Re: Some questions

[Patrick Oppenlander <patrick.oppenlander@gmail.com>]

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On Tue, May 1, 2018 at 1:31 AM, Rich Felker <dalias@libc.org> wrote:
> On Mon, Apr 30, 2018 at 03:29:39PM +1000, Patrick Oppenlander wrote:
>> Actually, my biggest issue with getcwd is that it allocates a PATH_MAX
>> sized buffer on the stack. That's painful on deeply embedded stuff.
>
> That's unrelated, and could/should be fixed by the attached patch I
> think.

Unfortunately that fails to build on arm with:

src/unistd/getcwd.c: In function 'getcwd':
src/unistd/getcwd.c:25:1: error: r7 cannot be used in asm here

I was also having a go at resolving the stack & the buffer size issue
and came up with the attached (untested) patch.

Patrick

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diff --git a/src/unistd/getcwd.c b/src/unistd/getcwd.c
index 103fbbb5..306dbc4f 100644
--- a/src/unistd/getcwd.c
+++ b/src/unistd/getcwd.c
@@ -3,17 +3,10 @@
 #include <limits.h>
 #include <string.h>
 #include "syscall.h"
+#include "libc.h"
 
-char *getcwd(char *buf, size_t size)
+static char *do_getcwd(char *buf, size_t size)
 {
-	char tmp[PATH_MAX];
-	if (!buf) {
-		buf = tmp;
-		size = PATH_MAX;
-	} else if (!size) {
-		errno = EINVAL;
-		return 0;
-	}
 	long ret = syscall(SYS_getcwd, buf, size);
 	if (ret < 0)
 		return 0;
@@ -21,5 +14,37 @@ char *getcwd(char *buf, size_t size)
 		errno = ENOENT;
 		return 0;
 	}
-	return buf == tmp ? strdup(buf) : buf;
+	return buf;
+}
+
+static char *getcwd_glibc(size_t size)
+{
+	char tmp[PATH_MAX];
+	if (!do_getcwd(tmp, sizeof tmp))
+		return 0;
+	size_t len = strlen(tmp) + 1;
+	if (!size)
+		size = len;
+	else if (size < len) {
+		errno = ERANGE;
+		return 0;
+	}
+	char *buf = malloc(size);
+	if (!buf) {
+		errno = ENOMEM;
+		return 0;
+	}
+	memcpy(buf, tmp, len);
+	return buf;
+}
+
+char *getcwd(char *buf, size_t size)
+{
+	if (!buf)
+		return getcwd_glibc(size);
+	if (!size) {
+		errno = EINVAL;
+		return 0;
+	}
+	return do_getcwd(buf, size);
 }