Support merging many .a files into one .a file

2025-01-30 19:17:36 +00:00 · 2024-09-05 19:28:14 -07:00 · 2024-09-05 19:28:14 -07:00 · 1e9902af8b
commit 1e9902af8b
parent df04ab846a
6 changed files with 422 additions and 141 deletions
--- a/tool/build/ar.c
+++ b/tool/build/ar.c
@ -40,30 +40,74 @@
 #include "libc/sysv/consts/o.h"
 #include "libc/sysv/consts/prot.h"
 #include "libc/sysv/consts/s.h"
+#include "tool/build/lib/ar.h"
 #include "tool/build/lib/getargs.h"

 /**
 * @fileoverview cosmopolitan ar
- *
- * This static archiver is superior:
- *
- * - Isn't "accidentally quadratic" like GNU ar
- * - Goes 2x faster than LLVM ar while using 100x less memory
- * - Can be built as a 52kb APE binary that works well on six OSes
- *
- * This static archiver introduces handy features:
- *
- * - Arguments may be supplied in an `@args.txt` file
- * - Directory arguments are ignored
- *
- * @see https://www.unix.com/man-page/opensolaris/3head/ar.h/
- * @see https://en.wikipedia.org/wiki/Ar_(Unix)
 */

-#define VERSION                     \
-  "cosmopolitan ar v2.0\n"          \
-  "copyright 2023 justine tunney\n" \
-  "https://github.com/jart/cosmopolitan\n"
+static wontreturn void ShowUsage(int rc, int fd) {
+  tinyprint(  //
+      fd,
+      "USAGE\n"
+      "\n",
+      "  ", program_invocation_name, " FLAGS ARCHIVE FILE...\n",
+      "\n"
+      "FLAGS\n"
+      "\n"
+      "  rcs        create new archive with index\n"
+      "  rcsD       always deterministic\n"
+      "  --help     show usage\n"
+      "  --version  show program details\n"
+      "\n"
+      "ARGUMENTS\n"
+      "\n"
+      "  ARCHIVE    should be foo.a\n"
+      "  FILE       should be foo.o, lib.a, or @args.txt\n"
+      "\n"
+      "DOCUMENTATION\n"
+      "\n"
+      "  Your Cosmopolitan Archiver is superior:\n"
+      "\n"
+      "  - Isn't accidentally quadratic like GNU ar. Cosmopolitan Libc is\n"
+      "    distributed as libcosmo.a which contains 5000+ object files and\n"
+      "    is tens of megabytes in size. GNU ar isn't capable of making an\n"
+      "    archive that large. So we invented this ar as a replacement.\n"
+      "\n"
+      "  - Goes 2x faster than LLVM ar thanks to modern system calls like\n"
+      "    copy_file_range(). This ar should also use 100x less memory.\n"
+      "\n"
+      "  - Can be built as a 96kb APE binary that works well on six OSes.\n"
+      "    Cosmopolitan uses the same dev tools on all OSes and archsr to\n"
+      "    ensure compilations are simple and deterministic for everyone.\n"
+      "\n"
+      "  This static archiver introduces handy features:\n"
+      "\n"
+      "  - Arguments may be supplied in an `@args.txt` file. This is useful\n"
+      "    for overcoming the `ARG_MAX` limit, which is especially important\n"
+      "    on Windows, where only very few command arguments can be passed.\n"
+      "    GNU Make can be easily configured to generate args files.\n"
+      "\n"
+      "  - You can merge many .a files into one big .a file. Args that end\n"
+      "    with .a will be opened as static archives. The .o files inside it\n"
+      "    will then be added to your new archive. It would be the same as if\n"
+      "    you passed all the .o files as args. This is fast. For example, to\n"
+      "    merge 37 .a files containing 5000 .o files takes ~38 milliseconds.\n"
+      "\n"
+      "  - Directory arguments are ignored. The biggest gotcha with makefiles\n"
+      "    that use wildcard globbing is that it can't detect when files are\n"
+      "    deleted, which means it can't invalidate the artifacts which had\n"
+      "    depended on that file, leading to nondeterminism and surprising\n"
+      "    build failures. The simplest way to solve that is to add the\n"
+      "    directory to the prerequisites list, since the directory modified\n"
+      "    time will be updated by the OS when files inside it are deleted.\n"
+      "    When doing this, it's simple and elegant to not need to filter\n"
+      "    the directory prerequisites before passing `$^` to `ar`.\n"
+      "\n",
+      NULL);
+  exit(rc);
+}

