Become a memory accounting amateur

userland/c/snprintf.c

@@ -0,0 +1,54 @@
+/* https://cirosantilli.com/linux-kernel-module-cheat#c
+ *
+ *  Like `sprintf`, but writes at most n bytes, so it is safer,
+ *  because it may not be possible or easy to calculate the resulting
+ *  size of a formated string.
+ *
+ *  The size given includes the null terminator. */
+
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+int main(void) {
+#if __STDC_VERSION__ >= 199901L
+    /* Common usage when string fits.
+     *
+     * Ensures that there will be no out or bounds access on out. */
+    {
+        int in = 1234;
+        char out[1024];
+        int snprintf_return;
+        snprintf_return = snprintf(out, sizeof(out), "ab%dcd", in);
+
+        /* The usual error checking. */
+        if (snprintf_return < 0) {
+            perror("snprintf");
+            exit(EXIT_FAILURE);
+        }
+        assert((size_t)snprintf_return < sizeof(out));
+
+        /* Assert because we know the return here.  */
+        assert(snprintf_return == 8);
+
+        /* What it actually copied. */
+        assert(strcmp(out, "ab1234cd") == 0);
+    }
+
+    /* Less common case where string does not fit. Error handling would
+     * normally follow in a real program. */
+    {
+        int in = 1234;
+        char out[6];
+        /* The return here is the same as before.
+         *
+         * Because it is >= than the imposed limit of 6, we know that
+         * the write failed to fully complete. */
+        assert(snprintf(out, sizeof(out), "ab%dcd", in) == 8);
+        assert(strcmp(out, "ab123") == 0);
+    }
+#endif
+    return EXIT_SUCCESS;
+}
+