[Qemu-devel] [PATCH] qemu-log: default to stderr for logging output

[Peter Maydell]

Switch the default for qemu_log logging output from "/tmp/qemu.log"
to stderr. This is an incompatible change in some sense, but logging
is mostly used for debugging purposes so it shouldn't affect production
use. The previous behaviour can be obtained by adding "-D /tmp/qemu.log"
to the command line.

This change requires us to:
 * update all the documentation/help text
 * make linux-user and bsd-user defer to qemu-log for the default
   logging destination rather than overriding it themselves
 * ensure that all logfile closing is done via qemu_log_close()
   and that that function doesn't close stderr
as well as the obvious change to the behaviour of do_qemu_set_log()
when no logfile name has been specified.

Signed-off-by: Peter Maydell <address@hidden>
---
Stefan suggested that qemu_log should default to stderr, and I
agree that it makes more sense than a random file in /tmp/.
As noted in the commit message, this is technically an incompatible
change.

This patchset sits on top of my recent 6 patch qemu_log
cleanup series.

 bsd-user/main.c    |   15 +++++++--------
 hmp-commands.hx    |    4 ++--
 include/qemu/log.h |    8 ++++++--
 linux-user/main.c  |   14 ++++----------
 qemu-doc.texi      |    8 ++++----
 qemu-log.c         |   29 +++++++++++------------------
 qemu-options.hx    |   10 +++++-----
 tcg/tci/README     |    2 +-
 8 files changed, 40 insertions(+), 50 deletions(-)

diff --git a/bsd-user/main.c b/bsd-user/main.c
index 097fbfe..1ec8636 100644
--- a/bsd-user/main.c
+++ b/bsd-user/main.c
@@ -34,8 +34,6 @@
 #include "qemu/timer.h"
 #include "qemu/envlist.h"
 
-#define DEBUG_LOGFILE "/tmp/qemu.log"
-
 int singlestep;
 #if defined(CONFIG_USE_GUEST_BASE)
 unsigned long mmap_min_addr;
@@ -691,8 +689,8 @@ static void usage(void)
            "-bsd type         select emulated BSD type FreeBSD/NetBSD/OpenBSD 
(default)\n"
            "\n"
            "Debug options:\n"
-           "-d options   activate log (default logfile=%s)\n"
-           "-D logfile   override default logfile location\n"
+           "-d options   activate log (default is to log to stderr)\n"
+           "-D logfile   write logs to 'logfile' rather than stderr\n"
            "-p pagesize  set the host page size to 'pagesize'\n"
            "-singlestep  always run in singlestep mode\n"
            "-strace      log system calls\n"
@@ -709,8 +707,7 @@ static void usage(void)
            ,
            TARGET_ARCH,
            interp_prefix,
-           x86_stack_size,
-           DEBUG_LOGFILE);
+           x86_stack_size);
     exit(1);
 }
 
@@ -733,7 +730,7 @@ int main(int argc, char **argv)
 {
     const char *filename;
     const char *cpu_model;
-    const char *log_file = DEBUG_LOGFILE;
+    const char *log_file = NULL;
     const char *log_mask = NULL;
     struct target_pt_regs regs1, *regs = &regs1;
     struct image_info info1, *info = &info1;
@@ -861,7 +858,9 @@ int main(int argc, char **argv)
     }
 
     /* init debug */
-    qemu_set_log_filename(log_file);
+    if (log_file) {
+        qemu_set_log_filename(log_file);
+    }
     if (log_mask) {
         int mask;
 
diff --git a/hmp-commands.hx b/hmp-commands.hx
index 64008a9..cef7708 100644
--- a/hmp-commands.hx
+++ b/hmp-commands.hx
@@ -295,14 +295,14 @@ ETEXI
         .name       = "log",
         .args_type  = "items:s",
         .params     = "item1[,...]",
-        .help       = "activate logging of the specified items to 
'/tmp/qemu.log'",
+        .help       = "activate logging of the specified items",
         .mhandler.cmd = do_log,
     },
 
 STEXI
 @item log @var{item1}[,...]
 @findex log
-Activate logging of the specified items to @file{/tmp/qemu.log}.
+Activate logging of the specified items.
 ETEXI
 
