On linux and macOS, directories can be nested to seemingly arbitrary depth, as demonstrated by the following C program. However, on macOS but not on linux, there seems to be a hard limit on the nesting level returned by getcwd, specifically a nesting level of 256. When that limit is reached, getcwd returns ENOENT, a rather strange error code. Where does this limit come from? Is there a way around it?
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/stat.h>
#include <sys/types.h>
void fail(char *msg) { perror(msg); exit(1); }
void create_nested_dirs(int n) {
int i;
char name[10];
char cwd[10000];
if (chdir("/tmp") < 0) fail("chdir(\"/tmp\")");
for (i=2; i<=n; i++) {
sprintf(name, "%09d", i);
printf("%s\n",name);
if (mkdir(name, 0777) < 0 && errno != EEXIST) fail("mkdir");
if (chdir(name) < 0) fail("chdir(name)");
if (getcwd(cwd, sizeof(cwd)) == NULL) fail("getcwd");
printf("cwd = \"%s\" strlen(cwd)=%d\n", cwd, strlen(cwd));
}
}
int main() {
long ret = pathconf("/", _PC_PATH_MAX);
printf("PATH_MAX is %ld\n", ret);
create_nested_dirs(300);
return 0;
}
Update
The above program was updated to print the value returned by pathconf("/", _PC_PATH_MAX) and to print the length of the path returned by getcwd.
On my machine running macOS Mojave 10.14, the PATH_MAX is 1024 and the longest string correctly returned by getcwd is 2542 characters long. Then a 2552 character long directory of nesting depth 256 is created by mkdir and then after a successful chdir to that directory a getcwd fails with ENOENT.
If the sprintf(name, "%09d", i); is changed to sprintf(name, "%03d", i); the paths are considerably shorter but the getcwd still fails when the directory nesting depth reaches 256.
So the limiting factor here is the nesting depth, not PATH_MAX.
My understanding of the source code here is that the meat of the work is done by the call fcntl(fd, F_GETPATH, b) so the problem may be in fcntl.
