Add a portable getpagesize() implementation

posix.cc

@@ -222,3 +222,16 @@ fmt::BufferedFile fmt::File::fdopen(const char *mode) {
   fd_ = -1;
   return file;
 }
+
+long fmt::getpagesize() {
+#ifdef _WIN32
+  SYSTEM_INFO si = {};
+  GetSystemInfo(&si);
+  return si.dwPageSize;
+#else
+  long size = FMT_POSIX_CALL(sysconf(_SC_PAGESIZE));
+  if (size < 0)
+    throw SystemError(errno, "cannot get memory page size");
+  return size;
+#endif
+}

posix.h

@@ -320,7 +320,10 @@ class File {
   // this File object from the file.
   BufferedFile fdopen(const char *mode);
 };
-}
+
+// Returns the memory page size.
+long getpagesize();
+}  // namespace fmt
 
 #if !FMT_USE_RVALUE_REFERENCES
 namespace std {

test/posix-test.cc

@@ -56,6 +56,7 @@ int fileno_count;
 std::size_t read_nbyte;
 std::size_t write_nbyte;
 bool increase_file_size;
+bool sysconf_error;
 }
 
 #define EMULATE_EINTR(func, error_result) \
@@ -80,6 +81,15 @@ int test::fstat(int fd, struct stat *buf) {
     buf->st_size = max_file_size();
   return result;
 }
+
+long test::sysconf(int name) {
+  long result = ::sysconf(name);
+  if (!sysconf_error)
+    return result;
+  // Simulate an error.
+  errno = EINVAL;
+  return -1;
+}
 #else
 errno_t test::sopen_s(
     int* pfh, const char *filename, int oflag, int shflag, int pmode) {
@@ -184,6 +194,20 @@ TEST(UtilTest, StaticAssert) {
   // a compile-time error.
 }
 
+TEST(UtilTest, GetPageSize) {
+#ifdef _WIN32
+  SYSTEM_INFO si = {};
+  GetSystemInfo(&si);
+  EXPECT_EQ(si.dwPageSize, fmt::getpagesize());
+#else
+  EXPECT_EQ(sysconf(_SC_PAGESIZE), fmt::getpagesize());
+  sysconf_error = true;
+  EXPECT_SYSTEM_ERROR(
+      fmt::getpagesize(), EINVAL, "cannot get memory page size");
+  sysconf_error = false;
+#endif
+}
+
 TEST(FileTest, OpenRetry) {
   write_file("test", "there must be something here");
   File *f = 0;

test/posix-test.h

@@ -45,6 +45,7 @@ typedef size_t size_t;
 typedef ssize_t ssize_t;
 int open(const char *path, int oflag, int mode);
 int fstat(int fd, struct stat *buf);
+long sysconf(int name);
 #else
 typedef unsigned size_t;
 typedef int ssize_t;