Added an extended version. My favorite.

.gitignore

@@ -2,4 +2,5 @@ chkgrp
 chkgrp-min
 chkgrp-native
 chkgrp-native-min
+chkgrp-native-min-extended
 

Makefile

@@ -3,22 +3,26 @@ all:
 	$(CC) -Wall -o ./chkgrp-min ./chkgrp-min.c
 	$(CC) -Wall -o ./chkgrp-native ./chkgrp-native.c
 	$(CC) -Wall -o ./chkgrp-native-min ./chkgrp-native-min.c
+	$(CC) -Wall -o ./chkgrp-native-min-extended ./chkgrp-native-min-extended.c
 
 clean:
 	rm -f ./chkgrp
 	rm -f ./chkgrp-min
 	rm -f ./chkgrp-native
 	rm -f ./chkgrp-native-min
+	rm -f ./chkgrp-native-min-extended
 
 install:
 	install -g 0 -o 0 -m 0655 ./chkgrp /usr/bin/
 	install -g 0 -o 0 -m 0655 ./chkgrp-min /usr/bin/
 	install -g 0 -o 0 -m 0655 ./chkgrp-native /usr/bin/
 	install -g 0 -o 0 -m 0655 ./chkgrp-native-min /usr/bin/
+	install -g 0 -o 0 -m 0655 ./chkgrp-native-min-extended /usr/bin/
 
 uninstall:
 	rm -f /usr/bin/chkgrp
 	rm -f /usr/bin/chkgrp-min
 	rm -f /usr/bin/chkgrp-native
 	rm -f /usr/bin/chkgrp-native-min
+	rm -f /usr/bin/chkgrp-native-min-extended
 

chkgrp-native-min-extended.c

@@ -0,0 +1,102 @@
+/**
+ * chkgrp-min-native-extended is a program that checks if a user is a member
+ * of a group.
+ *
+ * Author: Johannes Findeisen <you@hanez.org> - 2025
+ * Homepage: https://git.xw3.org/hanez/chkgrp
+ * License: Apache-2.0 (see LICENSE)
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <ctype.h>
+
+#define MAX_LINE 1024
+#define MAX_NAME 256
+
+int main(int argc, char *argv[])
+{
+    if (argc != 3)
+        return 2;
+
+    const char *username = argv[1];
+    const char *groupname = argv[2];
+
+    if (strlen(username) > MAX_NAME || strlen(groupname) > MAX_NAME)
+        return 2;
+
+    // Step 1: Find user in /etc/passwd to get their primary GID
+    FILE *passwd = fopen("/etc/passwd", "r");
+    if (!passwd)
+        return 2;
+
+    char line[MAX_LINE];
+    int user_gid = -1;
+    while (fgets(line, sizeof(line), passwd)) {
+        char *u = strtok(line, ":");
+        if (!u || strcmp(u, username) != 0)
+            continue;
+
+        strtok(NULL, ":"); // skip password
+        strtok(NULL, ":"); // skip UID
+        char *gid_str = strtok(NULL, ":");
+        if (!gid_str)
+            break;
+
+        user_gid = atoi(gid_str);
+        break;
+    }
+    fclose(passwd);
+    if (user_gid < 0)
+        return 2;
+
+    // Step 2: Find group in /etc/group and check membership or GID match
+    FILE *group = fopen("/etc/group", "r");
+    if (!group)
+        return 2;
+
+    int found = 0;
+    while (fgets(line, sizeof(line), group)) {
+        char *grp = strtok(line, ":");
+        if (!grp || strcmp(grp, groupname) != 0)
+            continue;
+
+        strtok(NULL, ":"); // skip password
+        char *gid_str = strtok(NULL, ":");
+        if (!gid_str)
+            break;
+
+        int group_gid = atoi(gid_str);
+        if (group_gid == user_gid) {
+            puts("Yes");
+            fclose(group);
+            return 0;
+        }
+
+        char *members = strtok(NULL, ":\n");
+        if (!members)
+            break;
+
+        char *m = strtok(members, ",");
+        while (m) {
+            if (!strcmp(m, username)) {
+                puts("Yes");
+                fclose(group);
+                return 0;
+            }
+            m = strtok(NULL, ",");
+        }
+
+        found = 1;
+        break;
+    }
+
+    fclose(group);
+    if (!found)
+        return 2;
+
+    puts("No");
+    return 1;
+}
+