subprocess: Avoid leaking POSIX name aliases beyond subprocess.h

The commit a32c0f3df4b6bcd1d2e93f19e8f380bb890cd507 introduced code to
silence MSVC's "warning C4996: The POSIX name for this item is
deprecated."

However, it exhibits several issues:
1. The aliases may leak into code outside the `subprocess.hpp` header.
2. They are unnecessarily applied when using the MinGW-w64 toolchain.
3. The fix is incomplete: downstream projects still see C4996 warnings.
4. The implementation lacks documentation.

This change addresses all of the above shortcomings.

Github-Pull: arun11299/cpp-subprocess#112
Rebased-From: 778543b2f2ca7f5d1c4f0241b635bbb265d750dd

Co-authored-by: Luke Dashjr <luke-jr+git@utopios.org>

Author: hebasto

src/util/subprocess.h

@@ -68,10 +68,6 @@ extern "C" {
   #include <windows.h>
   #include <io.h>
   #include <cwchar>
-
-  #define close _close
-  #define open _open
-  #define fileno _fileno
 #else
   #include <sys/wait.h>
   #include <unistd.h>
@@ -81,6 +77,22 @@ extern "C" {
   #include <sys/types.h>
 }
 
+// The Microsoft C++ compiler issues deprecation warnings
+// for the standard POSIX function names.
+// Its preferred implementations have a leading underscore.
+// See: https://learn.microsoft.com/en-us/cpp/c-runtime-library/compatibility.
+#if (defined _MSC_VER)
+  #define subprocess_close _close
+  #define subprocess_fileno _fileno
+  #define subprocess_open _open
+  #define subprocess_write _write
+#else
+  #define subprocess_close close
+  #define subprocess_fileno fileno
+  #define subprocess_open open
+  #define subprocess_write write
+#endif
+
 /*!
  * Getting started with reading this source code.
  * The source is mainly divided into four parts:
@@ -264,7 +276,7 @@ namespace util
 
     FILE *fp = _fdopen(os_fhandle, mode);
     if (fp == 0) {
-      _close(os_fhandle);
+      subprocess_close(os_fhandle);
       throw OSError("_fdopen", 0);
     }
 
@@ -383,7 +395,7 @@ namespace util
   {
     size_t nwritten = 0;
     while (nwritten < length) {
-      int written = write(fd, buf + nwritten, length - nwritten);
+      int written = subprocess_write(fd, buf + nwritten, length - nwritten);
       if (written == -1) return -1;
       nwritten += written;
     }
@@ -411,7 +423,7 @@ namespace util
 #ifdef __USING_WINDOWS__
     return (int)fread(buf, 1, read_upto, fp);
 #else
-    int fd = fileno(fp);
+    int fd = subprocess_fileno(fp);
     int rbytes = 0;
     int eintr_cnter = 0;
 
@@ -573,10 +585,10 @@ struct input
   explicit input(int fd): rd_ch_(fd) {}
 
   // FILE pointer.
-  explicit input (FILE* fp):input(fileno(fp)) { assert(fp); }
+  explicit input (FILE* fp):input(subprocess_fileno(fp)) { assert(fp); }
 
   explicit input(const char* filename) {
-    int fd = open(filename, O_RDONLY);
+    int fd = subprocess_open(filename, O_RDONLY);
     if (fd == -1) throw OSError("File not found: ", errno);
     rd_ch_ = fd;
   }
@@ -606,10 +618,10 @@ struct output
 {
   explicit output(int fd): wr_ch_(fd) {}
 
-  explicit output (FILE* fp):output(fileno(fp)) { assert(fp); }
+  explicit output (FILE* fp):output(subprocess_fileno(fp)) { assert(fp); }
 
   explicit output(const char* filename) {
-    int fd = open(filename, O_APPEND | O_CREAT | O_RDWR, 0640);
+    int fd = subprocess_open(filename, O_APPEND | O_CREAT | O_RDWR, 0640);
     if (fd == -1) throw OSError("File not found: ", errno);
     wr_ch_ = fd;
   }
@@ -637,10 +649,10 @@ struct error
 {
   explicit error(int fd): wr_ch_(fd) {}
 
