diff --git a/include/strings.h b/include/strings.h
new file mode 100644
index 0000000..294e983
--- /dev/null
+++ b/include/strings.h
@@ -0,0 +1,133 @@
+#ifndef VLIBC_STRINGS_H
+#define VLIBC_STRINGS_H
+
+/*
+ * vlibc — <strings.h>.
+ *
+ * BSD/XSI legacy string functions, gated by the active compatibility
+ * profile (see include/vlibc/features.h). This ENTIRE header is a legacy
+ * extension: nothing it declares is POSIX.1-2008 base, so at level 1
+ * (onlyposix) it is empty. POSIX mandates that <strings.h> (BSD legacy)
+ * and <string.h> (ISO C) remain separate headers; the two never share a
+ * declaration, so including both can never conflict.
+ *
+ *   strcasecmp / strncasecmp — XSI case-insensitive comparisons (ASCII
+ *                              'A'..'Z'/'a'..'z' fold only, byte-wise);
+ *   ffs / ffsl / ffsll       — XSI find-first-set-bit, 1-based;
+ *   bcmp / bcopy / bzero     — BSD legacy byte operations (bcopy takes
+ *                              (src, dst) — the arguments are REVERSED
+ *                              relative to memcpy/memmove);
+ *   index / rindex           — BSD legacy names for strchr / strrchr.
+ *
+ * This header includes <vlibc/features.h> itself, so the gate below always
+ * sees the configured VLIBC_LEVEL even when the caller included no vlibc
+ * header first, and <stddef.h> for size_t.
+ */
+
+#include <vlibc/features.h>
+
+#if VLIBC_LEVEL_GE(2)
+
+#include <stddef.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/*
+ * Compare s1 and s2 byte-wise, ignoring case: each byte is read as
+ * unsigned char and an ASCII 'A'..'Z' is folded to 'a'..'z' before the
+ * comparison; bytes at or above 0x80 pass through unmodified. Return
+ * negative, zero, or positive when s1 is less than, equal to, or greater
+ * than s2.
+ * pure: reads memory, no side effects.
+ */
+__attribute__((pure)) int
+strcasecmp(const char *s1, const char *s2);
+
+/*
+ * Compare at most n bytes of s1 and s2 as strcasecmp does, stopping early
+ * at the first difference or the first NUL; return negative, zero, or
+ * positive like strcasecmp. The NUL terminator of either string ends the
+ * comparison even when n is larger.
+ * pure: reads memory, no side effects.
+ */
+__attribute__((pure)) int
+strncasecmp(const char *s1, const char *s2, size_t n);
+
+/*
+ * Return the 1-based index of the least significant set bit of i, or 0
+ * when i has no set bit: ffs(0) == 0, ffs(1) == 1, ffs(8) == 4,
+ * ffs(INT_MIN) == 32.
+ * const: the result depends only on i.
+ */
+__attribute__((const)) int
+ffs(int i);
+
+/*
+ * Same as ffs for long: ffsl(0) == 0, ffsl(1L << 40) == 41,
+ * ffsl(LONG_MIN) == 64 (long is 64-bit on x86_64).
+ * const: the result depends only on i.
+ */
+__attribute__((const)) int
+ffsl(long i);
+
+/*
+ * Same as ffs for long long: ffsll(0) == 0, ffsll(LLONG_MIN) == 64.
+ * const: the result depends only on i.
+ */
+__attribute__((const)) int
+ffsll(long long i);
+
+/*
+ * Compare the first n bytes of s1 and s2 as unsigned char, like memcmp;
+ * unlike strcmp a NUL byte does not end the comparison. Return 0 when the
+ * n bytes are equal, nonzero otherwise.
+ * pure: reads memory, no side effects.
+ */
+__attribute__((pure)) int
+bcmp(const void *s1, const void *s2, size_t n);
+
+/*
+ * Copy n bytes from src to dst. The regions may overlap and the copy
+ * behaves like memmove. NOTE the argument order: source first, destination
+ * second — the reverse of memcpy/memmove.
+ * No intent attribute: it writes memory.
