diff --git a/SDKs/darwin/usr/include/fcntl.h b/SDKs/darwin/usr/include/fcntl.h
new file mode 100644
index 0000000..a5f91e3
--- /dev/null
+++ b/SDKs/darwin/usr/include/fcntl.h
@@ -0,0 +1,17 @@
+/* ===-- fcntl.h - stub SDK header for compiler-rt --------------------------===
+ *
+ *                     The LLVM Compiler Infrastructure
+ *
+ * This file is dual licensed under the MIT and the University of Illinois Open
+ * Source Licenses. See LICENSE.TXT for details.
+ *
+ * ===-----------------------------------------------------------------------===
+ *
+ * This is a stub SDK header file. This file is not part of the interface of
+ * this library nor an official version of the appropriate SDK header. It is
+ * intended only to stub the features of this header required by compiler-rt.
+ *
+ * ===-----------------------------------------------------------------------===
+ */
+
+#include <sys/fcntl.h>
diff --git a/SDKs/darwin/usr/include/stdio.h b/SDKs/darwin/usr/include/stdio.h
index 5bbd8a5..006652f 100644
--- a/SDKs/darwin/usr/include/stdio.h
+++ b/SDKs/darwin/usr/include/stdio.h
@@ -24,15 +24,18 @@
 typedef struct __sFILE FILE;
 typedef __SIZE_TYPE__ size_t;
 
-/* Determine the appropriate fopen() and fwrite() functions. */
+/* Determine the appropriate fdopen, fopen(), and fwrite() functions. */
 #if defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__)
 #  if defined(__i386)
+#    define __FDOPEN_NAME  "_fdopen$UNIX2003"
 #    define __FOPEN_NAME "_fopen$UNIX2003"
 #    define __FWRITE_NAME "_fwrite$UNIX2003"
 #  elif defined(__x86_64__)
+#    define __FDOPEN_NAME  "_fdopen"
 #    define __FOPEN_NAME "_fopen"
 #    define __FWRITE_NAME "_fwrite"
 #  elif defined(__arm)
+#    define __FDOPEN_NAME  "_fdopen"
 #    define __FOPEN_NAME "_fopen"
 #    define __FWRITE_NAME "_fwrite"
 #  else
@@ -40,9 +43,11 @@
 #  endif
 #elif defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__)
 #  if defined(__i386) || defined (__x86_64)
+#    define __FDOPEN_NAME  "_fdopen"
 #    define __FOPEN_NAME "_fopen"
 #    define __FWRITE_NAME "_fwrite"
 #  elif defined(__arm)
+#    define __FDOPEN_NAME  "_fdopen"
 #    define __FOPEN_NAME "_fopen"
 #    define __FWRITE_NAME "_fwrite"
 #  else
@@ -68,14 +73,13 @@
 int fclose(FILE *);
 int fflush(FILE *);
 FILE *fopen(const char * __restrict, const char * __restrict) __asm(__FOPEN_NAME);
+FILE *fdopen(int, const char *) __asm(__FDOPEN_NAME);
 int fprintf(FILE * __restrict, const char * __restrict, ...);
 size_t fwrite(const void * __restrict, size_t, size_t, FILE * __restrict)
   __asm(__FWRITE_NAME);
 size_t fread(void * __restrict, size_t, size_t, FILE * __restrict);
 long ftell(FILE *);
 int fseek(FILE *, long, int);
-void setbuf(FILE * __restrict, char * __restrict);
-
 int snprintf(char * __restrict, size_t, const char * __restrict, ...);
 
 #if defined(__cplusplus)
diff --git a/SDKs/darwin/usr/include/stdlib.h b/SDKs/darwin/usr/include/stdlib.h
index 7c973dc..b6d3171 100644
--- a/SDKs/darwin/usr/include/stdlib.h
+++ b/SDKs/darwin/usr/include/stdlib.h
@@ -27,5 +27,6 @@
 void free(void *);
 char *getenv(const char *);
 void *malloc(size_t);
+void *realloc(void *, size_t);
 
