From: Qing Cai <bsiice@xxxxxxx>
As stated in manpage SHMCTL(2), shm_nattch is "No. of current attaches"
(i.e., number of processes attached to the shared memeory). If an
application uses alsa-lib and invokes fork() from a thread of the
application, there may be the following execution sequence:
1. execute the following statement:
pcm_direct.c:110: dmix->shmptr = shmat(dmix->shmid, 0, 0)
(shm_nattch becomes 1)
2. invoke fork() in some thread.
(shm_nattch becomes 2)
3. execute the following statement:
pcm_direct.c:122: if (buf.shm_nattch == 1)
4. execute the following statement:
pcm_direct.c:131: if (dmix->shmptr->magic != SND_PCM_DIRECT_MAGIC)
(As stated in manpage SHMGET(2), "When a new shared memory segment
is created, its contents are initialized to zero values", so
dmix->shmptr->magic is 0)
5. execute the following statements:
pcm_direct.c:132: snd_pcm_direct_shm_discard(dmix)
pcm_direct.c:133: return -EINVAL
The above execution sequence will cause the following error:
unable to create IPC shm instance
This error causes multimedia application has no sound. This error rarely
occurs, probability is about 1%.
Signed-off-by: Qing Cai <bsiice@xxxxxxx>
Signed-off-by: Qing Cai <caiqing@xxxxxxxxxxx>
diff --git a/src/pcm/pcm_direct.c b/src/pcm/pcm_direct.c
index fd3877c..14de734 100644
--- a/src/pcm/pcm_direct.c
+++ b/src/pcm/pcm_direct.c
@@ -91,13 +91,19 @@ int snd_pcm_direct_semaphore_create_or_connect(snd_pcm_direct_t *dmix)
int snd_pcm_direct_shm_create_or_connect(snd_pcm_direct_t *dmix)
{
struct shmid_ds buf;
- int tmpid, err;
+ int tmpid, err, first_instance = 0;
retryget:
dmix->shmid = shmget(dmix->ipc_key, sizeof(snd_pcm_direct_share_t),
- IPC_CREAT | dmix->ipc_perm);
+ dmix->ipc_perm);
+ if (dmix->shmid < 0) {
+ if (errno == ENOENT)
+ if ((dmix->shmid = shmget(dmix->ipc_key, sizeof(snd_pcm_direct_share_t),
+ IPC_CREAT | IPC_EXCL | dmix->ipc_perm)) != -1)
+ first_instance = 1;
+ }
err = -errno;
- if (dmix->shmid < 0){
+ if (dmix->shmid < 0) {
if (errno == EINVAL)
if ((tmpid = shmget(dmix->ipc_key, 0, dmix->ipc_perm)) != -1)
if (!shmctl(tmpid, IPC_STAT, &buf))
@@ -119,7 +125,7 @@ retryget:
snd_pcm_direct_shm_discard(dmix);
return err;
}
- if (buf.shm_nattch == 1) { /* we're the first user, clear the segment */
+ if (first_instance) { /* we're the first user, clear the segment */
memset(dmix->shmptr, 0, sizeof(snd_pcm_direct_share_t));
if (dmix->ipc_gid >= 0) {
buf.shm_perm.gid = dmix->ipc_gid;
--
2.7.2
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