+ */
+void
+bcopy(const void *src, void *dst, size_t n);
+
+/*
+ * Fill n bytes at s with zero.
+ * No intent attribute: it writes memory.
+ */
+void
+bzero(void *s, size_t n);
+
+/*
+ * Return a pointer to the first occurrence of c (converted to unsigned
+ * char) in s, or NULL when absent. The terminating NUL is part of the
+ * string, so index(s, '\0') returns a pointer to it. Legacy name for
+ * strchr.
+ * pure: reads memory, no side effects.
+ */
+__attribute__((pure)) char *
+index(const char *s, int c);
+
+/*
+ * Return a pointer to the last occurrence of c (converted to unsigned
+ * char) in s, or NULL when absent. The terminating NUL is part of the
+ * string, so rindex(s, '\0') returns a pointer to it. Legacy name for
+ * strrchr.
+ * pure: reads memory, no side effects.
+ */
+__attribute__((pure)) char *
+rindex(const char *s, int c);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* VLIBC_LEVEL_GE(2) */
+
+#endif /* VLIBC_STRINGS_H */
diff --git a/src/string/strings_impl.c b/src/string/strings_impl.c
new file mode 100644
index 0000000..d1fb61a
--- /dev/null
+++ b/src/string/strings_impl.c
@@ -0,0 +1,202 @@
+#ifdef HAVE_CONFIG_H
+#include <config.h>
+#endif
+
+#include <string.h>
+#include <strings.h>
+
+#if VLIBC_LEVEL_GE(2)
+
+/*
+ * vlibc — <strings.h> legacy functions (todo 10).
+ *
+ * The whole file self-gates at VLIBC_LEVEL >= 2 (the build wiring pass may
+ * compile it unconditionally): at level 1 this translation unit produces
+ * nothing, because every function here is XSI/BSD legacy and none is
+ * POSIX.1-2008 base.
+ *
+ * bcopy/bzero/bcmp delegate to the project's own memmove/memset/memcmp
+ * (satisfied from the archive at link time) — bcopy MUST NOT be a bare
+ * memcpy alias, because its contract includes overlapping regions.
+ * index/rindex are written as direct byte scans (own code, not strchr
+ * wrappers) so their documented unsigned char conversion is explicit.
+ */
+
+/*
+ * Fold an ASCII uppercase letter to lowercase; every other byte passes
+ * through unchanged. Kept local and locale-free: the BSD comparisons must
+ * never depend on ctype tables or the active locale.
+ */
+static inline unsigned char
+fold_ascii_byte(unsigned char c)
+{
+    if (c >= 'A' && c <= 'Z')
+    {
+        return (unsigned char)(c + ('a' - 'A'));
+    }
+    return c;
+}
+
+/*
+ * Compare s1 and s2 byte-wise ignoring case. Equal-folded runs are skipped
+ * with a raw-byte fast path; the first differing byte pair is compared
+ * folded, so "A" and "a" order as equal and the returned sign is that of
+ * the folded difference. The scan stops at the first NUL in either string.
+ */
+int
+strcasecmp(const char *s1, const char *s2)
+{
+    const unsigned char *l = (const unsigned char *)s1;
+    const unsigned char *r = (const unsigned char *)s2;
+
+    for (; *l == *r; l++, r++)
+    {
+        if (*l == '\0')
+        {
+            return 0;
+        }
+    }
+
+    return (int)fold_ascii_byte(*l) - (int)fold_ascii_byte(*r);
+}
+
+/*
+ * Compare at most n bytes of s1 and s2 as strcasecmp does, stopping early
+ * at the first folded difference or the first NUL. When the comparison
+ * ends at a NUL, the shorter (equal-prefix) string sorts first.
