The API sets the priority for the current thread, not an arbitrary thread.

Implemented thread priority as the 'nice' value on Linux.  High priority threads require root permissions (you shouldn't give your game root permissions though!)

include/SDL_thread.h

@@ -157,9 +157,9 @@ extern DECLSPEC SDL_threadID SDLCALL SDL_ThreadID(void);
 extern DECLSPEC SDL_threadID SDLCALL SDL_GetThreadID(SDL_Thread * thread);
 
 /**
- *  Set the thread priority
+ *  Set the priority for the current thread
  */
-extern DECLSPEC int SDLCALL SDL_SetThreadPriority(SDL_Thread * thread, SDL_ThreadPriority priority);
+extern DECLSPEC int SDLCALL SDL_SetThreadPriority(SDL_ThreadPriority priority);
 
 /**
  *  Wait for a thread to finish.

src/audio/SDL_audio.c

@@ -329,6 +329,9 @@ SDL_RunAudio(void *devicep)
     int silence;
     Uint32 delay;
 
+    /* The audio mixing is always a high priority thread */
+    SDL_SetThreadPriority(SDL_THREAD_PRIORITY_HIGH);
+
     /* For streaming when the buffer sizes don't match up */
     Uint8 *istream;
     int istream_len = 0;
@@ -974,7 +977,6 @@ open_audio_device(const char *devname, int iscapture,
             SDL_SetError("Couldn't create audio thread");
             return 0;
         }
-        SDL_SetThreadPriority(device->thread, SDL_THREAD_PRIORITY_HIGH);
     }
 
     return id + 1;

src/thread/SDL_systhread.h

@@ -43,8 +43,8 @@ extern int SDL_SYS_CreateThread(SDL_Thread * thread, void *args);
 /* This function does any necessary setup in the child thread */
 extern void SDL_SYS_SetupThread(void);
 
-/* This function sets thread priority */
-extern int SDL_SYS_SetThreadPriority(SDL_Thread * thread, SDL_ThreadPriority priority);
+/* This function sets the current thread priority */
+extern int SDL_SYS_SetThreadPriority(SDL_ThreadPriority priority);
 
 /* This function waits for the thread to finish and frees any data
    allocated by SDL_SYS_CreateThread()

src/thread/SDL_thread.c

@@ -295,13 +295,9 @@ SDL_GetThreadID(SDL_Thread * thread)
 }
 
 int
-SDL_SetThreadPriority(SDL_Thread * thread, SDL_ThreadPriority priority)
+SDL_SetThreadPriority(SDL_ThreadPriority priority)
 {
-    if (!thread) {
-        SDL_SetError("SDL_SetThreadPriority() passed NULL thread");
-        return -1;
-    }
-    return SDL_SYS_SetThreadPriority(thread, priority);
+    return SDL_SYS_SetThreadPriority(priority);
 }
 
 void

src/thread/beos/SDL_systhread.c

@@ -93,14 +93,16 @@ SDL_ThreadID(void)
 int
 SDL_SYS_SetThreadPriority(SDL_Thread * thread, SDL_ThreadPriority priority)
 {
-    int32 new_priority = B_NORMAL_PRIORITY;
+    int32 value;
 
     if (priority == SDL_THREAD_PRIORITY_LOW) {
-        new_priority = B_LOW_PRIORITY;
+        value = B_LOW_PRIORITY;
     } else if (priority == SDL_THREAD_PRIORITY_HIGH) {
-        new_priority = B_URGENT_DISPLAY_PRIORITY;
+        value = B_URGENT_DISPLAY_PRIORITY;
+    } else {
+        value = B_NORMAL_PRIORITY;
     }
-    set_thread_priority(thread->handle, new_priority);
+    set_thread_priority(find_thread(NULL), value);
     return 0;
 }
 

src/thread/generic/SDL_systhread.c

@@ -46,7 +46,7 @@ SDL_ThreadID(void)
 }
 
 int
-SDL_SYS_SetThreadPriority(SDL_Thread * thread, SDL_ThreadPriority priority)
+SDL_SYS_SetThreadPriority(SDL_ThreadPriority priority)
 {
     return (0);
 }

