Only use available CPUs

Not all online CPUs may be available for the current process,
especially when CPU affinity is involved. In such cases too many
threads will be created, which will unnecessarily compete for CPU
time.

Use sched_getaffinity() (if available) to determine the correct
number of threads to create.

configure.ac

@@ -68,7 +68,7 @@ AC_SYS_LARGEFILE
 AC_FUNC_MALLOC
 AC_FUNC_REALLOC
 AC_FUNC_STRTOD
-AC_CHECK_FUNCS([memchr memmove memset strerror strtol])
+AC_CHECK_FUNCS([memchr memmove memset strerror strtol sched_getaffinity])
 AC_CHECK_HEADER([sys/endian.h],
                [
                  AC_CHECK_DECLS([htole64, le64toh], [], [], [

src/cpu.c

@@ -1,5 +1,28 @@
+#define _GNU_SOURCE
+
 #include <unistd.h>
 
+#include "config.h"
+
+#ifdef HAVE_SCHED_GETAFFINITY
+
+#include <sched.h>
+#include <stdio.h>
+
+size_t num_threads(void) {
+    cpu_set_t cpu_set;
+    CPU_ZERO(&cpu_set);
+
+    if (sched_getaffinity(0, sizeof cpu_set, &cpu_set) == -1)
+        return sysconf(_SC_NPROCESSORS_ONLN);
+    else
+        return CPU_COUNT(&cpu_set);
+}
+
+#else
+
 size_t num_threads(void) {
     return sysconf(_SC_NPROCESSORS_ONLN);
 }
+
+#endif