     {
diff --git a/include/qemu/log.h b/include/qemu/log.h
index 4527003..6b0db02 100644
--- a/include/qemu/log.h
+++ b/include/qemu/log.h
@@ -116,8 +116,12 @@ static inline void qemu_log_flush(void)
 /* Close the log file */
 static inline void qemu_log_close(void)
 {
-    fclose(qemu_logfile);
-    qemu_logfile = NULL;
+    if (qemu_logfile) {
+        if (qemu_logfile != stderr) {
+            fclose(qemu_logfile);
+        }
+        qemu_logfile = NULL;
+    }
 }
 
 /* Set up a new log file */
diff --git a/linux-user/main.c b/linux-user/main.c
index 8a61ea4..55e8326 100644
--- a/linux-user/main.c
+++ b/linux-user/main.c
@@ -35,8 +35,6 @@
 #include "qemu/envlist.h"
 #include "elf.h"
 
-#define DEBUG_LOGFILE "/tmp/qemu.log"
-
 char *exec_path;
 
 int singlestep;
@@ -3289,9 +3287,9 @@ static const struct qemu_argument arg_table[] = {
      "size",       "reserve 'size' bytes for guest virtual address space"},
 #endif
     {"d",          "QEMU_LOG",         true,  handle_arg_log,
-     "options",    "activate log"},
+     "options",    "activate log (default is to log to stderr)"},
     {"D",          "QEMU_LOG_FILENAME", true, handle_arg_log_filename,
-     "logfile",     "override default logfile location"},
+     "logfile",     "log to specified file rather than stderr"},
     {"p",          "QEMU_PAGESIZE",    true,  handle_arg_pagesize,
      "pagesize",   "set the host page size to 'pagesize'"},
     {"singlestep", "QEMU_SINGLESTEP",  false, handle_arg_singlestep,
@@ -3344,11 +3342,9 @@ static void usage(void)
     printf("\n"
            "Defaults:\n"
            "QEMU_LD_PREFIX  = %s\n"
-           "QEMU_STACK_SIZE = %ld byte\n"
-           "QEMU_LOG        = %s\n",
+           "QEMU_STACK_SIZE = %ld byte\n",
            interp_prefix,
-           guest_stack_size,
-           DEBUG_LOGFILE);
+           guest_stack_size);
 
     printf("\n"
            "You can use -E and -U options or the QEMU_SET_ENV and\n"
@@ -3432,7 +3428,6 @@ static int parse_args(int argc, char **argv)
 
 int main(int argc, char **argv, char **envp)
 {
-    const char *log_file = DEBUG_LOGFILE;
     struct target_pt_regs regs1, *regs = &regs1;
     struct image_info info1, *info = &info1;
     struct linux_binprm bprm;
@@ -3476,7 +3471,6 @@ int main(int argc, char **argv, char **envp)
 #endif
 
     /* init debug */
-    qemu_set_log_filename(log_file);
     optind = parse_args(argc, argv);
 
     /* Zero out regs */
diff --git a/qemu-doc.texi b/qemu-doc.texi
index 6d7f50d..747e052 100644
--- a/qemu-doc.texi
+++ b/qemu-doc.texi
@@ -2642,8 +2642,8 @@ Pre-allocate a guest virtual address space of the given 
size (in bytes).
 Debug options:
 
 @table @option
address@hidden -d
-Activate log (logfile=/tmp/qemu.log)
address@hidden -d item1,...
+Activate logging of the specified items (use '-d help' for a list of log items)
 @item -p pagesize
 Act as if the host page size was 'pagesize' bytes
 @item -g port
@@ -2781,8 +2781,8 @@ FreeBSD, NetBSD and OpenBSD (default).
 Debug options:
 
 @table @option
address@hidden -d
-Activate log (logfile=/tmp/qemu.log)
address@hidden -d item1,...
+Activate logging of the specified items (use '-d help' for a list of log items)
 @item -p pagesize
 Act as if the host page size was 'pagesize' bytes
 @item -singlestep
diff --git a/qemu-log.c b/qemu-log.c
index 2f47aaf..797f2af 100644
--- a/qemu-log.c
+++ b/qemu-log.c
@@ -20,12 +20,6 @@
 #include "qemu-common.h"
 #include "qemu/log.h"
 