 #define HEAP_SIZE (256L * 1024 * 1024)

@ -108,29 +152,6 @@ static wontreturn void SysDie(const char *path, const char *func) {
  exit(1);
 }

-static wontreturn void ShowUsage(int rc, int fd) {
-  tinyprint(fd, VERSION,
-            "\n"
-            "USAGE\n"
-            "\n",
-            "  ", program_invocation_name, " FLAGS ARCHIVE FILE...\n",
-            "\n"
-            "FLAGS\n"
-            "\n"
-            "  rcs        create new archive with index\n"
-            "  rcsD       always deterministic\n"
-            "  --help     show usage\n"
-            "  --version  show program details\n"
-            "\n"
-            "ARGUMENTS\n"
-            "\n"
-            "  ARCHIVE    should be foo.a\n"
-            "  FILE       should be foo.o or @args.txt\n"
-            "\n",
-            NULL);
-  exit(rc);
-}
-
 // allocates 𝑛 bytes of memory aligned on 𝑎 from .bss
 // - avoids binary bloat of mmap() and malloc()
 // - dies if out of memory or overflow occurs
@ -159,13 +180,11 @@ static void *balloc(size_t n, size_t a) {
  } else {
    c = 2ull << (__builtin_clzll(n - 1) ^ (sizeof(long long) * CHAR_BIT - 1));
  }
-  if (c < a || c > HEAP_SIZE || p + c > h + HEAP_SIZE) {
+  if (c < a || c > HEAP_SIZE || p + c > h + HEAP_SIZE)
    Die(program_invocation_name, "out of memory");
-  }
  used = p - h + c;
-  if (resizable) {
+  if (resizable)
    memcpy((char *)p - sizeof(c), &c, sizeof(c));
-  }
  return (void *)p;
 }

