C11 low-hanging fruit

[Gregor Richards <gr@purdue.edu>]

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Attached is a diff that adds the trivially-simple bits of C11. That is, 
those that require changes to the standard headers, but very little in 
the way of implementation. Included are:
  * stdalign.h , including backwards compatibility for GCC.
  * aligned_alloc
  * fopen(..., "...x")
  * conditional removal of gets

Excluded are:
  * threads.h (waiting to see glibc's ABI)
  * stdatomic.h (complicated, probably has ABI implications too, not 
even fully supported by GCC yet)
  * uchar.h (this is MOSTLY trivial since musl's wchar is Unicode 
anyway, but char32_t makes it a bit more than some weak aliases)
  * at_quick_exit, quick_exit (I suspect that our benevolent dictator 
will have a lot to say in exactly what makes an exit "quick" or not, so 
I'm not touching this)

I used __STDC_VERSION__ in the headers to control exposure of 
aligned_alloc and gets. aligned_alloc I additionally exposed with the 
non-standard feature test macro _ISOC11_SOURCE, for the simple reason 
that glibc does it; if you have a non-C11 compiler but C11 libc, 
-D_ISOC11_SOURCE is a much less gross way to get aligned_alloc than 
-D_GNU_SOURCE, anyway :)

With valediction,
  - Gregor Richards


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diff --git a/include/stdalign.h b/include/stdalign.h
new file mode 100644
--- /dev/null
+++ b/include/stdalign.h
@@ -0,0 +1,23 @@
+#ifndef _STDALIGN_H
+#define _STDALIGN_H
+
+/* this whole header only works in C11 or with compiler extensions */
+#if __STDC_VERSION__ < 201112L
+
+#if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 1)
+#define _Alignas(t) __attribute__((__aligned__(t)))
+#define _Alignof(t) __alignof__(t)
+
+#else
+#error stdalign.h is only supported with C11 or GNU extensions
+
+#endif
+
+#endif
+
+#define alignas _Alignas
+#define alignof _Alignof
+#define __alignas_is_defined 1
+#define __alignof_is_defined 1
+
+#endif
diff --git a/include/stdio.h b/include/stdio.h
--- a/include/stdio.h
+++ b/include/stdio.h
@@ -90,7 +90,11 @@
 int putchar(int);
 
 char *fgets(char *, int, FILE *);
+#if __STDC_VERSION__ < 201112L \
+ || defined(_XOPEN_SOURCE) || defined(_POSIX_C_SOURCE) \
+ || defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 char *gets(char *);
+#endif
 
 int fputs(const char *, FILE *);
 int puts(const char *);
diff --git a/include/stdlib.h b/include/stdlib.h
--- a/include/stdlib.h
+++ b/include/stdlib.h
@@ -102,6 +102,13 @@
 
 #endif
 
+#if __STDC_VERSION__ >= 201112L || defined(_GNU_SOURCE) || defined(_BSD_SOURCE) \
+ || defined(_ISOC11_SOURCE)
+
+void *aligned_alloc(size_t alignment, size_t size);
+
+#endif
+
 
 #if defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) \
  || defined(_BSD_SOURCE)
diff --git a/src/malloc/aligned_alloc.c b/src/malloc/aligned_alloc.c
new file mode 100644
--- /dev/null
+++ b/src/malloc/aligned_alloc.c
@@ -0,0 +1,15 @@
+#define _POSIX_C_SOURCE 200112L
+
+#include <errno.h>
+#include <stdlib.h>
+
+void *aligned_alloc(size_t alignment, size_t size)
+{
+	void *ret;
+	int err = posix_memalign(&ret, alignment, size);
+	if (err) {
+		errno = err;
+		return NULL;
+	}
+	return ret;
+}
diff --git a/src/stdio/fopen.c b/src/stdio/fopen.c
--- a/src/stdio/fopen.c
+++ b/src/stdio/fopen.c
@@ -6,6 +6,7 @@
 	int fd;
 	int flags;
 	int plus = !!strchr(mode, '+');
+	int excl = !!strchr(mode, 'x');
 
 	/* Check for valid initial mode character */
 	if (!strchr("rwa", *mode)) {
@@ -17,6 +18,7 @@
 	if (plus) flags = O_RDWR;
 	else if (*mode == 'r') flags = O_RDONLY;
 	else flags = O_WRONLY;
+	if (excl) flags |= O_EXCL;
 	if (*mode != 'r') flags |= O_CREAT;
 	if (*mode == 'w') flags |= O_TRUNC;
 	if (*mode == 'a') flags |= O_APPEND;