 #endif /* __STDLIB_H__ */
diff --git a/SDKs/darwin/usr/include/sys/fcntl.h b/SDKs/darwin/usr/include/sys/fcntl.h
new file mode 100644
index 0000000..b71706b
--- /dev/null
+++ b/SDKs/darwin/usr/include/sys/fcntl.h
@@ -0,0 +1,52 @@
+/* ===-- fcntl.h - stub SDK header for compiler-rt --------------------------===
+ *
+ *                     The LLVM Compiler Infrastructure
+ *
+ * This file is dual licensed under the MIT and the University of Illinois Open
+ * Source Licenses. See LICENSE.TXT for details.
+ *
+ * ===-----------------------------------------------------------------------===
+ *
+ * This is a stub SDK header file. This file is not part of the interface of
+ * this library nor an official version of the appropriate SDK header. It is
+ * intended only to stub the features of this header required by compiler-rt.
+ *
+ * ===-----------------------------------------------------------------------===
+ */
+
+#ifndef _SYS_FCNTL_H_
+#define _SYS_FCNTL_H_
+
+/* Determine the appropriate open function. */
+#if defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__)
+#  if defined(__i386)
+#    define __OPEN_NAME  "_open$UNIX2003"
+#  elif defined(__x86_64__)
+#    define __OPEN_NAME  "_open"
+#  elif defined(__arm)
+#    define __OPEN_NAME  "_open"
+#  else
+#    error "unrecognized architecture for targetting OS X"
+#  endif
+#elif defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__)
+#  if defined(__i386) || defined (__x86_64)
+#    define __OPEN_NAME  "_open"
+#  elif defined(__arm)
+#    define __OPEN_NAME  "_open"
+#  else
+#    error "unrecognized architecture for targetting iOS"
+#  endif
+#else
+#  error "unrecognized architecture for targetting Darwin"
+#endif
+
+#define O_RDONLY   0x0000    /* open for reading only */
+#define O_WRONLY   0x0001    /* open for writing only */
+#define O_RDWR     0x0002    /* open for reading and writing */
+#define O_ACCMODE  0x0003    /* mask for above modes */
+
+#define O_CREAT    0x0200    /* create if nonexistant */
+
+int open(const char *, int, ...) __asm(__OPEN_NAME);
+
+#endif /* !_SYS_FCNTL_H_ */
diff --git a/SDKs/darwin/usr/include/sys/mman.h b/SDKs/darwin/usr/include/sys/mman.h
new file mode 100644
index 0000000..84561f1
--- /dev/null
+++ b/SDKs/darwin/usr/include/sys/mman.h
@@ -0,0 +1,42 @@
+/* ===-- mman.h - stub SDK header for compiler-rt ---------------------------===
+ *
+ *                     The LLVM Compiler Infrastructure
+ *
+ * This file is dual licensed under the MIT and the University of Illinois Open
+ * Source Licenses. See LICENSE.TXT for details.
+ *
+ * ===-----------------------------------------------------------------------===
+ *
+ * This is a stub SDK header file. This file is not part of the interface of
+ * this library nor an official version of the appropriate SDK header. It is
+ * intended only to stub the features of this header required by compiler-rt.
+ *
+ * ===-----------------------------------------------------------------------===
+ */
+
+#ifndef __SYS_MMAN_H__
+#define __SYS_MMAN_H__
+
+typedef __SIZE_TYPE__ size_t;
+
+#define PROT_NONE     0x00
+#define PROT_READ     0x01
+#define PROT_WRITE    0x02
+#define PROT_EXEC     0x04
+
+#define MAP_SHARED    0x0001
+#define MAP_PRIVATE   0x0002
+
+#define MAP_FILE      0x0000
+#define MAP_ANON      0x1000
+
+#define MS_ASYNC      0x0001
+#define MS_INVALIDATE 0x0002
+#define MS_SYNC       0x0010
+
+void *mmap(void *addr, size_t len, int prot, int flags, int fd,
+           long long offset);
+int munmap(void *addr, size_t len);
+int msync(void *addr, size_t len, int flags);
+
+#endif /* __SYS_MMAN_H__ */
diff --git a/SDKs/linux/usr/include/fcntl.h b/SDKs/linux/usr/include/fcntl.h
new file mode 100644
index 0000000..a5f91e3
--- /dev/null
+++ b/SDKs/linux/usr/include/fcntl.h
@@ -0,0 +1,17 @@
+/* ===-- fcntl.h - stub SDK header for compiler-rt --------------------------===
+ *
+ *                     The LLVM Compiler Infrastructure
+ *
+ * This file is dual licensed under the MIT and the University of Illinois Open
+ * Source Licenses. See LICENSE.TXT for details.
+ *
+ * ===-----------------------------------------------------------------------===
+ *
+ * This is a stub SDK header file. This file is not part of the interface of
+ * this library nor an official version of the appropriate SDK header. It is
+ * intended only to stub the features of this header required by compiler-rt.
+ *
+ * ===-----------------------------------------------------------------------===
+ */
+
+#include <sys/fcntl.h>
diff --git a/SDKs/linux/usr/include/stdio.h b/SDKs/linux/usr/include/stdio.h
index 0ca10fa..fba5936 100644
--- a/SDKs/linux/usr/include/stdio.h
+++ b/SDKs/linux/usr/include/stdio.h
@@ -33,11 +33,11 @@
 extern int fclose(FILE *);
 extern int fflush(FILE *);
 extern FILE *fopen(const char * restrict, const char * restrict);
+extern FILE *fdopen(int, const char * restrict);
 extern int fprintf(FILE * restrict, const char * restrict, ...);
 extern size_t fwrite(const void * restrict, size_t, size_t, FILE * restrict);
 extern size_t fread(void * restrict, size_t, size_t, FILE * restrict);
 extern long ftell(FILE *);
 extern int fseek(FILE *, long, int);
-extern void setbuf(FILE * restrict, char * restrict);
 
