chdir: support stream and fd argument via fchdir.

* configure: new test for fchdir resulting in
HAVE_FCHDIR.

* sysif.c (get_fd): Define for HAVE_FCHDIR also.
(chdir_wrap): If HAVE_FCHDIR, handle non-string
arguments via fchdir, with help of get_fd.

* txr.1: Documented.

3 files changed, 47 insertions, 8 deletions

diff --git a/configure b/configure
index 720bcd77..51f7723c 100755
--- a/configure
+++ b/configure
@@ -2860,6 +2860,22 @@ else
   printf "no\n"
 fi
 
+printf "Checking for fchdir ... "
+cat > conftest.c <<!
+#include <unistd.h>
+
+int main(void)
+{
+  int e = fchdir(1);
+  return 0;
+}
+!
+if conftest ; then
+  printf "yes\n"
+  printf "#define HAVE_FCHDIR 1\n" >> config.h
+else
+  printf "no\n"
+fi
 
 printf "Checking for log2 ... "
 
diff --git a/sysif.c b/sysif.c
index 3b48af0b..e8fa1dac 100644
--- a/sysif.c
+++ b/sysif.c
@@ -452,7 +452,7 @@ static val mkdir_wrap(val path, val mode)
 }
 #endif
 
-#if HAVE_CHMOD || HAVE_CHOWN || HAVE_SYS_STAT || HAVE_FILE_STAMP_CHANGE
+#if HAVE_CHMOD || HAVE_CHOWN || HAVE_FCHDIR || HAVE_SYS_STAT || HAVE_FILE_STAMP_CHANGE
 static int get_fd(val stream, val self)
 {
   val fd_in = if3(integerp(stream), stream, stream_fd(stream));
@@ -576,9 +576,19 @@ static val ensure_dir(val path, val mode)
 static val chdir_wrap(val path)
 {
   val self = lit("chdir");
-  char *u8path = utf8_dup_to(c_str(path, self));
-  int err = chdir(u8path);
-  free(u8path);
+  int err;
+
+#if HAVE_FCHDIR
+  if (!stringp(path)) {
+    int fd = get_fd(path, self);
+    err = fchdir(fd);
+  } else
+#endif
+  {
+    char *u8path = utf8_dup_to(c_str(path, self));
+    err = chdir(u8path);
+    free(u8path);
+  }
 
   if (err < 0) {
     int eno = errno;
diff --git a/txr.1 b/txr.1
index 83b82a28..dde521b6 100644
--- a/txr.1
+++ b/txr.1
@@ -72513,17 +72513,30 @@ instead of
 
 .coNP Function @ chdir
 .synb
-.mets (chdir << path )
+.mets (chdir >> { path | < stream | << fd })
 .syne
 .desc
 .code chdir
-changes the current working directory to
-.metn path ,
-and returns
+changes the current working directory to the object specified
+by the argument, and returns
 .metn t ,
 or else throws an exception of type
 .codn file-error .
 
+If the argument is a string, it is interpreted as a
+.metn path ,
+in which case the POSIX
+.code chdir
+function is used. If the argument is a
+.meta stream
+then an integer file descriptor is retrieved from that stream using the
+.code fileno
+function. That descriptor can be specified directly as a
+.meta fd
+argument. In the case of these these two argument types, the
+.code fchdir
+function is used.
+
 .coNP Function @ pwd
 .synb
 .mets (pwd)