[gpsd-dev] [PATCH 6/6] Eliminates the one use of the portability-challenged alloca().

[Fred Wright]

In general, the use of alloca() is discouraged.  Here, there was
only one use, with a maximum size determinable at compile time,
so the code has been reworked to use an ordinary array (without
relying on the C99 variable-length array feature).  This always
allows for a GPS_PATH_MAX-sized device_name, but the amount of
space needed for that is relatively modest, and the behavior is
more reproducible with a fixed allocation.

TESTED:
Ran "scons build-all check" on OSX.  This code shouldn't be
OS-dependent.
---
 libgpsd_core.c | 25 +++++++++++++------------
 1 file changed, 13 insertions(+), 12 deletions(-)

diff --git a/libgpsd_core.c b/libgpsd_core.c
index 3fef4a8..eb4fddf 100644
--- a/libgpsd_core.c
+++ b/libgpsd_core.c
@@ -204,26 +204,28 @@ const char *gpsd_prettydump(struct gps_device_t *session)
                           session->lexer.outbuflen);
 }
 
+/* Define the possible hook strings here so we can get the length */
+#define HOOK_ACTIVATE "ACTIVATE"
+#define HOOK_DEACTIVATE "DEACTIVATE"
+
+#define MAX(a,b) (((a) > (b)) ? (a) : (b))
+#define HOOK_MAX MAX(sizeof(HOOK_ACTIVATE), sizeof(HOOK_DEACTIVATE))
+#define HOOK_CMD_MAX (sizeof(DEVICEHOOKPATH)-1 + 1 + GPS_PATH_MAX-1 + 1 \
+                      + HOOK_MAX-1 + 1)
 
 static void gpsd_run_device_hook(struct gpsd_errout_t *errout,
                                 char *device_name, char *hook)
 {
     struct stat statbuf;
+    char buf[HOOK_CMD_MAX];
+
     if (stat(DEVICEHOOKPATH, &statbuf) == -1)
        gpsd_log(errout, LOG_PROG,
                 "no %s present, skipped running %s hook\n",
                 DEVICEHOOKPATH, hook);
     else {
-       /* use alloca(), which is not malloc(), here because
-        * the pointer will never persist outside this small scope.
-        */
-       size_t bufsize = strlen(DEVICEHOOKPATH) + 1 + strlen(device_name) + 1 + 
strlen(hook) + 1;
-       char *buf = alloca(bufsize);
-       /* no need to check the return code of alloca()
-         * by definition, if alloca() fails the program segfaults
-         */
        int status;
-       (void)snprintf(buf, bufsize, "%s %s %s",
+       (void)snprintf(buf, sizeof(buf), "%s %s %s",
                       DEVICEHOOKPATH, device_name, hook);
        gpsd_log(errout, LOG_INF, "running %s\n", buf);
        status = system(buf);
@@ -233,7 +235,6 @@ static void gpsd_run_device_hook(struct gpsd_errout_t 
*errout,
            gpsd_log(errout, LOG_INF,
                     "%s returned %d\n", DEVICEHOOKPATH,
                     WEXITSTATUS(status));
-       /* buf automatically freed here as the stack pops */
     }
 }
 
@@ -356,7 +357,7 @@ void gpsd_deactivate(struct gps_device_t *session)
     if (session->mode == O_OPTIMIZE)
        gpsd_run_device_hook(&session->context->errout,
                             session->gpsdata.dev.path,
-                            "DEACTIVATE");
+                            HOOK_DEACTIVATE);
 #ifdef PPS_ENABLE
     session->pps_thread.report_hook = NULL; /* tell any PPS-watcher thread to 
die */
 #endif /* PPS_ENABLE */
@@ -543,7 +544,7 @@ int gpsd_activate(struct gps_device_t *session, const int 
mode)
 {
     if (mode == O_OPTIMIZE)
        gpsd_run_device_hook(&session->context->errout,
-                            session->gpsdata.dev.path, "ACTIVATE");
+                            session->gpsdata.dev.path, HOOK_ACTIVATE);
     session->gpsdata.gps_fd = gpsd_open(session);
     if (mode != O_CONTINUE)
        session->mode = mode;
-- 
2.9.3