Silence unused result warnings on newer compilers

Newer versions of GCC and Clang are not satisfied by a cast to void,
this fix is adapted from glibc's solution.

New wrapper function ignore_result should be used when a function with
explicit _unused_attribute_ wrapper is called whose result will not be
handled.

src/common.cpp

@@ -616,14 +616,14 @@ void debug_safe(int level, const char *msg, const char *param1, const char *para
         const char *end = strchr(cursor, '%');
         if (end == NULL) end = cursor + strlen(cursor);
 
-        (void)write(STDERR_FILENO, cursor, end - cursor);
+        ignore_result(write(STDERR_FILENO, cursor, end - cursor));
 
         if (end[0] == '%' && end[1] == 's') {
             // Handle a format string.
             assert(param_idx < sizeof params / sizeof *params);
             const char *format = params[param_idx++];
             if (!format) format = "(null)";
-            (void)write(STDERR_FILENO, format, strlen(format));
+            ignore_result(write(STDERR_FILENO, format, strlen(format)));
             cursor = end + 2;
         } else if (end[0] == '\0') {
             // Must be at the end of the string.
@@ -635,7 +635,7 @@ void debug_safe(int level, const char *msg, const char *param1, const char *para
     }
 
     // We always append a newline.
-    (void)write(STDERR_FILENO, "\n", 1);
+    ignore_result(write(STDERR_FILENO, "\n", 1));
 
     errno = errno_old;
 }

src/common.h

@@ -199,12 +199,12 @@ extern bool has_working_tty_timestamps;
 // from within a `switch` block. As of the time I'm writing this oclint doesn't recognize the
 // `__attribute__((noreturn))` on the exit_without_destructors() function.
 // TODO: we use C++11 [[noreturn]] now, does that change things?
-#define FATAL_EXIT()                        \
-    {                                       \
-        char exit_read_buff;                \
-        show_stackframe(L'E');              \
-        (void)read(0, &exit_read_buff, 1); \
-        exit_without_destructors(1);        \
+#define FATAL_EXIT()                       \
+    {                                      \
+        char exit_read_buff;               \
+        show_stackframe(L'E');             \
+        ignore_result(read(0, &exit_read_buff, 1)); \
+        exit_without_destructors(1);       \
     }
 
 /// Exit the program at once after emitting an error message and stack trace if possible.
@@ -876,4 +876,17 @@ enum {
     /// like an unrecognized flag, missing or too many arguments, an invalid integer, etc. But
     STATUS_INVALID_ARGS = 121,
 };
+
+/* Normally casting an expression to void discards its value, but GCC
+   versions 3.4 and newer have __attribute__ ((__warn_unused_result__))
+   which may cause unwanted diagnostics in that case.  Use __typeof__
+   and __extension__ to work around the problem, if the workaround is
+   known to be needed.  */
+#if 3 < __GNUC__ + (4 <= __GNUC_MINOR__)
+# define ignore_result(x) \
+    (__extension__ ({ __typeof__ (x) __x = (x); (void) __x; }))
+#else
+# define ignore_result(x) ((void) (x))
+#endif
+
 #endif

src/env_universal_common.cpp

@@ -1209,7 +1209,7 @@ class universal_notifier_named_pipe_t : public universal_notifier_t {
         // would cause us to hang!
         size_t read_amt = 64 * 1024;
         void *buff = malloc(read_amt);
-        (void)read(this->pipe_fd, buff, read_amt);
+        ignore_result(read(this->pipe_fd, buff, read_amt));
         free(buff);
     }
 
@@ -1308,7 +1308,7 @@ class universal_notifier_named_pipe_t : public universal_notifier_t {
             while (this->readback_amount > 0) {
                 char buff[64];
                 size_t amt_to_read = mini(this->readback_amount, sizeof buff);
-                (void)read(this->pipe_fd, buff, amt_to_read);
+                ignore_result(read(this->pipe_fd, buff, amt_to_read));
                 this->readback_amount -= amt_to_read;
             }
             assert(this->readback_amount == 0);

src/path.cpp

@@ -275,7 +275,7 @@ static void maybe_issue_path_warning(const wcstring &which_dir, const wcstring &
         debug(0, _(L"The error was '%s'."), strerror(saved_errno));
         debug(0, _(L"Please set $%ls to a directory where you have write access."), env_var);
     }
-    (void)write(STDERR_FILENO, "\n", 1);
+    ignore_result(write(STDERR_FILENO, "\n", 1));
 }
 
 static void path_create(wcstring &path, const wcstring &xdg_var, const wcstring &which_dir,

src/reader.cpp

@@ -696,14 +696,14 @@ void reader_write_title(const wcstring &cmd, bool reset_cursor_position) {
         for (size_t i = 0; i < lst.size(); i++) {
             fputws(lst.at(i).c_str(), stdout);
         }
-        (void)write(STDOUT_FILENO, "\a", 1);
+       ignore_result(write(STDOUT_FILENO, "\a", 1));
     }
 
     proc_pop_interactive();
     set_color(rgb_color_t::reset(), rgb_color_t::reset());
     if (reset_cursor_position && !lst.empty()) {
         // Put the cursor back at the beginning of the line (issue #2453).
-        (void)write(STDOUT_FILENO, "\r", 1);
+        ignore_result(write(STDOUT_FILENO, "\r", 1));
     }
 }
 
@@ -1291,7 +1291,7 @@ static void reader_flash() {
     }
 
     reader_repaint();
-    (void)write(STDOUT_FILENO, "\a", 1);
+    ignore_result(write(STDOUT_FILENO, "\a", 1));
 
     pollint.tv_sec = 0;
     pollint.tv_nsec = 100 * 1000000;
@@ -3248,7 +3248,7 @@ const wchar_t *reader_readline(int nchars) {
         reader_repaint_if_needed();
     }
 
-    (void)write(STDOUT_FILENO, "\n", 1);
+    ignore_result(write(STDOUT_FILENO, "\n", 1));
 
     // Ensure we have no pager contents when we exit.
     if (!data->pager.empty()) {

src/wutil.cpp

@@ -338,7 +338,7 @@ void safe_perror(const char *message) {
     safe_append(buff, safe_strerror(err), sizeof buff);
     safe_append(buff, "\n", sizeof buff);
 
-    (void)write(STDERR_FILENO, buff, strlen(buff));
+    ignore_result(write(STDERR_FILENO, buff, strlen(buff)));
     errno = err;
 }