Checking whether a file is symbolic link or not

rajesh_b · 01-06-2005, 06:57 AM

Hi i am writing a small function in c which will check whether the given file is symbolic link or not. But i m not getting correct result for symbolic link files as well as regular files. it is always showing as they r not symbolic files. when when i used readlink function to read the link it is giving the correct file name to which it points. and one more doubt is that if
S_ISLNK() macro can be used for to test whether a directory is symbolic link or it can be used to check only whether a file is symbolic link or not.
Here is my code:

#include <dirent.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include<stdio.h>
#include<stdlib.h>
int main()
{
char filename[100];
int res=-1;
int is_symlink(char *filename);
printf("Enter a file name:");
scanf("%s",filename);

}
int is_symlink(char *filename)
{
struct stat p_statbuf;
if(stat(filename,&p_statbuf)==0)
{

if(S_ISLNK(p_statbuf.st_mode)==1)
printf("it is symbolic link \n");

}
}

can any body help me what's wrong in the code.
Thanks in advance

Rajesh.

jlliagre · 01-06-2005, 07:55 AM

use lstat to get the real attributes, instead of stat, which returns the attributes of the file linked to.

Hko · 01-06-2005, 08:01 AM

Two mistakes in your program:

You included the prototype for the is_symlink() function in main(), but you didn't call it at all. So the is_symlink() never runs, leaving the only thing the program does to just read the file from user-input, without doing anything further. Just add a line "is_symlink(filename);" at the end of main() to fix this.
You call stat() to check if the entered file is a symlilnk. But stat() will follow the link and report information about the file a symlink points to. So stat() will never tell it's a symlink. For this purpose you need to use lstat(). (see the man page for stat, fstat, lstat)

Two other remarks:

If you would also print a message saying it's not a symlink, you would have noticed the is_symlink() function never executes. Even if that's not what you want, it makes sense when first trying to run/debug your program.

Using scanf() on a string of max 99 characters (plus the terminating '\0' character makes 100) is "dangerous". If the user enters more that a 100 characters, scanf() writes beyond the buffer (buffer-overflow bug), and your program will most probably crash with a segmentation fault. If not it will have a security bug at least.

Here's a changede version of your prograam with all 4 issues mentioned above fixed.

Code:

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

#define MAXFILENAMELEN 100

/* Prototype for function */
void is_symlink(const char *filename);

int main()
{
    int len;
    char filename[MAXFILENAMELEN];

    printf("Enter a file name: ");
    fgets(filename, MAXFILENAMELEN, stdin);
    
    /* Remove the newline from the end if it' there */
    len = strlen(filename);
    if (filename[len-1] == '\n') {
        filename[len-1] = '\0';
    }

    /* Call function to test if it is a symlink */
    is_symlink(filename);

    return 0;
}

void is_symlink(const char *filename)
{
    struct stat p_statbuf;

    if (lstat(filename, &p_statbuf) < 0) {  /* if error occured */
        perror("calling stat()");
        exit(1);  /* end progam here */
    }

    if (S_ISLNK(p_statbuf.st_mode) == 1) {
        printf("%s is a symbolic link\n", filename);
    } else {
        printf("%s is NOT a symbolic link\n", filename);
    }
}

rajesh_b · 01-06-2005, 10:07 AM

Hi
thanx for u r information. i solve the problem through u r help

Rajesh.