-#ifdef WIN32
-#define DEFAULT_LOGFILENAME "qemu.log"
-#else
-#define DEFAULT_LOGFILENAME "/tmp/qemu.log"
-#endif
-
 static char *logfilename;
 FILE *qemu_logfile;
 int qemu_loglevel;
@@ -56,14 +50,17 @@ void qemu_log_mask(int mask, const char *fmt, ...)
 /* enable or disable low levels log */
 void do_qemu_set_log(int log_flags, bool use_own_buffers)
 {
-    const char *fname = logfilename ?: DEFAULT_LOGFILENAME;
-
     qemu_loglevel = log_flags;
     if (qemu_loglevel && !qemu_logfile) {
-        qemu_logfile = fopen(fname, log_append ? "a" : "w");
-        if (!qemu_logfile) {
-            perror(fname);
-            _exit(1);
+        if (logfilename) {
+            qemu_logfile = fopen(logfilename, log_append ? "a" : "w");
+            if (!qemu_logfile) {
+                perror(logfilename);
+                _exit(1);
+            }
+        } else {
+            /* Default to stderr if no log file specified */
+            qemu_logfile = stderr;
         }
         /* must avoid mmap() usage of glibc by setting a buffer "by hand" */
         if (use_own_buffers) {
@@ -81,8 +78,7 @@ void do_qemu_set_log(int log_flags, bool use_own_buffers)
         }
     }
     if (!qemu_loglevel && qemu_logfile) {
-        fclose(qemu_logfile);
-        qemu_logfile = NULL;
+        qemu_log_close();
     }
 }
 
@@ -90,10 +86,7 @@ void qemu_set_log_filename(const char *filename)
 {
     g_free(logfilename);
     logfilename = g_strdup(filename);
-    if (qemu_logfile) {
-        fclose(qemu_logfile);
-        qemu_logfile = NULL;
-    }
+    qemu_log_close();
     qemu_set_log(qemu_loglevel);
 }
 
diff --git a/qemu-options.hx b/qemu-options.hx
index 046bdc0..d957565 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -2489,21 +2489,21 @@ Shorthand for -gdb tcp::1234, i.e. open a gdbserver on 
TCP port 1234
 ETEXI
 
 DEF("d", HAS_ARG, QEMU_OPTION_d, \
-    "-d item1,...    output log to /tmp/qemu.log (use '-d help' for a list of 
log items)\n",
+    "-d item1,...    enable logging of specified items (use '-d help' for a 
list of log items)\n",
     QEMU_ARCH_ALL)
 STEXI
address@hidden -d
address@hidden -d @var{item1}[,...]
 @findex -d
-Output log in /tmp/qemu.log
+Log specified items
 ETEXI
 
 DEF("D", HAS_ARG, QEMU_OPTION_D, \
-    "-D logfile      output log to logfile (instead of the default 
/tmp/qemu.log)\n",
+    "-D logfile      output log to logfile (instead of the default stderr)\n",
     QEMU_ARCH_ALL)
 STEXI
 @item -D @var{logfile}
 @findex -D
-Output log in @var{logfile} instead of /tmp/qemu.log
+Output log in @var{logfile} instead of to stderr
 ETEXI
 
 DEF("hdachs", HAS_ARG, QEMU_OPTION_hdachs, \
diff --git a/tcg/tci/README b/tcg/tci/README
index 6ac1ac9..dc57f07 100644
--- a/tcg/tci/README
+++ b/tcg/tci/README
@@ -52,7 +52,7 @@ The only difference from running QEMU with TCI to running 
without TCI
 should be speed. Especially during development of TCI, it was very
 useful to compare runs with and without TCI. Create /tmp/qemu.log by
 
-        qemu-system-i386 -d in_asm,op_opt,cpu -singlestep
+        qemu-system-i386 -d in_asm,op_opt,cpu -D /tmp/qemu.log -singlestep
 
 once with interpreter and once without interpreter and compare the resulting
 qemu.log files. This is also useful to see the effects of additional
-- 
1.7.9.5