 #endif /* __STDIO_H__ */
diff --git a/SDKs/linux/usr/include/stdlib.h b/SDKs/linux/usr/include/stdlib.h
index e6b8bf7..966b29d 100644
--- a/SDKs/linux/usr/include/stdlib.h
+++ b/SDKs/linux/usr/include/stdlib.h
@@ -30,5 +30,7 @@
   __attribute__((__warn_unused_result__));
 void *malloc(size_t) __attribute__((__nothrow__)) __attribute((__malloc__))
      __attribute__((__warn_unused_result__));
+void *realloc(void *, size_t) __attribute__((__nothrow__)) __attribute((__malloc__))
+     __attribute__((__warn_unused_result__));
 
 #endif /* __STDLIB_H__ */
diff --git a/SDKs/linux/usr/include/sys/fcntl.h b/SDKs/linux/usr/include/sys/fcntl.h
new file mode 100644
index 0000000..1512bf9
--- /dev/null
+++ b/SDKs/linux/usr/include/sys/fcntl.h
@@ -0,0 +1,29 @@
+/* ===-- fcntl.h - stub SDK header for compiler-rt --------------------------===
+ *
+ *                     The LLVM Compiler Infrastructure
+ *
+ * This file is dual licensed under the MIT and the University of Illinois Open
+ * Source Licenses. See LICENSE.TXT for details.
+ *
+ * ===-----------------------------------------------------------------------===
+ *
+ * This is a stub SDK header file. This file is not part of the interface of
+ * this library nor an official version of the appropriate SDK header. It is
+ * intended only to stub the features of this header required by compiler-rt.
+ *
+ * ===-----------------------------------------------------------------------===
+ */
+
+#ifndef _SYS_FCNTL_H_
+#define _SYS_FCNTL_H_
+
+#define O_RDONLY   0x0000
+#define O_WRONLY   0x0001
+#define O_RDWR     0x0002
+#define O_ACCMODE  0x0003
+
+#define O_CREAT    0x0200
+
+int open(const char *, int, ...);
+
+#endif /* _SYS_FCNTL_H_ */
diff --git a/SDKs/linux/usr/include/sys/mman.h b/SDKs/linux/usr/include/sys/mman.h
index 7c4d051..bfb7f8b 100644
--- a/SDKs/linux/usr/include/sys/mman.h
+++ b/SDKs/linux/usr/include/sys/mman.h
@@ -19,10 +19,28 @@
 
 typedef __SIZE_TYPE__ size_t;
 
-#define PROT_READ 0x1
-#define PROT_WRITE 0x2
-#define PROT_EXEC 0x4
+#define PROT_NONE     0x00
+#define PROT_READ     0x01
+#define PROT_WRITE    0x02
+#define PROT_EXEC     0x04
 