+ */
+int
+strncasecmp(const char *s1, const char *s2, size_t n)
+{
+    const unsigned char *l = (const unsigned char *)s1;
+    const unsigned char *r = (const unsigned char *)s2;
+
+    while (n != 0 && *l != '\0' && *r != '\0')
+    {
+        const unsigned char fl = fold_ascii_byte(*l);
+        const unsigned char fr = fold_ascii_byte(*r);
+
+        if (fl != fr)
+        {
+            return (int)fl - (int)fr;
+        }
+        l++;
+        r++;
+        n--;
+    }
+
+    /* n bytes compared: equal so far. */
+    if (n == 0)
+    {
+        return 0;
+    }
+
+    /* A NUL ended the scan: the one that is NUL here is the shorter. */
+    return (int)*l - (int)*r;
+}
+
+/*
+ * Return the 1-based index of the least significant set bit of i, or 0
+ * when i is 0. The zero case is guarded because __builtin_ctz is
+ * undefined for a zero argument.
+ */
+int
+ffs(int i)
+{
+    return i == 0 ? 0 : __builtin_ctz((unsigned int)i) + 1;
+}
+
+int
+ffsl(long i)
+{
+    return i == 0 ? 0 : __builtin_ctzl((unsigned long)i) + 1;
+}
+
+int
+ffsll(long long i)
+{
+    return i == 0 ? 0 : __builtin_ctzll((unsigned long long)i) + 1;
+}
+
+/*
+ * Copy n bytes from src to dst with memmove semantics: the regions may
+ * overlap and the copy behaves as if through a temporary. The argument
+ * order (src, dst) is the reverse of memmove's — kept as such because it
+ * is the BSD contract.
+ */
+void
+bcopy(const void *src, void *dst, size_t n)
+{
+    (void)memmove(dst, src, n);
+}
+
+/*
+ * Fill n bytes at s with zero.
+ */
+void
+bzero(void *s, size_t n)
+{
+    (void)memset(s, 0, n);
+}
+
+/*
+ * Compare the first n bytes of s1 and s2 as unsigned char, like memcmp;
+ * return 0 when equal, nonzero otherwise. A NUL byte does not end the
+ * comparison.
+ */
+int
+bcmp(const void *s1, const void *s2, size_t n)
+{
+    return memcmp(s1, s2, n);
+}
+
+/*
+ * Return a pointer to the first occurrence of c (converted to unsigned
+ * char) in s, or NULL when absent. The terminating NUL is part of the
+ * string, so index(s, '\0') returns a pointer to it. Legacy name for
+ * strchr.
+ */
+char *
+index(const char *s, int c)
+{
+    const unsigned char cc = (unsigned char)c;
+
+    for (;; s++)
+    {
+        if ((unsigned char)*s == cc)
+        {
+            return (char *)s;
+        }
+        if (*s == '\0')
+        {
+            return NULL;
+        }
+    }
+}
+
+/*
+ * Return a pointer to the last occurrence of c (converted to unsigned
+ * char) in s, or NULL when absent. The terminating NUL is part of the
+ * string, so rindex(s, '\0') returns a pointer to it. Legacy name for
+ * strrchr.
+ */
+char *
+rindex(const char *s, int c)
+{
+    const unsigned char cc = (unsigned char)c;
+    const char *last = NULL;
+
+    for (;; s++)
+    {
+        if ((unsigned char)*s == cc)
+        {
+            last = s;
+        }
+        if (*s == '\0')
+        {
+            return (char *)last;
+        }
+    }
+}
+
+#endif /* VLIBC_LEVEL_GE(2) */
diff --git a/tests/test_strings.c b/tests/test_strings.c
new file mode 100644
index 0000000..5b785f2
--- /dev/null
+++ b/tests/test_strings.c
@@ -0,0 +1,297 @@
+/*
+ * vlibc — strings.h legacy functions test (todo 10).
+ *
+ * Covers the whole <strings.h> surface end to end:
+ *
+ *   1. strcasecmp: ASCII case-insensitive compare ("AbC" vs "aBc" == 0),
+ *      ordering sign, empty strings, and high bytes (>= 0x80) compared
+ *      unmodified — no locale folding;
+ *   2. strncasecmp: the n bound (differences beyond n are invisible),
+ *      n == 0, and NUL-terminated early stop;
+ *   3. ffs/ffsl/ffsll: ffs(0) == 0, ffs(8) == 4, ffs(INT_MIN) == 32,
+ *      ffsl(LONG_MIN) == 64, ffsll(LLONG_MIN) == 64, plus low-bit spots;
+ *   4. bcopy: (src, dst) argument order (reversed vs memcpy), and the
+ *      discriminating overlap case dst == src + 1 — a forward-only copy
+ *      would smear the source byte and produce "aaaaaa" instead of
+ *      "aabcde";
+ *   5. bzero: zeroes the first n bytes and nothing beyond; n == 0 is a
+ *      no-op;
+ *   6. bcmp: equal/differ, does not stop at NUL, unsigned byte order;
+ *   7. index/rindex: first/last occurrence, the NUL counts as a match,
+ *      c converts through unsigned char, NULL when absent.
