basename and dirname changes the value of argument???

Applications Programming
1

Hi

I faced with some interesting behavior of basename and dirname functions from libgen.h: they changes the value of argument! Here is the declaration:

char  *basename(char *);
char  *dirname(char *);

It makes some tiresome to use them... I am new to C and maybe I do something wrong, but to make these functions work I have to create some temporary variables... Here is the code:

...
#include <libgen.h>
...
    char ef[PATH_MAX], eftmp[PATH_MAX], efbn[PATH_MAX] ... ;
...
    strcpy(eftmp, ef);
    strcpy(efbn, basename(eftmp));
...

In this example eftmp before basename() call doesn't equal to eftmp after! For me it looks a little bit strange...

If anyone share some experience on using them in more convenient fashion it will be great! Any comments also appreciated

Thanks in advance

2

From the man-page:

       Both dirname() and basename() may modify the contents of  path,  so  it
       may be desirable to pass a copy when calling one of these functions.

       These  functions  may  return  pointers  to statically allocated memory
       which may be overwritten by subsequent calls.  Alternatively, they  may
       return  a  pointer to some part of path, so that the string referred to
       by path should not be modified or freed until the pointer  returned  by
       the function is no longer required.

Meaning: SUSv2 only defined that the functions should return a pointer to a character array, but it doesn't define whether that pointer might be overwritten if the argument passed may be changed or not, but leaves that to the implementation.

You could change your code using malloc(), so that you wouldn't have to allocate memory statically but only if needed.

3

pludi
Thanks a lot for your answer! I'm using SunOS 5.10 and its manual pages say nothing about modifying function's arguments... So AFAICS, calling these function with duplicate arguments is the one way to preserve value of original variable...

I consider to start using malloc and stuff later, when I get some experience on pointer operations...

cheers

4

What platform are you on...both HPUX and AIX state in the man pages of basename/dirname saying that these functions may modify the pathname argument so it's a good idea to use strdup on the pathname before passing it as an argument to those functions.

5

Well, you can check it here - Solaris manual pages say nothing about changing the value of argument...

I still need to get some experience on pointer operations... This code returns some warnings:

#include <libgen.h>
...
    char ef[PATH_MAX], efbn[PATH_MAX] ... ;
...
    strcpy(efbn, basename(strdup(ef)));

check_end_file.c: In function `check_end_file':
check_end_file.c:46: warning: implicit declaration of function `strdup'
check_end_file.c:46: warning: passing arg 1 of `basename' makes pointer from integer without a cast

If basename(3C) takes pointer and strdup(3C) returns it, why does it "makes pointer from integer"?.. If I got it right, I need to pass to basename function a pointer to the beginning of some memory area, which hold char array, but strdup returns the same thing - "a pointer to a new string that is a duplicate of the string pointed to by s1"... I'm a little bit confused...
And what does mean "impicit declaration..."?

Thanks in advance

---------- Post updated at 01:12 AM ---------- Previous update was at 12:52 AM ----------

I've got it all. No further explanaition needed

thanks all

---------- Post updated at 01:30 AM ---------- Previous update was at 01:12 AM ----------

So after declaration of basename and strdup these warnings disappeared, but why some functions, returning non-integer values, such as readdir(3C) don't need to be declared?..