src/thread/nds/SDL_systhread.c

@@ -57,7 +57,7 @@ SDL_SYS_WaitThread(SDL_Thread * thread)
 }
 
 int
-SDL_SYS_SetThreadPriority(SDL_Thread * thread, SDL_ThreadPriority priority)
+SDL_SYS_SetThreadPriority(SDL_ThreadPriority priority)
 {
     return (0);
 }

src/thread/pthread/SDL_systhread.c

@@ -23,6 +23,11 @@
 
 #include <pthread.h>
 #include <signal.h>
+#ifdef linux
+#include <sys/time.h>
+#include <sys/resource.h>
+#include <sys/syscall.h>
+#endif
 
 #include "SDL_thread.h"
 #include "../SDL_thread_c.h"
@@ -34,6 +39,7 @@ static const int sig_list[] = {
     SIGVTALRM, SIGPROF, 0
 };
 
+
 static void *
 RunThread(void *data)
 {
@@ -92,12 +98,29 @@ SDL_ThreadID(void)
 }
 
 int
-SDL_SYS_SetThreadPriority(SDL_Thread * thread, SDL_ThreadPriority priority)
+SDL_SYS_SetThreadPriority(SDL_ThreadPriority priority)
 {
+#ifdef linux
+    int value;
+
+    if (priority == SDL_THREAD_PRIORITY_LOW) {
+        value = 19;
+    } else if (priority == SDL_THREAD_PRIORITY_HIGH) {
+        value = -20;
+    } else {
+        value = 0;
+    }
+    if (setpriority(PRIO_PROCESS, syscall(SYS_gettid), value) < 0) {
+        SDL_SetError("setpriority() failed");
+        return -1;
+    }
+    return 0;
+#else
     struct sched_param sched;
     int policy;
+    pthread_t thread = pthread_self();
 
-    if (pthread_getschedparam(thread->handle, &policy, &sched) < 0) {
+    if (pthread_getschedparam(thread, &policy, &sched) < 0) {
         SDL_SetError("pthread_getschedparam() failed");
         return -1;
     }
@@ -108,14 +131,14 @@ SDL_SYS_SetThreadPriority(SDL_Thread * thread, SDL_ThreadPriority priority)
     } else {
         int min_priority = sched_get_priority_min(policy);
         int max_priority = sched_get_priority_max(policy);
-        int priority = (min_priority + (max_priority - min_priority) / 2);
-        sched.sched_priority = priority;
+        sched.sched_priority = (min_priority + (max_priority - min_priority) / 2);
     }
-    if (pthread_setschedparam(thread->handle, policy, &sched) < 0) {
+    if (pthread_setschedparam(thread, policy, &sched) < 0) {
         SDL_SetError("pthread_setschedparam() failed");
         return -1;
     }
     return 0;
+#endif /* linux */
 }
 
 void

src/thread/windows/SDL_systhread.c

@@ -156,18 +156,18 @@ SDL_ThreadID(void)
 }
 
 int
-SDL_SYS_SetThreadPriority(SDL_Thread * thread, SDL_ThreadPriority priority)
+SDL_SYS_SetThreadPriority(SDL_ThreadPriority priority)
 {
-    BOOL result;
+    int value;
 
     if (priority == SDL_THREAD_PRIORITY_LOW) {
-        result = SetThreadPriority(thread->handle, THREAD_PRIORITY_LOWEST);
+        value = THREAD_PRIORITY_LOWEST;
     } else if (priority == SDL_THREAD_PRIORITY_HIGH) {
-        result = SetThreadPriority(thread->handle, THREAD_PRIORITY_HIGHEST);
+        value = THREAD_PRIORITY_HIGHEST;
     } else {
-        result = SetThreadPriority(thread->handle, THREAD_PRIORITY_NORMAL);
+        value = THREAD_PRIORITY_NORMAL;
     }
-    if (!result) {
+    if (!SetThreadPriority(GetCurrentThread(), value)) {
         WIN_SetError("SetThreadPriority()");
         return -1;
     }