@ -258,18 +277,24 @@ static void MakeArHeader(struct ar_hdr *h,  //
 // - uses copy_file_range() if possible
 // - returns number of bytes exchanged
 // - dies if operation fails
-static int64_t CopyFileOrDie(const char *inpath, int infd,  //
-                             const char *outpath, int outfd) {
-  int64_t toto;
+static void CopyFileOrDie(const char *inpath, int infd,    //
+                          const char *outpath, int outfd,  //
+                          size_t offset, size_t size) {
  char buf[512];
  size_t exchanged;
-  ssize_t got, wrote;
  enum { CFR, RW } mode;
-  for (mode = CFR, toto = 0;; toto += exchanged) {
+  ssize_t want, got, wrote;
+  if (offset)
+    if (lseek(infd, offset, SEEK_SET) == -1)
+      SysDie(inpath, "lseek");
+  for (mode = CFR; size; size -= exchanged) {
    if (mode == CFR) {
-      got = copy_file_range(infd, 0, outfd, 0, 4194304, 0);
+      want = 4194304;
+      if (want > size)
+        want = size;
+      got = copy_file_range(infd, 0, outfd, 0, want, 0);
      if (!got)
-        break;
+        Die(inpath, "unexpected eof");
      if (got != -1) {
        exchanged = got;
      } else if (errno == EXDEV ||       // different partitions
@ -282,9 +307,12 @@ static int64_t CopyFileOrDie(const char *inpath, int infd,  //
        SysDie(inpath, "copy_file_range");
      }
    } else {
-      got = read(infd, buf, sizeof(buf));
+      want = sizeof(buf);
+      if (want > size)
+        want = size;
+      got = read(infd, buf, want);
      if (!got)
-        break;
+        Die(inpath, "unexpected eof");
      if (got == -1)
        SysDie(inpath, "read");
      wrote = write(outfd, buf, got);
@ -295,7 +323,51 @@ static int64_t CopyFileOrDie(const char *inpath, int infd,  //
      exchanged = wrote;
    }
  }
-  return toto;
+}
+
+static void AppendName(const char *name, struct Args *names,
+                       struct Bytes *filenames) {
+  struct ar_hdr header1;
+  char bnbuf[PATH_MAX + 1];
+  strlcpy(bnbuf, name, sizeof(bnbuf));
+  char *aname = StrCat(basename(bnbuf), "/");
+  if (strlen(aname) <= sizeof(header1.ar_name)) {
+    AppendArg(names, aname);
+  } else {
+    char ibuf[21];
+    FormatUint64(ibuf, filenames->i);
+    AppendArg(names, StrCat("/", ibuf));
+    AppendBytes(filenames, aname, strlen(aname));
+    AppendBytes(filenames, "\n", 1);
+  }
+}
+
+static void AppendSymbols(const char *path, const Elf64_Ehdr *elf,
+                          size_t elfsize, struct Bytes *symbols,
+                          struct Ints *symnames, int objid) {
+  if (!IsElf64Binary(elf, elfsize))
+    Die(path, "not an elf64 binary");
+  char *strs = GetElfStringTable(elf, elfsize, ".strtab");
+  if (!strs)
+    Die(path, "elf .strtab not found");
+  Elf64_Xword symcount;
+  Elf64_Shdr *symsec = GetElfSymbolTable(elf, elfsize, SHT_SYMTAB, &symcount);
+  Elf64_Sym *syms = GetElfSectionAddress(elf, elfsize, symsec);
+  if (!syms)
+    Die(path, "elf symbol table not found");
+  for (Elf64_Xword j = symsec->sh_info; j < symcount; ++j) {
+    if (!syms[j].st_name)
+      continue;
+    if (syms[j].st_shndx == SHN_UNDEF)
+      continue;
+    if (syms[j].st_shndx == SHN_COMMON)
+      continue;
+    const char *symname = GetElfString(elf, elfsize, strs, syms[j].st_name);
+    if (!symname)
+      Die(path, "elf symbol name corrupted");
+    AppendBytes(symbols, symname, strlen(symname) + 1);
+    AppendInt(symnames, objid);
+  }
 }

 int main(int argc, char *argv[]) {
@ -309,24 +381,26 @@ int main(int argc, char *argv[]) {

  // handle hardcoded flags
  if (argc == 2) {
-    if (IsEqual(argv[1], "-n")) {
+    if (IsEqual(argv[1], "-n"))
      exit(0);
-    }
    if (IsEqual(argv[1], "-h") ||  //
        IsEqual(argv[1], "-?") ||  //
        IsEqual(argv[1], "--help")) {
      ShowUsage(0, 1);
    }
    if (IsEqual(argv[1], "--version")) {
-      tinyprint(1, VERSION, NULL);
+      tinyprint(1,
+                "cosmopolitan ar v3.0\n"
+                "copyright 2024 justine tunney\n"
+                "https://github.com/jart/cosmopolitan\n",
+                NULL);
      exit(0);
    }
  }

  // get flags and output path
-  if (argc < 3) {
-    ShowUsage(1, 2);
-  }
+  if (argc < 3)
+    Die(argv[0], "missing argument");
  char *flags = argv[1];
  const char *outpath = argv[2];

