v.pref, v.builder: support `-no-std` (skips passing `-std=c99` to the C backend)

cmd/v/help/build-c.txt

@@ -35,7 +35,7 @@ see also `v help build`.
       Supported only on `linux` targets currently.
 
    -bare-builtin-dir <bare-builtin-dir>
-      Use with `-freestanding`. This specifies the directory to the 
+      Use with `-freestanding`. This specifies the directory to the
       implementation of some basic builtin functions. The list is as follows:
          bare_print(buf &byte, len u64)
             Print len charecters from the buffer pointed to by buf to stdout.
@@ -50,7 +50,7 @@ see also `v help build`.
          __free(ptr &C.void)
             Free the block of memory ptr allocated by malloc.
          realloc(old_area &C.void, new_size size_t) &C.void
-            Allocates a new area of size new_size, copies old_area 
+            Allocates a new area of size new_size, copies old_area
             to the new area, and returns a pointer to the new area.
          [export: 'calloc']
          __calloc(nmemb size_t, size size_t) &C.void
@@ -75,20 +75,20 @@ see also `v help build`.
          vsnprintf(str &char, size size_t, format &char, ap va_list) int
             See `man vsnprintf`.
          bare_backtrace() string
-            Return a backtrace that can be printed. If backtraces are not 
+            Return a backtrace that can be printed. If backtraces are not
             supported, return a message stating that backtraces do not work.
          [export: 'exit']
          __exit(code int)
             Exit with code code. code is allowed to be ignored.
 
-         The module decleration should be `builtin`. The default Linux 
+         The module decleration should be `builtin`. The default Linux
          implementation can be found in `vlib/builtin/linux_bare`.
 
    -os <os>, -target-os <os>
       Change the target OS that V tries to compile for.
       By default, the target OS is the host system.
       When OS is `cross`, V will attempt to output cross-platform C code.
-	  
+
       Here is a list of the operating systems, supported by V:
       (CI tests runs on every commit/PR for each of these):
             `windows`, `linux`, `macos`
@@ -137,7 +137,7 @@ see also `v help build`.
       and unsafe{free(x)} calls manually in this mode).
       Some short lived applications, like compilers and other CLI tools are
       more performant without autofree.
-      
+
    -gc <mode>
       Use and link an optional garbage collector. Only the Boehm–Demers–Weiser
       garbage collector is supported currently with the following sub-options:
@@ -152,12 +152,12 @@ see also `v help build`.
       You need to install a `libgc-dev` package first, or install it manually from:
 
           https://github.com/ivmai/bdwgc
-          
+
       Note, `-gc boehm` is complementary to -autofree. The Boehm garbage
       collector is conservative, and it may make your program significantly
       slower if it does many small allocations in a loop. This option
       is intended *mainly* for reducing the memory usage of programs, that
-      process large amounts of text in *batch mode* on low/limited memory 
+      process large amounts of text in *batch mode* on low/limited memory
       environments like small VPSes, and for which a few ms of garbage
       collection pauses from time to time *do not matter much*.
 
@@ -237,12 +237,19 @@ see also `v help build`.
       you use -no-rsp, V will pass the C compiler options directly to the C
       compiler, on the command line, without writing an .rsp file first.
 
-   -assert aborts
+   -no-std
+      By default, V passes -std=c99 to the C backend, but some compilers do
+      not support that, even though they may be able to compile the produced
+      code, or have other options that can be tuned to allow it.
+      Passing -no-std will remove that flag, and you can then use -cflags ''
+      to pass the other options for your specific C compiler.
+
+-assert aborts
       Call abort() after an assertion failure. Debuggers usually
       install signal handlers for SIGABRT, so your program will stop and you
       will get a backtrace. If you are running your program outside of a
       debugger, you will most likely get a core dump file.
-      
+
    -assert backtraces
       Call print_backtrace() after an assertion failure. Note that
       backtraces are not implemented yet on all combinations of

vlib/v/builder/cc.v

@@ -186,7 +186,10 @@ fn (mut v Builder) setup_ccompiler_options(ccompiler string) {
 	mut debug_options := ['-g']
 	mut optimization_options := ['-O2']
 	// arguments for the C compiler
-	ccoptions.args = [v.pref.cflags, '-std=gnu99']
+	ccoptions.args = [v.pref.cflags]
+	if !v.pref.no_std {
+		ccoptions.args << '-std=c99'
+	}
 	ccoptions.wargs = [
 		'-Wall',
 		'-Wextra',

vlib/v/pref/pref.v

@@ -173,6 +173,7 @@ pub mut:
 	fatal_errors        bool        // unconditionally exit after the first error with exit(1)
 	reuse_tmpc          bool        // do not use random names for .tmp.c and .tmp.c.rsp files, and do not remove them
 	no_rsp              bool        // when true, pass C backend options directly on the CLI (do not use `.rsp` files for them, some older C compilers do not support them)
+	no_std              bool        // when true, do not pass -std=c99 to the C backend
 	use_color           ColorOutput // whether the warnings/errors should use ANSI color escapes.
 	is_parallel         bool
 	error_limit         int
@@ -478,6 +479,9 @@ pub fn parse_args(known_external_commands []string, args []string) (&Preferences
 			'-no-rsp' {
 				res.no_rsp = true
 			}
+			'-no-std' {
+				res.no_std = true
+			}
 			'-keepc' {
 				res.reuse_tmpc = true
 			}