feat: add onUsr1 and printErr

user/pipecat.c

@@ -0,0 +1,56 @@
+#include "ulib.h"
+
+/*
+ * Middle stage of shell.c's new N-stage pipeline live demo
+ * (`pipesrc | pipecat | pipesink`) -- the first process in this kernel with
+ * BOTH ends of two DIFFERENT real inter-process pipes baked into its fds at
+ * once (fd 0 from jobctlSpawnPiped's stdinHandle, fd 1 from its
+ * stdoutHandle). user/hcat.c already proved a piped stdout alongside a
+ * piped stdin, but that stdin was always a shell-fed here-string
+ * (jobctlSpawnPipedHereString), never a second real process-to-process
+ * pipe the way this one's is -- this program is the actual proof that
+ * shell.c's runPipelineStages can wire a genuine middle pipeline stage
+ * where NEITHER end is the console. From this program's own point of view
+ * there is nothing special about either fd: it is simply "cat" -- read
+ * stdin to real EOF, then write exactly those bytes to stdout -- with no
+ * idea either end is a pipe rather than a real file or console, the same
+ * shape as hcat.c's/pipesink.c's own read loops.
+ *
+ * Diagnostics go to fd 2 (never fd 1), same reason as every other filter in
+ * this pipeline family: fd 1 here is the real payload pipesink.c compares
+ * byte-for-byte, and print()/printDec() (ulib.h) are both hardcoded to fd 1.
+ */
+
+static void printErr(const char* text)
+{
+    userWrite(2, text, stringLength(text));
+}
+
+int main(void)
+{
+    printErr("pipecat: reading piped stdin\n");
+
+    char buffer[64];
+    unsigned long total = 0;
+    for (;;) {
+        long got = userRead(0, buffer + total, sizeof(buffer) - 1 - total);
+        if (got < 0) {
+            printErr("pipecat: FAIL read\n");
+            return 1;
+        }
+        if (got == 0)
+            break;
+        total += (unsigned long)got;
+        if (total >= sizeof(buffer) - 1)
+            break;
+    }
+
+    long written = userWrite(1, buffer, total);
+    if (written != (long)total) {
+        printErr("pipecat: FAIL short/failed write\n");
+        return 1;
+    }
+
+    printErr("pipecat: forwarded payload\n");
+    return 0;
+}