-  explicit error(FILE* fp):error(fileno(fp)) { assert(fp); }
+  explicit error(FILE* fp):error(subprocess_fileno(fp)) { assert(fp); }
 
   explicit error(const char* filename) {
-    int fd = open(filename, O_APPEND | O_CREAT | O_RDWR, 0640);
+    int fd = subprocess_open(filename, O_APPEND | O_CREAT | O_RDWR, 0640);
     if (fd == -1) throw OSError("File not found: ", errno);
     wr_ch_ = fd;
   }
@@ -803,28 +815,28 @@ class Streams
   void cleanup_fds()
   {
     if (write_to_child_ != -1 && read_from_parent_ != -1) {
-      close(write_to_child_);
+      subprocess_close(write_to_child_);
     }
     if (write_to_parent_ != -1 && read_from_child_ != -1) {
-      close(read_from_child_);
+      subprocess_close(read_from_child_);
     }
     if (err_write_ != -1 && err_read_ != -1) {
-      close(err_read_);
+      subprocess_close(err_read_);
     }
   }
 
   void close_parent_fds()
   {
-    if (write_to_child_ != -1)  close(write_to_child_);
-    if (read_from_child_ != -1) close(read_from_child_);
-    if (err_read_ != -1)        close(err_read_);
+    if (write_to_child_ != -1)  subprocess_close(write_to_child_);
+    if (read_from_child_ != -1) subprocess_close(read_from_child_);
+    if (err_read_ != -1)        subprocess_close(err_read_);
   }
 
   void close_child_fds()
   {
-    if (write_to_parent_ != -1)  close(write_to_parent_);
-    if (read_from_parent_ != -1) close(read_from_parent_);
-    if (err_write_ != -1)        close(err_write_);
+    if (write_to_parent_ != -1)  subprocess_close(write_to_parent_);
+    if (read_from_parent_ != -1) subprocess_close(read_from_parent_);
+    if (err_write_ != -1)        subprocess_close(err_write_);
   }
 
   FILE* input()  { return input_.get(); }
@@ -1161,8 +1173,8 @@ inline void Popen::execute_process() noexcept(false)
   child_pid_ = fork();
 
   if (child_pid_ < 0) {
-    close(err_rd_pipe);
-    close(err_wr_pipe);
+    subprocess_close(err_rd_pipe);
+    subprocess_close(err_wr_pipe);
     throw OSError("fork failed", errno);
   }
 
@@ -1172,14 +1184,14 @@ inline void Popen::execute_process() noexcept(false)
     stream_.close_parent_fds();
 
     //Close the read end of the error pipe
-    close(err_rd_pipe);
+    subprocess_close(err_rd_pipe);
 
     detail::Child chld(this, err_wr_pipe);
     chld.execute_child();
   }
   else
   {
-    close (err_wr_pipe);// close child side of pipe, else get stuck in read below
+    subprocess_close(err_wr_pipe);// close child side of pipe, else get stuck in read below
 
     stream_.close_child_fds();
 
@@ -1188,7 +1200,7 @@ inline void Popen::execute_process() noexcept(false)
 
       FILE* err_fp = fdopen(err_rd_pipe, "r");
       if (!err_fp) {
-          close(err_rd_pipe);
+          subprocess_close(err_rd_pipe);
           throw OSError("fdopen failed", errno);
       }
       int read_bytes = util::read_atmost_n(err_fp, err_buf, SP_MAX_ERR_BUF_SIZ);
@@ -1278,13 +1290,13 @@ namespace detail {
 
       // Close the duped descriptors
       if (stream.read_from_parent_ != -1 && stream.read_from_parent_ > 2)
-        close(stream.read_from_parent_);
+        subprocess_close(stream.read_from_parent_);
 
       if (stream.write_to_parent_ != -1 && stream.write_to_parent_ > 2)
-        close(stream.write_to_parent_);
+        subprocess_close(stream.write_to_parent_);
 
       if (stream.err_write_ != -1 && stream.err_write_ > 2)
-        close(stream.err_write_);
+        subprocess_close(stream.err_write_);
 
       // Replace the current image with the executable
       sys_ret = execvp(parent_->exe_name_.c_str(), parent_->cargv_.data());
@@ -1311,15 +1323,15 @@ namespace detail {
 #ifdef __USING_WINDOWS__
     util::configure_pipe(&this->g_hChildStd_IN_Rd, &this->g_hChildStd_IN_Wr, &this->g_hChildStd_IN_Wr);
     this->input(util::file_from_handle(this->g_hChildStd_IN_Wr, "w"));
-    this->write_to_child_ = _fileno(this->input());
+    this->write_to_child_ = subprocess_fileno(this->input());
 
     util::configure_pipe(&this->g_hChildStd_OUT_Rd, &this->g_hChildStd_OUT_Wr, &this->g_hChildStd_OUT_Rd);
     this->output(util::file_from_handle(this->g_hChildStd_OUT_Rd, "r"));
-    this->read_from_child_ = _fileno(this->output());
+    this->read_from_child_ = subprocess_fileno(this->output());
 
     util::configure_pipe(&this->g_hChildStd_ERR_Rd, &this->g_hChildStd_ERR_Wr, &this->g_hChildStd_ERR_Rd);
     this->error(util::file_from_handle(this->g_hChildStd_ERR_Rd, "r"));
-    this->err_read_ = _fileno(this->error());
+    this->err_read_ = subprocess_fileno(this->error());
 #else
 
     if (write_to_child_ != -1)  input(fdopen(write_to_child_, "wb"));