Where is PATH_MAX defined in Linux?

Question

Which header file should I invoke with #include to be able to use PATH_MAX as an int for sizing a string?

I want to be able to declare:

char *current_path[PATH_MAX];

But when I do so my compiler (Clang/LLVM on Linux) issues the following error:

recursive_find6.c:29:20: error: use of undeclared identifier 'PATH_MAX'
char *current_path[PATH_MAX];
                   ^

I tried doing a google search but still no luck.

#include <limits.h> Does NOT fix the problem/error.

Am I also correct that the value of PATH_MAX is an int?

Answer 1

This is a good question. I am doing some simple C programming and encountered this problem. On your particular Linux/Unix, to to /usr/include directory, here are all the header files for your system.

find . -name "*.h" | xargs grep PATH_MAX 

You should see several headers defining PATH_MAX. And you can see that this value is defined differently in different places. Here is a listing from my Ubuntu (I also manually removed some false positive hits from the grep program).

./x86_64-linux-gnu/bits/posix1_lim.h:#define _POSIX_PATH_MAX      256
./X11/InitialI.h:#ifndef PATH_MAX
./X11/InitialI.h:#define PATH_MAX 512
./X11/InitialI.h:#ifndef PATH_MAX
./X11/InitialI.h:#define PATH_MAX MAXPATHLEN
./X11/InitialI.h:#define PATH_MAX 1024
./X11/Xos.h:#  define PATH_MAX 4096
./X11/Xwindows.h:#if defined(WIN32) && (!defined(PATH_MAX) || PATH_MAX < 1024)
./X11/Xwindows.h:# undef PATH_MAX
./X11/Xwindows.h:# define PATH_MAX 1024
./X11/Xosdefs.h:#  ifndef PATH_MAX
./X11/Xosdefs.h:#   define PATH_MAX 4096
./X11/Xosdefs.h:#  ifndef PATH_MAX
./X11/Xosdefs.h:#   define PATH_MAX 1024
./X11/extensions/XKBsrv.h:#define   PATH_MAX MAXPATHLEN
./X11/extensions/XKBsrv.h:#define   PATH_MAX 1024
./python2.7/osdefs.h:#ifndef PATH_MAX
./python2.7/osdefs.h:#define PATH_MAX MAXPATHLEN
./python2.7/osdefs.h:#if defined(PATH_MAX) && PATH_MAX > 1024
./python2.7/osdefs.h:#define MAXPATHLEN PATH_MAX
./linux/limits.h:#define PATH_MAX        4096   /* # chars in a path name including nul */
./linux/btrfs.h:#define BTRFS_INO_LOOKUP_PATH_MAX 4080
./linux/un.h:#define UNIX_PATH_MAX  108

Looks like /linux/limits.h has the largest defined number. I do concern about the portability of this file. So to be safe you might simply defined this MACRO yourself with a different name, say PATHLEN (4080 is long enough for most practical situations). If this is for a large software package, you also have control. This is my opinion.

Answer 2

Its in linux/limits.h.
#define PATH_MAX 4096 /* # chars in a path name including nul */

#include <linux/limits.h>

char current_path[PATH_MAX];

PATH_MAX has some flaws as mentioned in this blog (thanks paulsm4)

Answer 3

Be aware, that it is still unclear if PATH_MAX defines a maximum length with or without a trailing nul byte. It may be one or the other on different operating systems. If you can't or don't want to check which case it is during compilation, it's safer to force artificial limit of PATH_MAX - 1. Better safe than sorry. (Obviously, you still need to reserve at least PATH_MAX bytes of memory to buffer the string.)