+#define MAP_SHARED    0x0001
+#define MAP_PRIVATE   0x0002
+
+#define MAP_FILE      0x0000
+#define MAP_ANON      0x1000
+
+#define MS_ASYNC      0x0001
+#define MS_INVALIDATE 0x0002
+#define MS_SYNC       0x0010
+
+extern void *mmap(void *addr, size_t len, int prot, int flags, int fd,
+                  long long offset)
+  __attribute__((__nothrow__));
+extern int munmap(void *addr, size_t len)
+  __attribute__((__nothrow__));
+extern int msync(void *addr, size_t len, int flags)
+  __attribute__((__nothrow__));
 extern int mprotect (void *__addr, size_t __len, int __prot)
   __attribute__((__nothrow__));
 
diff --git a/lib/profile/GCDAProfiling.c b/lib/profile/GCDAProfiling.c
index 53ecff4..71dd9bd 100644
--- a/lib/profile/GCDAProfiling.c
+++ b/lib/profile/GCDAProfiling.c
@@ -20,10 +20,12 @@
 |*
 \*===----------------------------------------------------------------------===*/
 
+#include <fcntl.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
+#include <sys/mman.h>
 #include <sys/types.h>
 #ifdef _WIN32
 #include <direct.h>
@@ -50,11 +52,13 @@
 /*
  * Buffer that we write things into.
  */
-#define WRITE_BUFFER_SIZE (1 << 12)
-static char write_buffer[WRITE_BUFFER_SIZE];
-static int64_t cur_file_pos = 0;
-static uint32_t cur_offset = 0;
+#define NEW_WRITE_BUFFER_SIZE (128 * 1024)
+static char *new_write_buffer = NULL;
+static uint64_t cur_buffer_size = 0;
+static uint64_t cur_pos = 0;
+static uint64_t file_size = 0;
 static int new_file = 0;
+static int fd = -1;
 
 /*
  * A list of functions to write out the data.
@@ -82,30 +86,20 @@
 static struct flush_fn_node *flush_fn_head = NULL;
 static struct flush_fn_node *flush_fn_tail = NULL;
 
-static void flush_buffer() {
-  /* Flush the buffer to file. */
-  fwrite(write_buffer, cur_offset, 1, output_file);
-
-  if (new_file) {
-    cur_offset = 0;
-    return;
-  }
-
-  /* Read in more of the file. */
-  fread(write_buffer,1, WRITE_BUFFER_SIZE, output_file);
-  fseek(output_file, cur_file_pos, SEEK_SET);
-
-  cur_offset = 0;
+static void resize_write_buffer(uint64_t size) {
+  if (!new_file) return;
+  size += cur_pos;
+  if (size <= cur_buffer_size) return;
+  size = (size - 1) / NEW_WRITE_BUFFER_SIZE + 1;
+  size *= NEW_WRITE_BUFFER_SIZE;
+  new_write_buffer = realloc(new_write_buffer, size);
+  cur_buffer_size = size;
 }
 
 static void write_bytes(const char *s, size_t len) {
-  if (cur_offset + len > WRITE_BUFFER_SIZE)
-    flush_buffer();
-
-  cur_file_pos += len;
-
-  for (; len > 0; --len)
-    write_buffer[cur_offset++] = *s++;
+  resize_write_buffer(len);
+  memcpy(&new_write_buffer[cur_pos], s, len);
+  cur_pos += len;
 }
 
 static void write_32bit_value(uint32_t i) {
@@ -130,30 +124,22 @@
 static uint32_t read_32bit_value() {
   uint32_t val;
 
-  if (cur_offset + 4 > WRITE_BUFFER_SIZE)
-    flush_buffer();
-
   if (new_file)
     return (uint32_t)-1;
 
-  val = *(uint32_t*)&write_buffer[cur_offset];
-  cur_file_pos += 4;
-  cur_offset += 4;
+  val = *(uint32_t*)&new_write_buffer[cur_pos];
+  cur_pos += 4;
   return val;
 }
 
 static uint64_t read_64bit_value() {
   uint64_t val;
 
-  if (cur_offset + 8 > WRITE_BUFFER_SIZE)
-    flush_buffer();
-
   if (new_file)
-    return (uint32_t)-1;
+    return (uint64_t)-1;
 
-  val = *(uint64_t*)&write_buffer[cur_offset];
-  cur_file_pos += 8;
-  cur_offset += 8;
+  val = *(uint64_t*)&new_write_buffer[cur_pos];
+  cur_pos += 8;
   return val;
 }
 