+ *
+ * With `-f`, only the plan's failure scenario runs: index("abc", 'z') and
+ * rindex("abc", 'z') must BOTH be NULL (character absent); a non-NULL
+ * result is the defect. Exits 0 when both are NULL as expected.
+ *
+ * No host libc headers are included (the -Iinclude path would shadow
+ * GCC's internal headers); all output goes through raw SYS_write via
+ * <vlibc/internal/test.h>. Not yet wired into make check (the build
+ * wiring todo owns that).
+ *
+ * The whole body mirrors the header's gate: every <strings.h> function is
+ * level 2, so at level 1 this TU compiles to a no-op runner (the test_ctype
+ * precedent). Compile the real test with -DVLIBC_LEVEL=2.
+ *
+ * Standalone build:
+ *   gcc -Iinclude -DVLIBC_LEVEL=2 -std=c23 -Wall -Wextra -pedantic \
+ *       -o /tmp/t10 tests/test_strings.c src/string/strings_impl.c \
+ *       src/string/{memmove,memset,memcmp}.c
+ */
+
+#include <strings.h>
+
+#include <limits.h>
+
+#include <vlibc/internal/test.h>
+
+#if VLIBC_LEVEL_GE(2)
+
+/* ---- test functions (one per checked behavior) ---- */
+
+static int
+test_strcasecmp(void)
+{
+    static const unsigned char hi[] = {0xe1, 0x00};
+    static const unsigned char lo[] = {0x61, 0x00};
+
+    TEST_ASSERT_EQ(strcasecmp("AbC", "aBc"), 0);
+    TEST_ASSERT_EQ(strcasecmp("", ""), 0);
+    TEST_ASSERT_EQ(strcasecmp("cafe", "CAFE"), 0);
+    TEST_ASSERT_TRUE(strcasecmp("abc", "abd") < 0);
+    TEST_ASSERT_TRUE(strcasecmp("abd", "abc") > 0);
+    TEST_ASSERT_TRUE(strcasecmp("", "a") < 0);
+    TEST_ASSERT_TRUE(strcasecmp("a", "") > 0);
+    TEST_ASSERT_TRUE(strcasecmp("Z", "a") > 0); /* folds 'Z' to 'z' */
+    TEST_ASSERT_TRUE(strcasecmp("A", "z") < 0);
+    /* Bytes >= 0x80 pass through unmodified and sort above ASCII. */
+    TEST_ASSERT_EQ(strcasecmp((const char *)hi, (const char *)hi), 0);
+    TEST_ASSERT_TRUE(strcasecmp((const char *)hi, (const char *)lo) > 0);
+    return 0;
+}
+
+static int
+test_strncasecmp(void)
+{
+    TEST_ASSERT_EQ(strncasecmp("AbCd", "aBcD", 4), 0);
+    TEST_ASSERT_EQ(strncasecmp("abcX", "abcY", 3), 0); /* diff beyond n */
+    TEST_ASSERT_TRUE(strncasecmp("abcX", "abcY", 4) < 0);
+    TEST_ASSERT_EQ(strncasecmp("x", "y", 0), 0); /* n == 0 */
+    TEST_ASSERT_EQ(strncasecmp("", "", 5), 0);
+    TEST_ASSERT_EQ(strncasecmp("a\0b", "a\0c", 3), 0);   /* stops at NUL */
+    TEST_ASSERT_TRUE(strncasecmp("abc", "abcd", 4) < 0); /* shorter lhs */
+    TEST_ASSERT_TRUE(strncasecmp("abcd", "abc", 4) > 0);
+    TEST_ASSERT_EQ(strncasecmp("hello", "HELLO world", 5), 0);
+    return 0;
+}
+
+static int
+test_ffs(void)
+{
+    TEST_ASSERT_EQ(ffs(0), 0);
+    TEST_ASSERT_EQ(ffs(1), 1);
+    TEST_ASSERT_EQ(ffs(2), 2);
+    TEST_ASSERT_EQ(ffs(8), 4);
