Don't depend on internals of FILE*

closes alandefreitas#4

Author: mrexodia

source/matplot/CMakeLists.txt

@@ -89,6 +89,15 @@ target_link_libraries(matplot PUBLIC nodesoup cimg)
 # https://cmake.org/cmake/help/v3.14/manual/cmake-compile-features.7.html#requiring-language-standards
 target_compile_features(matplot PUBLIC cxx_std_17)
 
+include(CheckSymbolExists)
+
+# Some hack to not depend on FILE* internals
+# https://github.com/alandefreitas/matplotplusplus/issues/4
+check_symbol_exists(__fbufsize "stdio_ext.h" HAVE_FBUFSIZE)
+if (HAVE_FBUFSIZE)
+    target_compile_definitions(matplot PRIVATE MATPLOT_HAS_FBUFSIZE)
+endif()
+
 if (BUILD_FOR_DOCUMENTATION_IMAGES)
     message("Building matplot for documentation images. wait() commands will be ignored. ~figure will save the files.")
     target_compile_definitions(matplot PUBLIC MATPLOT_BUILD_FOR_DOCUMENTATION_IMAGES)

source/matplot/backend/gnuplot.cpp

@@ -10,6 +10,23 @@
 #include <matplot/util/popen.h>
 #include <matplot/util/common.h>
 
+#ifdef MATPLOT_HAS_FBUFSIZE
+
+#include <stdio_ext.h>
+
+static size_t gnuplot_pipe_capacity(FILE *f) {
+    size_t sz = __fbufsize(f);
+    return sz != 0 ? sz : matplot::backend::gnuplot::pipe_capacity_worst_case;
+}
+
+#else
+
+static size_t gnuplot_pipe_capacity(FILE *) {
+    return matplot::backend::gnuplot::pipe_capacity_worst_case;
+}
+
+#endif // MATPLOT_HAS_FBUFSIZE
+
 namespace matplot::backend {
     bool gnuplot::consumes_gnuplot_commands() {
         return true;
@@ -220,7 +237,7 @@ namespace matplot::backend {
         if (!pipe_) {
             return;
         }
-        size_t pipe_capacity = (pipe_->_bf._base != nullptr) ? pipe_->_bf._size : pipe_capacity_worst_case;
+        size_t pipe_capacity = gnuplot_pipe_capacity(pipe_);
         if (command.size() + bytes_in_pipe_ > pipe_capacity) {
             flush_commands();
             bytes_in_pipe_ = 0;

source/matplot/backend/gnuplot.h

@@ -77,7 +77,7 @@ namespace matplot::backend {
         static constexpr bool trace_commands = false;
 #endif
 
-        static constexpr size_t pipe_capacity_worst_case = 4096;
+        static constexpr size_t pipe_capacity_worst_case = BUFSIZ;
         /// File formats for figures and properties of terminals
         static constexpr std::array<std::pair<std::string_view,std::string_view>,33> extension_terminal() {
             return std::array<std::pair<std::string_view,std::string_view>,33>