UNIX for Advanced & Expert Users

       -L, --dereference
              when showing file information for a symbolic link, show information for  the
              file the link references rather than for the link itself

#include <sys/types.h>
#include <signal.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/stat.h>
#include <stdlib.h>
#include <limits.h>
#include <string.h>
#include <stddef.h>
#include <pwd.h>

#define ck_null(x) if((x)==NULL){perror(""); exit(EXIT_FAILURE);}
#define ck_ltz(x) if((x)==(-1)){perror(""); exit(EXIT_FAILURE);}
#ifndef PATH_MAX
#define PATH_MAX 255
#endif
#define DEPTH 16

size_t signal_arglen =0;
char signal_buffer[PATH_MAX+1]={0x0};

void sig_int(int signo)
{
	write(STDERR_FILENO,signal_buffer,signal_arglen);
	exit(EXIT_FAILURE);
}

void expand_path(char *path)
{
	char *prepend=malloc(2*PATH_MAX);
    char *ptr=path;    
	ck_null(prepend);

	if(memcmp(path,"~/",2)==0 || memcmp(path,"./",2)==0)
	{
		struct passwd *pw=NULL;
	    char tmp[PATH_MAX+1]={0x0};	    
	    
		ptr++;       
		switch(*path)
		{
			case '.':				
				sprintf(prepend,"%s%s", getcwd(tmp,PATH_MAX), ptr);
				break;
			case '~':  /* not normally executed ~/ expanded by shell */
				pw=getpwuid(getuid() );
				sprintf(prepend,"%s%s", pw->pw_dir, ptr);
				break;
			default:
				break;			
		}
		strcpy(path,prepend);
    }
    free(prepend);	
}


void get_link_info(char *path)
{
	char buf[PATH_MAX+1]={0x0};
	ssize_t buflen=readlink(path,buf,PATH_MAX);
	
	if(buflen>0)
	{
	    char *p=strrchr(path,'/');	
	    if(p==NULL) 
	        p=path;	   	   
	    buf[buflen]=0x0;
	    strcpy(p,buf);
    }	
}

void recurse_path(char elements[][PATH_MAX],char *path)
{
	int i=0;
    char *p=path;
    char *q=NULL;
        
    if(*p=='/') i=(-1); 
    if(strchr(path,'/')==NULL)
    {
    	strcpy(elements[0],path);	
    }
    else
    {  
		for(p=path;*p;p++)
		{
			if(*p=='/')			
				q=elements[++i];				
    	    *q++=*p;
		}	
     }
}

char  *get_path_elements(char *dest, char *path)
{
	char *working=calloc(1,PATH_MAX*2);
	int i=0;
	char elements[DEPTH][PATH_MAX]={0x0};
	
	ck_null(working);	
	recurse_path(elements,path);
	while(*elements[i])	
	{
		strcat(working,elements[i++]);
		get_link_info(working);	
	}
	strcpy(dest,working);
	free(working);
	return dest;
}

int main(int argc, char *argv[])
{
	char path[PATH_MAX*2]={0x0};
	char working_path[PATH_MAX*2]={0x0};
	int i=1;
    char *p=argv[i];

	sprintf(signal_buffer,"%s interrupt\n",argv[0]);
    signal_arglen=strlen(signal_buffer);	
	signal(SIGINT,sig_int);
	if (argc >1)
	{
	    while(*p)
	    {
	    	strcpy(working_path,p);
	    	expand_path(working_path);
	    	printf("%s\n",
	    	        get_path_elements(path, working_path));
	    	i++;
	    	p=argv[i];
	    }
	}
	else
	{
		while(fgets(working_path,sizeof(working_path),stdin)!=NULL)
		{
			expand_path(working_path);
	    	printf("%s\n",
	    	        get_path_elements(path, working_path));		
		}
	}
	return 0;
}