diff --git a/options/ansi/include/string.h b/options/ansi/include/string.h
index d8bce017..224c576d 100644
--- a/options/ansi/include/string.h
+++ b/options/ansi/include/string.h
@@ -57,6 +57,8 @@ char *stpcpy(char *__restrict, const char *__restrict);
 
 // GNU extensions.
 int strverscmp(const char *l0, const char *r0);
+int ffsl(long i);
+int ffsll(long long i);
 
 #ifdef __cplusplus
 }
diff --git a/options/posix/generic/strings-stubs.cpp b/options/posix/generic/strings-stubs.cpp
index 59a5354a..137c1c1d 100644
--- a/options/posix/generic/strings-stubs.cpp
+++ b/options/posix/generic/strings-stubs.cpp
@@ -13,9 +13,61 @@ char *rindex(const char *s, int c) {
 	return strrchr(s, c);
 }
 
-int ffs(int) {
-	__ensure(!"Not implemented");
-	__builtin_unreachable();
+namespace {
+
+	template<typename T>
+	int ffs_generic(T i) {
+		//Non-portably assume a byte has 8 bits; fine in all plausible cases.
+		for(int b = 0; b < sizeof(T) * 8;)
+			if(i & (static_cast<T>(0x1) << b++))
+				return b;
+
+		return 0;
+	}
+
+}
+
+#ifdef __has_builtin
+#	if __has_builtin(__builtin_ffs)
+#		define __mlibc_ffs __builtin_ffs
+#	endif
+#	if __has_builtin(__builtin_ffsl)
+#		define __mlibc_ffsl __builtin_ffsl
+#	endif
+#	if __has_builtin(__builtin_ffsll)
+#		define __mlibc_ffsll __builtin_ffsll
+#	endif
+#endif
+
+int ffs(int i) {
+#ifdef __mlibc_ffs
+	return __mlibc_ffs(i);
+#else
+	return ffs_generic<int>(i);
+#endif
+}
+
+/*
+	Both ffsl() and ffsll() are glibc extensions
+	defined in string.h. They are however implemented
+	here because of similarity in logic and
+	shared code.
+*/
+
+int ffsl(long i) {
+#ifdef __mlibc_ffsl
+	return __mlibc_ffsl(i);
+#else
+	return ffs_generic<long>(i);
+#endif
+}
+
+int ffsll(long long i) {
+#ifdef __mlibc_ffsll
+	return __mlibc_ffsll(i);
+#else
+	return ffs_generic<long long>(i);
+#endif
 }
 
 int strcasecmp(const char *a, const char *b) {
diff --git a/tests/glibc/ffsl-ffsll.c b/tests/glibc/ffsl-ffsll.c
new file mode 100644
index 00000000..5bea030d
--- /dev/null
+++ b/tests/glibc/ffsl-ffsll.c
@@ -0,0 +1,20 @@
+#include <assert.h>
+#include <string.h>
+
+int main(void){
+    long test1 = 1L << (sizeof(long) * 8 - 1);
+    long long test2 = 1LL << (sizeof(long long) * 8 - 1);
+
+    assert(ffsl(test1) == sizeof(long) * 8);
+    assert(ffsll(test2) == sizeof(long long) * 8);
+    assert(ffsl(0) == 0);
+    assert(ffsll(0) == 0);
+    assert(ffsl(test1) == ffsll(test1));
+    if(sizeof(long) < sizeof(long long)){
+        assert(ffsl(test1) < ffsll(test2));
+    } else {
+        assert(ffsl(test2) == ffsll(test2));
+    }
+
+    return 0;
+}
diff --git a/tests/meson.build b/tests/meson.build
index f59e6f1a..ca217fd1 100644
--- a/tests/meson.build
+++ b/tests/meson.build
@@ -39,7 +39,8 @@ posix_test_cases = [
 	'system', # This test should be in the ANSI tests, but it depends on sys/wait.h
 	'sigsuspend',
 	'sigaltstack',
-	'realpath'
+	'realpath',
+	'ffs'
 ]
 
 posix_fail_test_cases = [
@@ -47,7 +48,8 @@ posix_fail_test_cases = [
 ]
 
 glibc_test_cases = [
-	'getopt'
+	'getopt',
+	'ffsl-ffsll'
 ]
 
 test_sources = [
diff --git a/tests/posix/ffs.c b/tests/posix/ffs.c
new file mode 100644
index 00000000..23f89bc2
--- /dev/null
+++ b/tests/posix/ffs.c
@@ -0,0 +1,10 @@
+#include <assert.h>
+#include <strings.h>
+
+int main(void){
+    assert(ffs(0x8000) == 16);
+    assert(ffs(0x0) == 0);
+    assert(ffs(0x8000000000000000) == 0);
+
+    return 0;
+}