第4章 文件系统：编写pwd

/* * spwd.c: a simplified version of pwd * * starts in current directory an recursively * climbs up to root of filesystem, prints top part * then prints current part * * uses readdir() to get info about each thing * * bug: prints an empty string if run from "/" * */#include <stdio.h>#include <sys/types.h>#include <sys/stat.h>#include <dirent.h>ino_t get_inode(char *);void printpathto(ino_t);void inum_to_name(ino_t, char *, int);int main(){    printpathto(get_inode("."));        /* print path to here */    putchar('\n');              /* then and newline */    return 0;}void printpathto(ino_t this_inode)    /*     * prints path leading down to an object with this inode     * kindof recursive     */{    ino_t my_inode;    char its_name[BUFSIZ];    if (get_inode("..") != this_inode)    {        chdir("..");                    /* up one dir */        inum_to_name(this_inode, its_name, BUFSIZ); /* get its name */        my_inode = get_inode(".");          /* print head */        printpathto(my_inode);              /* recursively */        printf("/%s", its_name);            /* now print */        /* name of this */    }}void inum_to_name(ino_t inode_to_find, char *namebuf, int buflen)    /*     * looks through current directory for a file with this inode     * number and copies its name into namebuf     */{    DIR *dir_ptr;               /* the directory */    struct dirent *direntp;         /* each entry */    dir_ptr = opendir(".");    if(dir_ptr == NULL) {        perror(".");        exit(1);    }    /*     * search directory for a file with specified inum     */    while ((direntp = readdir(dir_ptr)) != NULL)        if (direntp->d_ino == inode_to_find)        {            strncpy(namebuf, direntp->d_name, buflen);            namebuf[buflen-1] = '\0';/* just in case */            closedir(dir_ptr);            return ;        }    fprintf(stderr, "error looking for inum %d\n", inode_to_find);    exit(1);}ino_t get_inode(char *fname)    /*     * returns inode number of the file     */{    struct stat info;    if (stat(fname, &info) == -1) {        fprintf(stderr, "Cannot stat ");        perror(fname);        exit(1);    }    return info.st_ino;}