@ -347,8 +421,8 @@ int main(int argc, char *argv[]) {

  struct Args args = {reballoc(0, 4096, sizeof(char *))};
  struct Args names = {reballoc(0, 4096, sizeof(char *))};
-  struct Ints modes = {reballoc(0, 4096, sizeof(int))};
  struct Ints sizes = {reballoc(0, 4096, sizeof(int))};
+  struct Ints foffsets = {reballoc(0, 4096, sizeof(int))};
  struct Ints symnames = {reballoc(0, 16384, sizeof(int))};
  struct Bytes symbols = {reballoc(0, 131072, sizeof(char))};
  struct Bytes filenames = {reballoc(0, 16384, sizeof(char))};
@ -365,63 +439,42 @@ int main(int argc, char *argv[]) {
      continue;
    if (endswith(arg, ".pkg"))
      continue;
-    if (stat(arg, &st))
-      SysDie(arg, "stat");
-    if (S_ISDIR(st.st_mode))
-      continue;
-    if (!st.st_size)
-      Die(arg, "file is empty");
-    if (st.st_size > 0x7ffff000)
-      Die(arg, "file too large");
-    if ((fd = open(arg, O_RDONLY)) == -1)
-      SysDie(arg, "open");
-    AppendArg(&args, StrDup(arg));
-    AppendInt(&sizes, st.st_size);
-    AppendInt(&modes, st.st_mode);
-    char bnbuf[PATH_MAX + 1];
-    strlcpy(bnbuf, arg, sizeof(bnbuf));
-    char *aname = StrCat(basename(bnbuf), "/");
-    if (strlen(aname) <= sizeof(header1.ar_name)) {
-      AppendArg(&names, aname);
+    if (endswith(arg, ".a")) {
+      struct Ar ar;
+      struct ArFile arf;
+      openar(&ar, arg);
+      while (readar(&ar, &arf)) {
+        AppendArg(&args, StrDup(arg));
+        AppendInt(&sizes, arf.size);
+        AppendInt(&foffsets, arf.offset);
+        AppendName(arf.name, &names, &filenames);
+        AppendSymbols(arg, arf.data, arf.size, &symbols, &symnames, objectid++);
+      }
+      closear(&ar);
    } else {
-      char ibuf[21];
-      FormatUint64(ibuf, filenames.i);
-      AppendArg(&names, StrCat("/", ibuf));
-      AppendBytes(&filenames, aname, strlen(aname));
-      AppendBytes(&filenames, "\n", 1);
+      if (stat(arg, &st))
+        SysDie(arg, "stat");
+      if (S_ISDIR(st.st_mode))
+        continue;
+      if (!st.st_size)
+        Die(arg, "file is empty");
+      if (st.st_size > 0x7ffff000)
+        Die(arg, "file too large");
+      if ((fd = open(arg, O_RDONLY)) == -1)
+        SysDie(arg, "open");
+      AppendArg(&args, StrDup(arg));
+      AppendInt(&sizes, st.st_size);
+      AppendInt(&foffsets, 0);
+      AppendName(arg, &names, &filenames);
+      void *elf = mmap(0, st.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
+      if (elf == MAP_FAILED)
+        SysDie(arg, "mmap");
+      AppendSymbols(arg, elf, st.st_size, &symbols, &symnames, objectid++);
+      if (munmap(elf, st.st_size))
+        SysDie(arg, "munmap");
+      if (close(fd))
+        SysDie(arg, "close");
    }
-    size_t mapsize = st.st_size;
-    void *elf = mmap(0, mapsize, PROT_READ, MAP_PRIVATE, fd, 0);
-    if (elf == MAP_FAILED)
-      SysDie(arg, "mmap");
-    if (!IsElf64Binary(elf, mapsize))
-      Die(arg, "not an elf64 binary");
-    char *strs = GetElfStringTable(elf, mapsize, ".strtab");
-    if (!strs)
-      Die(arg, "elf .strtab not found");
-    Elf64_Xword symcount;
-    Elf64_Shdr *symsec = GetElfSymbolTable(elf, mapsize, SHT_SYMTAB, &symcount);
-    Elf64_Sym *syms = GetElfSectionAddress(elf, mapsize, symsec);
-    if (!syms)
-      Die(arg, "elf symbol table not found");
-    for (Elf64_Xword j = symsec->sh_info; j < symcount; ++j) {
-      if (!syms[j].st_name)
-        continue;
-      if (syms[j].st_shndx == SHN_UNDEF)
-        continue;
-      if (syms[j].st_shndx == SHN_COMMON)
-        continue;
-      const char *symname = GetElfString(elf, mapsize, strs, syms[j].st_name);
-      if (!symname)
-        Die(arg, "elf symbol name corrupted");
-      AppendBytes(&symbols, symname, strlen(symname) + 1);
-      AppendInt(&symnames, objectid);
-    }
-    if (munmap(elf, mapsize))
-      SysDie(arg, "munmap");
-    if (close(fd))
-      SysDie(arg, "close");
-    ++objectid;
  }
  getargs_destroy(&ga);

