feat(string): add ssh_string_cmp function for comparing ssh_strings

Signed-off-by: Praneeth Sarode <praneethsarode@gmail.com>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>

include/libssh/libssh.h

@@ -851,6 +851,7 @@ LIBSSH_API char *ssh_string_to_char(ssh_string str);
 #define SSH_STRING_FREE_CHAR(x) \
     do { if ((x) != NULL) { ssh_string_free_char(x); x = NULL; } } while(0)
 LIBSSH_API void ssh_string_free_char(char *s);
+LIBSSH_API int ssh_string_cmp(ssh_string s1, ssh_string s2);
 
 LIBSSH_API int ssh_getpass(const char *prompt, char *buf, size_t len, int echo,
     int verify);

src/libssh.map

@@ -491,5 +491,6 @@ LIBSSH_AFTER_4_10_0
         ssh_get_supported_methods;
         sshsig_sign;
         sshsig_verify;
+        ssh_string_cmp;
 } LIBSSH_4_10_0;
 

src/string.c

@@ -27,8 +27,8 @@
 #include <limits.h>
 
 #ifndef _WIN32
-#include <netinet/in.h>
 #include <arpa/inet.h>
+#include <netinet/in.h>
 #endif
 
 #include "libssh/priv.h"
@@ -244,6 +244,56 @@ struct ssh_string_struct *ssh_string_copy(struct ssh_string_struct *s)
     return new;
 }
 
+/**
+ * @brief Compare two SSH strings.
+ *
+ * @param[in] s1        The first SSH string to compare.
+ * @param[in] s2        The second SSH string to compare.
+ *
+ * @return              0 if the strings are equal,
+ *                      < 0 if s1 is less than s2,
+ *                      > 0 if s1 is greater than s2.
+ */
+int ssh_string_cmp(struct ssh_string_struct *s1, struct ssh_string_struct *s2)
+{
+    size_t len1, len2, min_len;
+    int cmp;
+
+    /* Both are NULL */
+    if (s1 == NULL && s2 == NULL) {
+        return 0;
+    }
+
+    /* Only one is NULL - NULL is considered "less than" non-NULL */
+    if (s1 == NULL) {
+        return -1;
+    } else if (s2 == NULL) {
+        return 1;
+    }
+
+    /* Get lengths */
+    len1 = ssh_string_len(s1);
+    len2 = ssh_string_len(s2);
+    min_len = MIN(len1, len2);
+
+    /* Compare data up to the shorter length */
+    if (min_len > 0) {
+        cmp = memcmp(s1->data, s2->data, min_len);
+        if (cmp != 0) {
+            return cmp;
+        }
+    }
+
+    /* If common prefix is equal, compare lengths */
+    if (len1 < len2) {
+        return -1;
+    } else if (len1 > len2) {
+        return 1;
+    }
+
+    return 0;
+}
+
 /**
  * @brief Destroy the data in a string so it couldn't appear in a core dump.
  *