+    TEST_ASSERT_EQ(ffs(0x8000), 16);
+    TEST_ASSERT_EQ(ffs(-1), 1); /* all bits set: lowest bit is bit 0 */
+    TEST_ASSERT_EQ(ffs(INT_MIN), 32);
+
+    TEST_ASSERT_EQ(ffsl(0), 0);
+    TEST_ASSERT_EQ(ffsl(1L << 40), 41);
+    TEST_ASSERT_EQ(ffsl(LONG_MIN), 64); /* long is 64-bit on x86_64 */
+
+    TEST_ASSERT_EQ(ffsll(0), 0);
+    TEST_ASSERT_EQ(ffsll(0x100000000LL), 33);
+    TEST_ASSERT_EQ(ffsll(LLONG_MIN), 64);
+    return 0;
+}
+
+static int
+test_bcopy(void)
+{
+    char buf[16] = "abcdef";
+    char buf2[16] = "abcdef";
+
+    /* Non-overlapping copy; NOTE the (src, dst) argument order. */
+    bcopy(buf, buf2, 7);
+    TEST_ASSERT_STREQ(buf2, "abcdef");
+
+    /* Overlap, dst == src + 1: only a backward (memmove-style) copy can
+     * produce this; a forward copy smears src[0] over the whole range. */
+    bcopy(buf, buf + 1, 5);
+    TEST_ASSERT_STREQ(buf, "aabcde");
+
+    /* Overlap, dst inside src at +2: both directions agree, sanity only. */
+    {
+        char buf3[16] = "abcdef";
+
+        bcopy(buf3, buf3 + 2, 4);
+        TEST_ASSERT_STREQ(buf3, "ababcd");
+    }
+
+    /* Overlap, dst before src (backward region): forward copy is correct. */
+    {
+        char buf4[16] = "abcdef";
+
+        bcopy(buf4 + 2, buf4, 4);
+        TEST_ASSERT_STREQ(buf4, "cdefef");
+    }
+    return 0;
+}
+
+static int
+test_bzero(void)
+{
+    char buf[8];
+    int i;
+
+    for (i = 0; i < 8; i++)
+    {
+        buf[i] = 'A';
+    }
+    bzero(buf, 4);
+    TEST_ASSERT_EQ(buf[0], '\0');
+    TEST_ASSERT_EQ(buf[3], '\0');
+    TEST_ASSERT_EQ(buf[4], 'A'); /* untouched past n */
+
+    bzero(buf, 0); /* n == 0: nothing */
+    TEST_ASSERT_EQ(buf[0], '\0');
+    TEST_ASSERT_EQ(buf[4], 'A');
+    return 0;
+}
+
+static int
+test_bcmp(void)
+{
+    static const unsigned char hi[] = {0xff, 0x00};
+    static const unsigned char lo[] = {0x7f, 0x00};
+
+    TEST_ASSERT_EQ(bcmp("abc", "abc", 3), 0);
+    TEST_ASSERT_TRUE(bcmp("abc", "abd", 3) != 0);
+    TEST_ASSERT_EQ(bcmp("abc", "abd", 2), 0);
+    TEST_ASSERT_EQ(bcmp("", "", 0), 0);
+    /* Bytes compare as unsigned char; a NUL does not end the comparison. */
+    TEST_ASSERT_TRUE(bcmp(hi, lo, 1) != 0);
+    TEST_ASSERT_EQ(bcmp(hi, hi, 2), 0);
+    TEST_ASSERT_EQ(bcmp("a\0b", "a\0c", 3) != 0, 1);
+    return 0;
+}
+
+static int
+test_index(void)
+{
+    char s[] = "hello";
+
+    TEST_ASSERT_EQ(index(s, 'l') - s, 2);
+    TEST_ASSERT_EQ(index(s, 'h') - s, 0);
+    TEST_ASSERT_EQ(index(s, '\0') - s, 5); /* the NUL counts */
+    TEST_ASSERT_NULL(index(s, 'z'));
+    /* c converts through unsigned char: 0x100 is byte 0, the NUL. */
+    TEST_ASSERT_EQ(index(s, 0x100) - s, 5);
+    return 0;
+}
+
+static int
