Am 17.07.2014 14:45, schrieb Nguyễn Thái Ngọc Duy:
> This array 'cwd' is used to store the result from getcwd() and chdir()
> back. PATH_MAX is the right constant for the job.
PATH_MAX may be better than 1024, but there can't really be a correct
constant. The actual limit depends on the file system.
> On systems with
> longer PATH_MAX (eg. 4096 on Linux), hard coding 1024 fails stuff,
> e.g. "git init".
Out of curiosity, I just created a path with over 130000 characters on
Linux. It's not actually usable but it shows that 4096 is not a real
limit on Linux. Here's how I created that insane path:
a=1234567890
x=$a$a$a$a$a
y=$x$x$x$x$x
cd /tmp
while true
do
mkdir $y || break
cd $y || break
done
pwd >/tmp/y
cd /tmp
wc -c <y
Another experiment with the program listed below shows that getcwd() on
Linux works fine with such paths if the provided buffer is large
enough, even though PATH_MAX is 4096:
#include <limits.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
int main(int argc, char **argv)
{
char cwd[200000];
printf("PATH_MAX = %d\n", PATH_MAX);
if (getcwd(cwd, sizeof(cwd)))
printf("strlen(getcwd()) = %zu\n", strlen(cwd));
return 0;
}
> Make it static too to reduce stack usage.
Why is that needed? Is recursion involved? (Didn't find any in the
function itself after a very brief look.)
There is get_current_dir_name(), a GNU extension that allocates the
necessary memory. Perhaps we can use it instead and provide a
compatibility implementation based on getcwd() for platforms that don't
have it?
But then there's also this advice from the getcwd(3) manpage on OpenBSD:
"These routines have traditionally been used by programs to save the
name of a working directory for the purpose of returning to it. A much
faster and less error-prone method of accomplishing this is to open the
current directory (.) and use the fchdir(2) function to return."
So, how about something like this?
---
abspath.c | 15 ++++++++++-----
1 file changed, 10 insertions(+), 5 deletions(-)
diff --git a/abspath.c b/abspath.c
index ca33558..7fff13a 100644
--- a/abspath.c
+++ b/abspath.c
@@ -38,10 +38,10 @@ static const char *real_path_internal(const char *path, int
die_on_error)
/*
* If we have to temporarily chdir(), store the original CWD
- * here so that we can chdir() back to it at the end of the
+ * here so that we can fchdir() back to it at the end of the
* function:
*/
- char cwd[1024] = "";
+ int cwd_fd = -1;
int buf_index = 1;
@@ -80,7 +80,9 @@ static const char *real_path_internal(const char *path, int
die_on_error)
}
if (*buf) {
- if (!*cwd && !getcwd(cwd, sizeof(cwd))) {
+ if (cwd_fd < 0)
+ cwd_fd = open(".", O_RDONLY);
+ if (cwd_fd < 0) {
if (die_on_error)
die_errno("Could not get current
working directory");
else
@@ -142,8 +144,11 @@ static const char *real_path_internal(const char *path,
int die_on_error)
retval = buf;
error_out:
free(last_elem);
- if (*cwd && chdir(cwd))
- die_errno("Could not change back to '%s'", cwd);
+ if (cwd_fd >= 0) {
+ if (fchdir(cwd_fd))
+ die_errno("Could not change back to the original
working directory");
+ close(cwd_fd);
+ }
return retval;
}
--
2.0.2
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