Fix the pwrite64 system call. (emscripten-core#4838)

There was an implementation of the `pwrite64` syscall in Emscripten,
but when invoked it crashed because of ReferenceError.

This commit replaces the `nbyte` variable with the `count` one and
adds some tests for this syscall.

Author: atrosinenko

AUTHORS

@@ -271,3 +271,4 @@ a license to everyone to use it as detailed in LICENSE.)
 * Vladimir Davidovich <[email protected]>
 * Christophe Gragnic <[email protected]>
 * Murphy McCauley <[email protected]>
+* Anatoly Trosinenko <[email protected]>

src/library_syscall.js

@@ -838,11 +838,8 @@ var SyscallsLibrary = {
     return FS.read(stream, {{{ heapAndOffset('HEAP8', 'buf') }}}, count, offset);
   },
   __syscall181: function(which, varargs) { // pwrite64
-#if SYSCALL_DEBUG
-    Module.printErr('warning: untested syscall');
-#endif
     var stream = SYSCALLS.getStreamFromFD(), buf = SYSCALLS.get(), count = SYSCALLS.get(), zero = SYSCALLS.getZero(), offset = SYSCALLS.get64();
-    return FS.write(stream, {{{ heapAndOffset('HEAP8', 'buf') }}}, nbyte, offset);
+    return FS.write(stream, {{{ heapAndOffset('HEAP8', 'buf') }}}, count, offset);
   },
   __syscall183: function(which, varargs) { // getcwd
     var buf = SYSCALLS.get(), size = SYSCALLS.get();

tests/unistd/io.c

@@ -156,6 +156,16 @@ int main() {
   printf("errno: %d\n\n", errno);
   errno = 0;
 
+  printf("pwrite to the middle of file: %d\n", pwrite(f, writeBuffer + 2, 3, 17));
+  printf("errno: %d\n", errno);
+  printf("seek: %d\n\n", lseek(f, 0, SEEK_CUR));
+  errno = 0;
+
+  printf("pwrite past end of file: %d\n", pwrite(f, writeBuffer, 5, 32));
+  printf("errno: %d\n", errno);
+  printf("seek: %d\n\n", lseek(f, 0, SEEK_CUR));
+  errno = 0;
+
   int bytesRead;
   printf("seek: %d\n", lseek(f, 0, SEEK_SET));
   printf("read after write: %d\n", bytesRead = read(f, readBuffer, sizeof readBuffer));

tests/unistd/io.out

@@ -66,7 +66,15 @@ seek: 23
 write after end of file: 8
 errno: 0
 
+pwrite to the middle of file: 3
+errno: 0
+seek: 31
+
+pwrite past end of file: 5
+errno: 0
+seek: 31
+
 seek: 0
-read after write: 31
+read after write: 37
 errno: 0
-final: wri4567890wri\0\0\0\0\0\0\0\0\0\0writeme\0
+final: wri4567890wri\0\0\0\0ite\0\0\0writeme\0\0write