How to check if file exists ?

[jabka]

some time ago i wrote a small post about how to check if file exists
the code is using open() but today i got a comment that i should use stat() so i read the man file and tried the next.
The question is :
is it good enough explanation and is it the right approach ?

/************************************************************************
 *
 * Purpose: This program will demonstrate the use of stat() on linux 
 *
 * Author:  Jabka Atu
 *
 * Date:    13 October 2007
 *          
 *
 * Notes:   I used  M J Leslie's orignal example to learn and 
 *          used his file as skelton for this
 *
 *
 ************************************************************************/


#include <sys/stat.h>			/* declare the 'stat' structure	*/
#include <sys/types.h>			
#include <unistd.h>		
#include <stdio.h>			    /* printf			*/
#include <time.h>

#define  FILENAME "/tmp"		/* PUT YOUR FILE NAME HERE	*/

/************************************************************************/



void file_stat(char * filename);


main()
{
  file_stat(FILENAME);
}



void file_stat(char * filename)
{
  struct stat stat_p;		/* 'stat_p' is a pointer to a structure
				 			 * of type 'stat'.  			*/

							/* Get stats for file and place them in
				 			 * the structure.			*/
  /*
     To use Macro you should do the next:
     write the macro and as it argument use the defined types
     for example:
     Macro = S_ISDIR(mode)
     Argumant = mode
     usage:
     S_ISDIR(stat_p.st_mode); // st_mode is a predifined field
 
  	 now the MACROs for state are (thnx to http://linux.die.net/man/2/stat )
  
  
  		S_ISREG(m)
    		is it a regular file? 
		S_ISDIR(m)
    		directory? 
		S_ISCHR(m)
    		character device? 
		S_ISBLK(m)
	    	block device? 
		S_ISFIFO(m)
    		FIFO (named pipe)? 
		S_ISLNK(m)
    		symbolic link? (Not in POSIX.1-1996.) 
		S_ISSOCK(m)
  			socket? (Not in POSIX.1-1996.)
   */  
  
  stat (filename, &stat_p);
  printf("This will print several tests on your file \n");
  printf("The answer is in boolean \n 0 - No \n 1 - yes\n");
			
  printf("\tIs it \n\texsits ?\t\t%d \n",S_ISREG(stat_p.st_mode));
  printf("\tDirectory ?     \t%d \n",S_ISDIR(stat_p.st_mode));
  printf("\tCharecter Device  \t%d\n",S_ISCHR(stat_p.st_mode));
  printf("\tblock device?   \t%d\n",S_ISBLK(stat_p.st_mode));
  printf("\tFIFO (named pipe)? \t%d \n",S_ISFIFO(stat_p.st_mode));
  printf("\tsocket?     \t\t%d\n",S_ISSOCK(stat_p.st_mode));
}

/************************************************************************/

[Salem]

Well it depends how portable you want the answer to be.
If you're OK restricting yourself to POSIX systems, then stat() should be fine.

[dwks]

If you want ANSI-compatible code, the closest you can get is to try opening the file for reading.

[jabka]

so i should work with fopen() and work with errno ?

[dwks]

I don't think errno is ANSI standard. You can just check to see if fopen() returned NULL or not.

Of course, this method has many flaws. You might not have the permissions to open the file, for example, even though it may exist.

[jabka]

i goggled a bit and asked on irc (and got a stfu) and found in msdn the use of _stat()

also im trying to figure if fopen will set errno as i needed .
so i guess errno is ansi

[dwks]

http://www.opengroup.org/onlinepubs/...ons/fopen.html

[Prelude]

>I don't think errno is ANSI standard.
IIRC, errno isn't required to be set by anything in the stdio library except fsetpos, fgetpos, and ftell.

[cpjust]

If you use open() to check if a file exists and you want to create it if it doesn't exist, you'll probably want to use the O_EXCL flag in the call to open() to prevent a race condition.

[dwks]

If you're using open(), you're using POSIX code, and you might as well use stat(). Unless you want to use the file afterwards or something.

[jabka]

Howdy,..

after a few hints and stfu's i found a solution (or think i did ).

using perror(NULL) to print the error (it will check only the lest errno error but it is a good begning) and using fopen() (it is an ANSI ):

/*
 *
 * An ANSI solution for checking if file exists.
 *
 * Thnx to dwks , Tzafrir , salem , Mika0x65 and more for the guidance ..
 */
 
#include <stdio.h>
#include <errno.h>

#define FILENAME "myfile"



int main(){
	FILE *fp = NULL;
        fp = fopen(FILENAME,"r+");
	if (fp == NULL)
	{
		perror(NULL); //will print the text relative to errno
	}
	return 0;
}

i don't have windows to check if it will work and profuce the correct error (nonexistent and no privileges )

[matsp]

I'm pretty sure that "extern errno" is not correct. It should be fine to just include <errno.h> and nothing else. Saying something like "extern errno" won't work when errno is a macro that replaces with a function call, uses an array index by thread ID or a dig into some TLS (Thread Local Storage) variable.

The reason errno isn't just simply a global variable is that people who use multithreading want errno to still work "as expected", and if you have ONE global variable used by multiple threads, it needs to have locks and protection to prevent one function call from overwriting the result with another function call in another thread. That would be complex and convoluted (and basicly every function that uses errno may need this locking mechanism, since it's very difficult to figure out what particular scenarios the lock is REALLY necessary and which cases are not).

So, instead the implementation inside the C-library separates a different errno per thread in some way or another. How this is done will vary depending on the OS and compiler vendor.

--
Mats

[jabka]

noted thank you .
so i actualy don't need to do extern errno in any C or C++ code ?

[Salem]

Including errno.h will do the right thing for you.

[cpjust]

If you're using open(), you're using POSIX code, and you might as well use stat(). Unless you want to use the file afterwards or something.

stat() is fine if you aren't actually trying to create a file, but if you want to create a file only if it doesn't already exist, stat() won't help you because you still need to call open() after stat() and in between those calls someone else could quickly slip in and create the file which could lead to security issues...