From bb5f12b2fa4ae4887fb51bfeb60107d751a5636d Mon Sep 17 00:00:00 2001
From: Simon Rose <simon.rose@ess.eu>
Date: Tue, 24 Feb 2026 14:53:33 +0000
Subject: [PATCH] Add command to show errlog buffer contents

---
 modules/libcom/src/error/errlog.c         | 28 +++++++++++++++++++++++
 modules/libcom/src/error/errlog.h         |  9 --------
 modules/libcom/src/error/errlogPvt.h      | 26 +++++++++++++++++++++
 modules/libcom/src/iocsh/libComRegister.c | 17 ++++++++++++++
 4 files changed, 71 insertions(+), 9 deletions(-)
 create mode 100644 modules/libcom/src/error/errlogPvt.h

diff --git a/modules/libcom/src/error/errlog.c b/modules/libcom/src/error/errlog.c
index b5678fcba..c4362f763 100644
--- a/modules/libcom/src/error/errlog.c
+++ b/modules/libcom/src/error/errlog.c
@@ -641,6 +641,34 @@ void errlogBufResize(int bufsize, int maxMsgSize)
     epicsMutexUnlock(pvt.bufSizeLock);
 }
 
+void errlogShow(int level)
+{
+    epicsMutexMustLock(pvt.bufSizeLock);
+    printf("Error log:\n");
+    printf("  buffer size: %zu\n", pvt.bufSize);
+    printf("  max message size: %zu\n", pvt.maxMsgSize);
+    epicsMutexUnlock(pvt.bufSizeLock);
+
+    if (level > 0)
+    {
+        epicsMutexMustLock(pvt.listenerLock);
+        printf("  number of listeners: %u\n", pvt.listenerList.count);
+        epicsMutexUnlock(pvt.listenerLock);
+    }
+
+    if (level > 1)
+    {
+        epicsMutexMustLock(pvt.msgQueueLock);
+        printf("  buffer(log) contents:\n");
+        printf("%s\n", pvt.log->base);
+        printf("%*s^\n", (int)pvt.log->pos, "");
+        printf("  buffer(print) contents:\n");
+        printf("%s\n", pvt.print->base);
+        printf("%*s^\n", (int)pvt.print->pos, "");
+        epicsMutexUnlock(pvt.msgQueueLock);
+    }
+}
+
 void errlogFlush(void)
 {
     /* wait for both buffers to be handled to know that all currently
diff --git a/modules/libcom/src/error/errlog.h b/modules/libcom/src/error/errlog.h
index aea8437da..a204bec87 100644
--- a/modules/libcom/src/error/errlog.h
+++ b/modules/libcom/src/error/errlog.h
@@ -224,15 +224,6 @@ LIBCOM_API int errlogInit(int bufsize);
  */
 LIBCOM_API int errlogInit2(int bufsize, int maxMsgSize);
 
-/**
- * errlogBufResize can be used to reinitialize the error logging system with a larger buffer and maximum message size.
- * The default buffer size is 1280 bytes, and the default maximum message size is 256.
- *
- * \param bufsize The desired buffer size
- * \param maxMsgSize The desired max message size
- */
-LIBCOM_API void errlogBufResize(int bufsize, int maxMsgSize);
-
 /** Wakes up the errlog task and then waits until all messages are flushed from the queue. */
 LIBCOM_API void errlogFlush(void);
 
diff --git a/modules/libcom/src/error/errlogPvt.h b/modules/libcom/src/error/errlogPvt.h
new file mode 100644
index 000000000..c65220769
--- /dev/null
+++ b/modules/libcom/src/error/errlogPvt.h
@@ -0,0 +1,26 @@
+/*************************************************************************\
+* Copyright (c) 2026 European Spallation Source ERIC
+* SPDX-License-Identifier: EPICS
+* EPICS BASE is distributed subject to a Software License Agreement found
+* in file LICENSE that is included with this distribution.
+\*************************************************************************/
+
+#ifndef INC_errlogPvt_H
+#define INC_errlogPvt_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * errlogShow is used to display the contents of the error log buffers
+ *
+ * \param level How much detail to print out
+ */
+void errlogShow(int level);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /*INC_errlogPvt_H*/
diff --git a/modules/libcom/src/iocsh/libComRegister.c b/modules/libcom/src/iocsh/libComRegister.c
index 844ee14e2..cbf262608 100644
--- a/modules/libcom/src/iocsh/libComRegister.c
+++ b/modules/libcom/src/iocsh/libComRegister.c
@@ -22,6 +22,7 @@
 #include "osiUnistd.h"
 #include "logClient.h"
 #include "errlog.h"
+#include "errlogPvt.h"
 #include "taskwd.h"
 #include "registry.h"
 #include "epicsGeneralTime.h"
@@ -306,6 +307,21 @@ static void errlogBufResizeCallFunc(const iocshArgBuf *args)
     errlogBufResize(args[0].ival, args[1].ival);
 }
 
+/* errlogShow */
+static const iocshArg errlogShowArg0 = { "level",iocshArgInt};
+static const iocshArg * const errlogShowArgs[1] = {&errlogShowArg0};
+static const iocshFuncDef errlogShowFuncDef = {
+    "errlogShow",1,errlogShowArgs,
+    "Show the contents of the error log private data\n"
+    "level 0 - Show the size of the buffers and max message size\n"
+    "      1 - Show the number of listeners\n"
+    "      2 - Show the contents of the buffer\n"
+};
+static void errlogShowCallFunc(const iocshArgBuf *args)
+{
+    errlogShow(args[0].ival);
+}
+
 /* errlog */
 IOCSH_STATIC_FUNC void errlog(const char *message)
 {
@@ -517,6 +533,7 @@ void epicsStdCall libComRegister(void)
     iocshRegister(&errlogInitFuncDef,errlogInitCallFunc);
     iocshRegister(&errlogInit2FuncDef,errlogInit2CallFunc);
     iocshRegister(&errlogBufResizeFuncDef,errlogBufResizeCallFunc);
+    iocshRegister(&errlogShowFuncDef,errlogShowCallFunc);
     iocshRegister(&errlogFuncDef, errlogCallFunc);
     iocshRegister(&iocLogPrefixFuncDef, iocLogPrefixCallFunc);