@ -461,45 +514,37 @@ int main(int argc, char *argv[]) {
  MakeArHeader(&header1, "/", 0, tablebufsize + ROUNDUP(symbols.i, 2));
  MakeArHeader(&header2, "//", 0, ROUNDUP(filenames.i, 2));
  WRITE32BE(tablebuf, symnames.i);
-  for (size_t i = 0; i < symnames.i; ++i) {
+  for (size_t i = 0; i < symnames.i; ++i)
    WRITE32BE(tablebuf + 4 + i * 4, offsets[symnames.p[i]]);
-  }

  // write output archive
  int outfd;
-  if ((outfd = creat(outpath, 0644)) == -1) {
+  if ((outfd = creat(outpath, 0644)) == -1)
    SysDie(outpath, "creat");
-  }
-  if (ftruncate(outfd, outsize)) {
+  if (ftruncate(outfd, outsize))
    SysDie(outpath, "ftruncate");
-  }
-  if ((outsize = writev(outfd, iov, ARRAYLEN(iov))) == -1) {
+  if ((outsize = writev(outfd, iov, ARRAYLEN(iov))) == -1)
    SysDie(outpath, "writev[1]");
-  }
  for (size_t i = 0; i < args.i; ++i) {
    const char *inpath = args.p[i];
-    if ((fd = open(inpath, O_RDONLY)) == -1) {
-      SysDie(inpath, "open");
-    }
+    if (!(i && IsEqual(inpath, args.p[i - 1])))
+      if ((fd = open(inpath, O_RDONLY)) == -1)
+        SysDie(inpath, "open");
    iov[0].iov_base = "\n";
    outsize += (iov[0].iov_len = outsize & 1);
    iov[1].iov_base = &header1;
    outsize += (iov[1].iov_len = sizeof(struct ar_hdr));
-    MakeArHeader(&header1, names.p[i], modes.p[i], sizes.p[i]);
-    if (writev(outfd, iov, 2) == -1) {
+    MakeArHeader(&header1, names.p[i], 0100644, sizes.p[i]);
+    if (writev(outfd, iov, 2) == -1)
      SysDie(outpath, "writev[2]");
-    }
    outsize += sizes.p[i];
-    if (CopyFileOrDie(inpath, fd, outpath, outfd) != sizes.p[i]) {
-      Die(inpath, "file size changed");
-    }
-    if (close(fd)) {
-      SysDie(inpath, "close");
-    }
+    CopyFileOrDie(inpath, fd, outpath, outfd, foffsets.p[i], sizes.p[i]);
+    if (!(i + 1 < args.i && IsEqual(inpath, args.p[i + 1])))
+      if (close(fd))
+        SysDie(inpath, "close");
  }
-  if (close(outfd)) {
+  if (close(outfd))
    SysDie(outpath, "close");
-  }

