Implement SAL_DEBUG_BACKTRACE for Linux

...by using glibc's backtrace_symbols(3).  If this is ever implemented for other
platforms, backtrace_symbols' interface may turn out to be awkward; but just use
it directly for now.  Also, the output from backtrace_symbols isn't too useful
in itself, as for non-exported symbols it only prints soname+offset, but some
addr2line postprocessing can turn that into something half-way decent.

Change-Id: I58cc7912aa7d8031729fc116a82a409c1c16977a

sal/osl/unx/backtrace.c

@@ -94,6 +94,11 @@ int backtrace( void **buffer, int max_frames )
     return i;
 }
 
+char ** backtrace_symbols(void * const * buffer, int size)
+{
+    return NULL; /*TODO*/
+}
+
 void backtrace_symbols_fd( void **buffer, int size, int fd )
 {
     FILE    *fp = fdopen( fd, "w" );
@@ -171,6 +176,11 @@ int backtrace( void **buffer, int max_frames )
     return i;
 }
 
+char ** backtrace_symbols(void * const * buffer, int size)
+{
+    return NULL; /*TODO*/
+}
+
 void backtrace_symbols_fd( void **buffer, int size, int fd )
 {
     FILE    *fp = fdopen( fd, "w" );
@@ -239,6 +249,11 @@ int backtrace( void **buffer, int max_frames )
     return i;
 }
 
+char ** backtrace_symbols(void * const * buffer, int size)
+{
+    return NULL; /*TODO*/
+}
+
 void backtrace_symbols_fd( void **buffer, int size, int fd )
 {
     FILE    *fp = fdopen( fd, "w" );
@@ -282,6 +297,11 @@ int backtrace( void **buffer, int max_frames )
     return 0;
 }
 
+char ** backtrace_symbols(void * const * buffer, int size)
+{
+    return NULL; /*TODO*/
+}
+
 void backtrace_symbols_fd( void **buffer, int size, int fd )
 {
 }

sal/osl/unx/backtrace.h

@@ -28,6 +28,8 @@ extern "C" {
 
 int backtrace( void **buffer, int max_frames );
 
+char ** backtrace_symbols(void * const * buffer, int size);
+
 void backtrace_symbols_fd( void **buffer, int size, int fd );
 
 /* no frame.h on FreeBSD */

sal/osl/unx/backtraceapi.cxx

@@ -9,13 +9,50 @@
 
 #include <sal/config.h>
 
+#include <cassert>
+#include <cstdlib>
+#include <memory>
+
+#include <o3tl/runtimetooustring.hxx>
+#include <rtl/ustrbuf.hxx>
 #include <rtl/ustring.hxx>
+
+#include "backtrace.h"
 #include "backtraceasstring.hxx"
 
-// FIXME: no-op for now; it needs implementing, cf. above.
-OUString osl::detail::backtraceAsString(int /*maxNoStackFramesToDisplay*/)
+namespace {
+
+struct FreeGuard {
+    FreeGuard(char ** theBuffer): buffer(theBuffer) {}
+
+    ~FreeGuard() { std::free(buffer); }
+
+    char ** buffer;
+};
+
+}
+
+OUString osl::detail::backtraceAsString(int maxNoStackFramesToDisplay)
 {
-    return OUString();
+    assert(maxNoStackFramesToDisplay >= 0);
+    if (maxNoStackFramesToDisplay == 0) {
+        return OUString();
+    }
+    auto b1 = std::unique_ptr<void *[]>(new void *[maxNoStackFramesToDisplay]);
+    int n = backtrace(b1.get(), maxNoStackFramesToDisplay);
+    FreeGuard b2(backtrace_symbols(b1.get(), n));
+    b1.reset();
+    if (b2.buffer == nullptr) {
+        return OUString();
+    }
+    OUStringBuffer b3;
+    for (int i = 0; i != n; ++i) {
+        if (i != 0) {
+            b3.append("\n");
+        }
+        b3.append(o3tl::runtimeToOUString(b2.buffer[i]));
+    }
+    return b3.makeStringAndClear();
 }
 
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