用作数组时，实际上是读取数组名，也就是一个地址，加上一个偏移地址。这个偏移地址的计算是就是：下标的值*元素类型的大小。这样就可打到要取得数的位置，不会判断下标是否是负数。

下面看一个例子：

#include <stdio.h> int add(int *a,int *b){    printf("a=%d, b=%d\n",*a,*b);    printf("a[-1]=%d, a[0]=%d,a[1]=%d\n",(a[-1]),a[0],a[1]);    printf("b[-1]=%d, b[0]=%d,b[1]=%d\n",(b[-1]),b[0],b[1]); } int main(void){     int a=2,b=3;     add(&a,&b); }

程序的输出结果是：

aaron@aaron:~/Documents/clearning$ ./a.out a=2, b=3a[-1]=0, a[0]=2,a[1]=3b[-1]=2, b[0]=3,b[1]=134513824

因为，gcc 对局部变量a,b是按顺序分配的，这样通过适当的偏移值就可访问变量。

而对于ＡＰＵＥ中的程序的用法，ptr的作用是递归深入时，加下一层的路径，当递归返回时，减去下一层的路径。　

附程序：

/*************************************************************************> File Name: 4_7TranverseDir.c> Author: aaron> Mail: [e-mail] > Created Time: Tue 18 Feb 2014 10:56:44 AM CST    递归降序遍历目录层次结构，并按文件类型计数。 ************************************************************************//* funciton type that is called for each filename */#include "apue.h"#include <dirent.h>#include <limits.h>typedef int Myfunc(const char *, const struct stat *,int);static Myfunc myfunc;static int    myftw(char *, Myfunc *);static int    dopath(Myfunc *);static long   nreg, ndir, nblk, nchr, nfifo, nslink, nsock, ntot;int main(int argc,char *argv[]){    int ret;        if(argc!=2)        err_quit("usage: ftw <starting-pathname>");    ret=myftw(argv[1],myfunc);   // do it all     ntot=nreg+ndir+nblk+nchr+nfifo+nslink+nsock;    if(ntot==0){        ntot=1;     //avoid divide by 0; print 0 for all counts     }    printf("regualr files =%7ld, %5.2f%%\n",nreg, nreg*100.0/ntot);    printf("directories   =%7ld, %5.2f%%\n",ndir, ndir*100.0/ntot);    printf("block special =%7ld, %5.2f%%\n",nblk, nblk*100.0/ntot);    printf("char special  =%7ld, %5.2f%%\n",nchr, nchr*100.0/ntot);    printf("FFIO          =%7ld, %5.2f%%\n",nfifo, nfifo*100.0/ntot);    printf("symbolic links=%7ld, %5.2f%%\n",nslink, nslink*100.0/ntot);    printf("sockets       =%7ld, %5.2f%%\n",nsock,nsock*100.0/ntot);    exit(ret);}/* Descend through the hierarchy, starting at "pathname". * The caller's func() is called for every file. */#define FTW_F  1        /* file other than directory */ #define FTW_D  2        /* directory */ #define FTW_DNR 3      /* directory that can't be read */ #define FTW_NS  4       /* file that we can't stat*/ static char *fullpath;         /* contains full pathname for every file */static int myftw(char *pathname, Myfunc *func){    int len;    fullpath=path_alloc(&len);              /* malloc's for PATH_MAX+1 bytes */    strncpy(fullpath,pathname,len);         /* protect against */    fullpath[len-1]=0;                      /* buff overrun */    return(dopath(func));}/* * Descend through the hierarchy, starting at "fullpath". * If "fullpath" is anything other than a directory, we lstat() it, * call func(), and return. For a directory, we call ourself  * recursively for each name in the directory. */ /*we return whatever func( ) returns */ static int dopath(Myfunc* func){    struct stat     statbuf;    struct dirent   *dirp;    DIR             *dp;    int             ret;    char            *ptr;        if(lstat(fullpath, &statbuf)<0)                    //stat error        return (func(fullpath,&statbuf,FTW_NS));    if(S_ISDIR(statbuf.st_mode)==0)                     // not a directory.        return (func(fullpath,&statbuf,FTW_F));/* * It's a directory. First call func() for the directory, * then process each filename in the directory. */    if((ret=func(fullpath,&statbuf,FTW_D))!=0)                                         return(ret);    ptr=fullpath+strlen(fullpath);                      // point to end of fullpath.    *ptr++='/';    *ptr=0;    if((dp=opendir(fullpath))==NULL)                /* can't read directory */        return(func(fullpath, &statbuf, FTW_DNR));        while((dirp=readdir(dp))!=NULL){        if(strcmp(dirp->d_name,".")==0||strcmp(dirp->d_name,"..")==0){            continue;        }        strcpy(ptr,dirp->d_name);               //append name after slash        if((ret=dopath(func))!=0)               // recursively            break;    }    ptr[-1]=0;                              // erase everything from slash onwards     if (closedir(dp)<0)                err_ret("can't close directory %s", fullpath);    return (ret);         } static int myfunc(const char *pathname, const struct stat *statptr, int type){    switch(type){        case FTW_F:        switch(statptr->st_mode & S_IFMT){            case S_IFREG:  nreg++;  break;            case S_IFBLK:  nblk++;  break;            case S_IFCHR:  nchr++;  break;            case S_IFIFO:  nfifo++;  break;            case S_IFLNK:  nslink++;  break;            case S_IFSOCK:  nsock++;  break;            case S_IFDIR:                 err_dump("for S_IFDIR for %s", pathname);   // directories should have type=FTW_D */        }        break;                case FTW_D:            ndir++;            break;        case FTW_DNR:            err_ret("can't read directory %s", pathname);            break;        case FTW_NS:            err_ret("stat error for %s", pathname);            break;        default:            err_dump("unknown type %d for pathname %s", type, pathname);    }    return 0;}char *path_alloc(int* size){    char *p = NULL;    if(!size) return NULL;    p = malloc(256);    if(p)    *size = 256;    else    *size = 0;    return p;}