  return 0;
 }
--- a/tool/build/lib/ar.c
+++ b/tool/build/lib/ar.c
@ -0,0 +1,166 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│ vi: set et ft=c ts=2 sts=2 sw=2 fenc=utf-8                               :vi │
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2024 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "tool/build/lib/ar.h"
+#include "libc/ar.h"
+#include "libc/calls/calls.h"
+#include "libc/calls/struct/stat.h"
+#include "libc/ctype.h"
+#include "libc/fmt/conv.h"
+#include "libc/runtime/runtime.h"
+#include "libc/stdio/stdio.h"
+#include "libc/str/str.h"
+#include "libc/sysv/consts/map.h"
+#include "libc/sysv/consts/o.h"
+#include "libc/sysv/consts/prot.h"
+
+/**
+ * @fileoverview static archive reader
+ *
+ * This file implements an API similar to opendir() for raeding the .o
+ * files within your .a file. This works by mapping the .a file into
+ * memory and then yielding pointers into the map where embedded files
+ * reside, along with their decoded filenames.
+ *
+ * To try this library:
+ *
+ *     make -j o//tool/decode/ar2 o//libc/str/str.a
+ *     o//tool/decode/ar2 o//libc/str/str.a
+ *
+ * This implementation currently dies on error. The intent is to make
+ * this as simple of an abstraction as possible for simple command line
+ * utilities like o//tool/build/ar. It shouldn't be considered a serious
+ * general purpose library. Another thing it can't do is decode symbol
+ * table information, since Cosmopolitan Ar currently doesn't need it.
+ */
+
+void openar(struct Ar *ar, const char *path) {
+  memset(ar, 0, sizeof(*ar));
+  ar->path = path;
+  if ((ar->fd = open(path, O_RDONLY)) == -1) {
+    perror(ar->path);
+    exit(1);
+  }
+  if (fstat(ar->fd, &ar->st)) {
+    perror(ar->path);
+    exit(1);
+  }
+  ar->map = mmap(0, ar->st.st_size, PROT_READ, MAP_PRIVATE, ar->fd, 0);
+  if (ar->map == MAP_FAILED) {
+    perror(ar->path);
+    exit(1);
+  }
+  if (!startswith(ar->map, ARMAG)) {
+    tinyprint(2, ar->path, ": not an ar file\n", NULL);
+    exit(1);
+  }
+  ar->offset = SARMAG;
+}
+
+void closear(struct Ar *ar) {
+  if (munmap(ar->map, ar->st.st_size)) {
+    perror(ar->path);
+    exit(1);
+  }
+  if (close(ar->fd)) {
+    perror(ar->path);
+    exit(1);
+  }
+}
+
+bool readar(struct Ar *ar, struct ArFile *arf) {
+  for (;;) {
+    ar->offset += 1;
+    ar->offset &= -2;
+    if (ar->offset + sizeof(struct ar_hdr) > ar->st.st_size)
+      return false;
+
+    struct ar_hdr *hdr = (struct ar_hdr *)(ar->map + ar->offset);
+    ar->offset += sizeof(struct ar_hdr);
+
+    char ar_fmag[sizeof(hdr->ar_fmag) + 1] = {0};
+    memcpy(ar_fmag, hdr->ar_fmag, sizeof(hdr->ar_fmag));
+    if (strcmp(ar_fmag, ARFMAG)) {
+      tinyprint(2, ar->path, ": corrupt ar file fmag\n", NULL);
+      exit(1);
+    }
+
+    char ar_name[sizeof(hdr->ar_name) + 1] = {0};
+    memcpy(ar_name, hdr->ar_name, sizeof(hdr->ar_name));
+    for (int j = sizeof(hdr->ar_name) - 1 + 1; j-- && isspace(ar_name[j]);)
+      ar_name[j] = '\0';
+
+    char ar_size[sizeof(hdr->ar_size) + 1] = {0};
+    memcpy(ar_size, hdr->ar_size, sizeof(hdr->ar_size));
+    int size = atoi(ar_size);
+    if (size < 0 || ar->offset + size > ar->st.st_size) {
+      tinyprint(2, ar->path, ": ar size overlaps eof\n", NULL);
+      exit(1);
+    }
+
+    // symbol table
+    if (!strcmp(ar_name, "/")) {
+      ar->offset += size;
+      continue;
+    }
+
+    // filename table
+    if (!strcmp(ar_name, "//")) {
+      ar->filenames = ar->map + ar->offset;
+      ar->filenames_size = size;
+      ar->offset += size;
+      continue;
+    }
+
+    // get name of object file
+    size_t len;
+    const char *e;
+    if (ar_name[0] == '/') {
+      int off = atoi(ar_name + 1);
+      if (off < 0 || off >= ar->filenames_size) {
+        tinyprint(2, ar->path, ": ar filename not found\n", NULL);
+        exit(1);
+      }
+      if (!(e = memchr(ar->filenames + off, '\n', ar->filenames_size - off))) {
+        tinyprint(2, ar->path, ": ar filename overlaps end\n", NULL);
+        exit(1);
+      }
+      if ((len = e - (ar->filenames + off)) >= PATH_MAX) {
+        tinyprint(2, ar->path, ": ar filename too long\n", NULL);
+        exit(1);
+      }
+      memcpy(arf->name, ar->filenames + off, len);
+      arf->name[len] = '\0';
+      if (len && arf->name[len - 1] == '/')
+        arf->name[--len] = '\0';
+    } else if ((len = strlen(ar_name)) && ar_name[len - 1] == '/') {
+      memcpy(arf->name, ar_name, len - 1);
+      arf->name[len - 1] = '\0';
+    } else {
+      tinyprint(2, ar->path, ": unsupported ar name: ", ar_name, "\n", NULL);
+      exit(1);
+    }
+
+    // return pointer to embedded file
+    arf->size = size;
+    arf->offset = ar->offset;
+    arf->data = ar->map + ar->offset;
+    ar->offset += size;
+    return true;
+  }
+}
--- a/tool/build/lib/ar.h
+++ b/tool/build/lib/ar.h
@ -0,0 +1,29 @@
+#ifndef COSMOPOLITAN_TOOL_BUILD_LIB_AR_H_
+#define COSMOPOLITAN_TOOL_BUILD_LIB_AR_H_
+#include "libc/calls/struct/stat.h"
+#include "libc/limits.h"
+COSMOPOLITAN_C_START_
+
+struct Ar {
+  const char *path;
+  int fd;
+  struct stat st;
+  char *map;
+  size_t offset;
+  const char *filenames;
+  size_t filenames_size;
+};
+
+struct ArFile {
+  void *data;
+  size_t size;
+  size_t offset;
+  char name[PATH_MAX];
+};
+
+void openar(struct Ar *, const char *);
+void closear(struct Ar *);
+bool readar(struct Ar *, struct ArFile *);
+
+COSMOPOLITAN_C_END_
+#endif /* COSMOPOLITAN_TOOL_BUILD_LIB_AR_H_ */
--- a/tool/decode/BUILD.mk
+++ b/tool/decode/BUILD.mk
@ -40,6 +40,7 @@ TOOL_DECODE_DIRECTDEPS =				\
 	THIRD_PARTY_MUSL				\
 	THIRD_PARTY_TZ					\
 	THIRD_PARTY_XED					\
+	TOOL_BUILD_LIB					\
 	TOOL_DECODE_LIB

