reject an empty entry in the -u list

Makefile.am

@@ -132,6 +132,7 @@ DIALECT_NEUTRAL_TESTS = tests/case-00-hello.bash \
 	tests/case-20-handle-missing-files.bash \
 	tests/case-20-offset-field.bash \
 	tests/case-20-repeat-count.bash \
+	tests/case-20-uid-empty-entry.bash \
 	tests/case-21-exit-Q-status.bash \
 	tests/case-22-empty-process-name.bash \
 	tests/case-23-multi-address-hostname.bash \

src/arg.c

@@ -2019,7 +2019,7 @@ int enter_uid(struct lsof_context *ctx, /* context */
     char *s, *st;
     uid_t uid;
 
-    if (!us) {
+    if (!us || !*us) {
         (void)fprintf(stderr, "%s: no UIDs specified\n", Pn);
         return (1);
     }
@@ -2063,6 +2063,17 @@ int enter_uid(struct lsof_context *ctx, /* context */
             s++;
         if (j)
             continue;
+        if (!lnml) {
+
+            /*
+             * An empty entry (e.g. from ",99" or "^") would otherwise be
+             * taken as UID 0.
+             */
+            (void)fprintf(stderr, "%s: empty -u UID or login name: ", Pn);
+            safestrprt(us, stderr, 1);
+            err = 1;
+            continue;
+        }
         if (nn) {
             lnm[lnml++] = '\0';
             if ((pw = getpwnam(lnm)) == NULL) {

tests/case-20-uid-empty-entry.bash

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+# An empty entry in a -u list must not be taken as UID 0.
+source tests/common.bash
+
+{
+    for spec in "" ",99999" "^"; do
+        output=$($lsof -u "$spec" -F u 2>/dev/null)
+        status=$?
+        if echo "$output" | grep -q '^u0$'; then
+            echo "lsof -u $spec selected UID 0" >> $report
+            exit 1
+        fi
+        if [ $status -eq 0 ]; then
+            echo "lsof -u $spec should fail" >> $report
+            exit 1
+        fi
+    done
+    exit 0
+} > $report 2>&1