@@ -210,6 +196,21 @@
   }
 }
 
+static void map_file() {
+  fseek(output_file, 0L, SEEK_END);
+  file_size = ftell(output_file);
+
+  new_write_buffer = mmap(0, file_size, PROT_READ | PROT_WRITE,
+                          MAP_FILE | MAP_SHARED, fd, 0);
+}
+
+static void unmap_file() {
+  msync(new_write_buffer, file_size, MS_SYNC);
+  munmap(new_write_buffer, file_size);
+  new_write_buffer = NULL;
+  file_size = 0;
+}
+
 /*
  * --- LLVM line counter API ---
  */
@@ -220,18 +221,21 @@
  */
 void llvm_gcda_start_file(const char *orig_filename, const char version[4]) {
   char *filename = mangle_filename(orig_filename);
+  const char *mode = "r+b";
 
   /* Try just opening the file. */
-  output_file = fopen(filename, "r+b");
+  new_file = 0;
+  fd = open(filename, O_RDWR);
 
-  if (!output_file) {
+  if (fd == -1) {
     /* Try opening the file, creating it if necessary. */
     new_file = 1;
-    output_file = fopen(filename, "w+b");
-    if (!output_file) {
+    mode = "w+b";
+    fd = open(filename, O_RDWR | O_CREAT);
+    if (fd == -1) {
       /* Try creating the directories first then opening the file. */
       recursive_mkdir(filename);
-      output_file = fopen(filename, "w+b");
+      fd = open(filename, O_RDWR | O_CREAT);
       if (!output_file) {
         /* Bah! It's hopeless. */
         fprintf(stderr, "profiling:%s: cannot open\n", filename);
@@ -241,14 +245,19 @@
     }
   }
 
-  setbuf(output_file, 0);
+  output_file = fdopen(fd, mode);
 
   /* Initialize the write buffer. */
-  memset(write_buffer, 0, WRITE_BUFFER_SIZE);
-  fread(write_buffer, 1, WRITE_BUFFER_SIZE, output_file);
-  fseek(output_file, 0L, SEEK_SET);
-  cur_file_pos = 0;
-  cur_offset = 0;
+  new_write_buffer = NULL;
+  cur_buffer_size = 0;
+  cur_pos = 0;
+
+  if (new_file) {
+    resize_write_buffer(NEW_WRITE_BUFFER_SIZE);
+    memset(new_write_buffer, 0, NEW_WRITE_BUFFER_SIZE);
+  } else {
+    map_file();
+  }
 
   /* gcda file, version, stamp LLVM. */
   write_bytes("adcg", 4);
@@ -315,12 +324,11 @@
 void llvm_gcda_emit_arcs(uint32_t num_counters, uint64_t *counters) {
   uint32_t i;
   uint64_t *old_ctrs = NULL;
-  int64_t save_cur_file_pos = cur_file_pos;
   uint32_t val = 0;
+  uint64_t save_cur_pos = cur_pos;
 
   if (!output_file) return;
 
-  flush_buffer();
   val = read_32bit_value();
 
   if (val != (uint32_t)-1) {
@@ -339,15 +347,10 @@
     old_ctrs = malloc(sizeof(uint64_t) * num_counters);
     for (i = 0; i < num_counters; ++i)
       old_ctrs[i] = read_64bit_value();
-
-    /* Reset the current buffer and file position. */
-    memset(write_buffer, 0, WRITE_BUFFER_SIZE);
-    cur_file_pos = save_cur_file_pos;
-    cur_offset = 0;
-
-    fseek(output_file, cur_file_pos, SEEK_SET);
   }
 
+  cur_pos = save_cur_pos;
+
   /* Counter #1 (arcs) tag */
   write_bytes("\0\0\xa1\1", 4);
   write_32bit_value(num_counters * 2);
@@ -369,9 +372,17 @@
   /* Write out EOF record. */
   if (!output_file) return;
   write_bytes("\0\0\0\0\0\0\0\0", 8);
-  flush_buffer();
+
+  if (new_file) {
+    fwrite(new_write_buffer, cur_pos, 1, output_file);
+    free(new_write_buffer);
+  } else {
+    unmap_file();
+  }
+
   fclose(output_file);
   output_file = NULL;
+  new_write_buffer = NULL;
 
 #ifdef DEBUG_GCDAPROFILING
   fprintf(stderr, "llvmgcda: -----\n");