pwd.c

#include <stdio.h>
#include <stdlib.h>
#include <dirent.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <string.h>
#include <unistd.h>

#define MAX_DIR_DEPTH 256

ino_t get_ino_byname( char* );
char* find_name_byino( ino_t ino );

int main( int argc, char* argv[] ) {
    char* dir_stack[MAX_DIR_DEPTH];
    unsigned current_depth = 0;

    while( 1 ) {
        ino_t current_ino = get_ino_byname( "." );
        ino_t parrent_ino = get_ino_byname( ".." );

        if( current_ino == parrent_ino ) {
            break;
        }

        chdir( ".." );
        dir_stack[current_depth++] = find_name_byino( current_ino );
        if( current_depth >= MAX_DIR_DEPTH ) {
            fprintf( stderr, "Directory tree is too deep.\n" );
            exit( -1 );
        }
    }

    int i = 0;
    for( i = current_depth - 1; i >= 0; i-- ) {
        fprintf( stdout, "/%s", dir_stack[i] );
    }
    fprintf( stdout, "%s\n", current_depth == 0 ? "/" : "" );

    return 0;
}

ino_t get_ino_byname( char* filename ) {
    struct stat file_stat;
    if( 0 != stat( filename, &file_stat ) ) {
        perror( "stat" );
        exit( -1 );
    }
    return file_stat.st_ino;
}

char* find_name_byino( ino_t ino ) {
    DIR* dp = NULL;
    struct dirent* dptr = NULL;
    char* filename = NULL;
    if( NULL == ( dp = opendir( "." ) ) ) {
        fprintf( stderr, "Can not open Current Directory\n" );
        exit( -1 );
    }
    else {
        while( NULL != ( dptr = readdir( dp ) ) ) {
            if( dptr->d_ino == ino ) {
                filename = strdup( dptr->d_name );
                break;
            }
        }
        closedir( dp );
    }

    return filename;
}