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[dev] Add memrchr

--- a/include/string.h

+++ b/include/string.h

@@ -19,6 +19,7 @@

 extern int strncmp(const char *s1, const char *s2, size_t n);

 extern size_t strxfrm(char * restrict s1, const char * restrict s2, size_t n);

 extern void *memchr(const void *s, int c, size_t n);

+extern void *memrchr(const void *s, int c, size_t n);

 extern char *strchr(const char *s, int c);

 extern size_t strcspn(const char *s1, const char *s2);

 extern char *strpbrk(const char *s1, const char *s2);

--- a/src/libc/string/Makefile

+++ b/src/libc/string/Makefile

@@ -4,6 +4,7 @@

 include ../rules.mk

 OBJS = memchr.o\

+       memrchr.o \

        memcmp.o\

        memcpy.o\

        memmove.o\

--- /dev/null

+++ b/src/libc/string/memrchr.c

@@ -1,0 +1,13 @@

+#include <string.h>

+

+#undef memrchr

+

+void *

+memrchr(const void *s, int c, size_t n)

+{

+	unsigned char *p = s;

+

+	for (p += n-1; n > 0 && *p != c; --p)

+		n--;

+	return (n == 0) ? NULL : p;

+}