 TOOL_DECODE_DEPS :=					\
--- a/tool/decode/ar.c
+++ b/tool/decode/ar.c
@ -110,9 +110,8 @@ static void Print(void) {
  printf("\n");
  printf("\t.long\t%-*.u# %s\n", 35, entries, "symbol table entries");
  table = 8 + 60 + 4;
-  for (i = 0; i < entries; ++i) {
+  for (i = 0; i < entries; ++i)
    printf("\t.long\t%#-*.x# %u\n", 35, READ32BE(data + table + i * 4), i);
-  }
  symbols = table + entries * 4;
  symbolslen = arsize - (entries + 1) * 4;
  for (i = o = 0; o < symbolslen; ++i, o += n + 1) {
--- a/tool/decode/ar2.c
+++ b/tool/decode/ar2.c
@ -0,0 +1,41 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│ vi: set et ft=c ts=2 sts=2 sw=2 fenc=utf-8                               :vi │
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2024 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/elf/elf.h"
+#include "libc/stdio/stdio.h"
+#include "tool/build/lib/ar.h"
+
+void ProcessFile(const char *path) {
+  struct Ar ar;
+  struct ArFile arf;
+  openar(&ar, path);
+  while (readar(&ar, &arf)) {
+    printf("%s: %s", path, arf.name);
+    if (IsElf64Binary(arf.data, arf.size))
+      printf(" is elf");
+    else
+      printf(" is not elf!!");
+    printf("\n");
+  }
+  closear(&ar);
+}
+
+int main(int argc, char *argv[]) {
+  for (int i = 1; i < argc; ++i)
+    ProcessFile(argv[i]);
+}