port: set shm size with ftruncate before writing

[lttng-ust.git] / libringbuffer / shm.c

diff --git a/libringbuffer/shm.c b/libringbuffer/shm.c
index 9160c2184f9874841e30802bab2288be30f36894..566ed467fe2f715bc60f56449b222964c8b51acd 100644 (file)
--- a/libringbuffer/shm.c
+++ b/libringbuffer/shm.c
@@ -128,19 +128,27 @@ struct shm_object *_shm_object_table_alloc_shm(struct shm_object_table *table,
        }
        memcpy(obj->wait_fd, waitfd, sizeof(waitfd));
 
-       /* create shm */
+       /*
+        * Set POSIX shared memory object size
+        *
+        * First, use ftruncate() to set its size, some implementations won't
+        * allow writes past the size set by ftruncate.
+        * Then, use write() to fill it with zeros, this allows us to fully
+        * allocate it and detect a shortage of shm space without dealing with
+        * a SIGBUS.
+        */
 
        shmfd = stream_fd;
-       ret = zero_file(shmfd, memory_map_size);
-       if (ret) {
-               PERROR("zero_file");
-               goto error_zero_file;
-       }
        ret = ftruncate(shmfd, memory_map_size);
        if (ret) {
                PERROR("ftruncate");
                goto error_ftruncate;
        }
+       ret = zero_file(shmfd, memory_map_size);
+       if (ret) {
+               PERROR("zero_file");
+               goto error_zero_file;
+       }
        /*
         * Also ensure the file metadata is synced with the storage by using
         * fsync(2).