Merged with chkgrp and renamed to chkusr.

chkgrp.c

@@ -0,0 +1,77 @@
+/**
+ * chkgrp 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 <pwd.h>
+#include <grp.h>
+#include <unistd.h>
+#include <errno.h>
+#include <limits.h>
+
+#ifndef LOGIN_NAME_MAX
+#define LOGIN_NAME_MAX 256
+#endif
+
+int main(int argc, char *argv[])
+{
+    if (argc != 3) {
+        fprintf(stderr, "Usage: %s <username> <groupname>\n", argv[0]);
+        return 2;
+    }
+
+    const char *username = argv[1];
+    const char *groupname = argv[2];
+
+    // Runtime check for max name length
+    long max_name_len = sysconf(_SC_LOGIN_NAME_MAX);
+    if (max_name_len <= 0 || max_name_len > 1024) {
+        max_name_len = LOGIN_NAME_MAX;
+    }
+
+    if (strlen(username) > (size_t)max_name_len) {
+        fprintf(stderr, "Error: Username too long (max %ld characters).\n",
+                max_name_len);
+        return 2;
+    }
+
+    if (strlen(groupname) > (size_t)max_name_len) {
+        fprintf(stderr, "Error: Group name too long (max %ld characters).\n",
+                max_name_len);
+        return 2;
+    }
+
+    struct passwd *pw = getpwnam(username);
+    if (!pw) {
+        fprintf(stderr, "Error: User '%s' not found.\n", username);
+        return 2;
+    }
+
+    struct group *gr = getgrnam(groupname);
+    if (!gr) {
+        fprintf(stderr, "Error: Group '%s' not found.\n", groupname);
+        return 2;
+    }
+
+    if (pw->pw_gid == gr->gr_gid) {
+        puts("Yes");
+        return 0;
+    }
+
+    for (char **members = gr->gr_mem; *members != NULL; members++) {
+        if (strcmp(*members, username) == 0) {
+            puts("Yes");
+            return 0;
+        }
+    }
+
+    puts("No");
+    return 1;
+}
+