+test_rindex(void)
+{
+    char s[] = "hello";
+
+    TEST_ASSERT_EQ(rindex(s, 'l') - s, 3);
+    TEST_ASSERT_EQ(rindex(s, 'h') - s, 0);
+    TEST_ASSERT_EQ(rindex(s, '\0') - s, 5); /* the NUL counts */
+    TEST_ASSERT_NULL(rindex(s, 'z'));
+    return 0;
+}
+
+/* ---- failure mode: the plan's absent-character scenario ---- */
+
+static int
+test_absent_char(void)
+{
+    TEST_ASSERT_NULL(index("abc", 'z'));
+    TEST_ASSERT_NULL(rindex("abc", 'z'));
+    return 0;
+}
+
+/* ---- register + runner ---- */
+
+static const struct vlibc_test tests[] = {
+    {"strcasecmp", test_strcasecmp},
+    {"strncasecmp", test_strncasecmp},
+    {"ffs", test_ffs},
+    {"bcopy", test_bcopy},
+    {"bzero", test_bzero},
+    {"bcmp", test_bcmp},
+    {"index", test_index},
+    {"rindex", test_rindex},
+};
+
+/*
+ * Own main (not TEST_MAIN): supports the -f failure mode. Everything else
+ * follows the TEST_MAIN contract — same output shape, 0 on all-pass.
+ */
+int
+main(int argc, char **argv)
+{
+    const size_t count = sizeof tests / sizeof tests[0];
+    size_t passed = 0;
+    size_t i;
+
+    if (argc > 1 && argv[1][0] == '-' && argv[1][1] == 'f' && argv[1][2] == '\0')
+    {
+        int before = vlibc_test_failures;
+
+        vlibc_test_say(1, "RUN absent-char: ");
+        if (test_absent_char() == 0 && vlibc_test_failures == before)
+        {
+            vlibc_test_say(1, "PASS\n");
+            vlibc_test_say(1, "SUMMARY: 1/1 passed, 0 assertion failure(s)\n");
+            return 0;
+        }
+        vlibc_test_say(1, "FAIL\n");
+        return 1;
+    }
+
+    for (i = 0; i < count; i++)
+    {
+        int before = vlibc_test_failures;
+
+        vlibc_test_say(1, "RUN ");
+        vlibc_test_say(1, tests[i].name);
+        vlibc_test_say(1, ": ");
+        if (tests[i].run() == 0 && vlibc_test_failures == before)
+        {
+            vlibc_test_say(1, "PASS\n");
+            passed++;
+        }
+        else
+        {
+            vlibc_test_say(1, "FAIL\n");
+        }
+    }
+
+    vlibc_test_say(1, "SUMMARY: ");
+    vlibc_test_say_dec(1, (unsigned long)passed);
+    vlibc_test_say(1, "/");
+    vlibc_test_say_dec(1, (unsigned long)count);
+    vlibc_test_say(1, " passed, ");
+    vlibc_test_say_dec(1, (unsigned long)vlibc_test_failures);
+    vlibc_test_say(1, " assertion failure(s)\n");
+
+    return passed == count ? 0 : 1;
+}
+
+#else /* !VLIBC_LEVEL_GE(2) */
+
+/*
+ * Level 1: every <strings.h> function is gated at level 2, so there is
+ * nothing to run. Keep the TU compilable at any configured profile.
+ */
+int
+main(void)
+{
+    vlibc_test_say(1, "SKIP: <strings.h> is level 2, not available here\n");
+    return 0;
+}
+
+#endif /* VLIBC_LEVEL_GE(2) */