Added zip_entry_noallocreadwithoffset

README.md

@@ -208,6 +208,27 @@ zip_stream_close(zip);
 free(buf);
 ```
 
+* Extract a partial zip entry
+
+```c
+unsigned char buf[16];
+size_t bufsize = sizeof(buf);
+
+struct zip_t *zip = zip_open("foo.zip", 0, 'r');
+{
+    zip_entry_open(zip, "foo-1.txt");
+    {
+        size_t offset = 4;
+        ssize_t nread = zip_entry_noallocreadwithoffset(zip, offset, bufsize, (void *)buf);
+    }
+
+    zip_entry_close(zip);
+}
+zip_close(zip);
+
+free(buf);
+```
+
 * List of all zip entries
 
 ```c

src/zip.c

@@ -114,7 +114,7 @@ struct zip_entry_mark_t {
   size_t lf_length;
 };
 
-static const char *const zip_errlist[33] = {
+static const char *const zip_errlist[35] = {
     NULL,
     "not initialized\0",
     "invalid entry name\0",
@@ -148,6 +148,8 @@ static const char *const zip_errlist[33] = {
     "cannot initialize reader\0",
     "cannot initialize writer\0",
     "cannot initialize writer from reader\0",
+    "invalid argument\0",
+    "cannot initialize reader iterator\0",
 };
 
 const char *zip_strerror(int errnum) {
@@ -1654,6 +1656,76 @@ ssize_t zip_entry_noallocread(struct zip_t *zip, void *buf, size_t bufsize) {
   return (ssize_t)zip->entry.uncomp_size;
 }
 
+ssize_t zip_entry_noallocreadwithoffset(struct zip_t *zip,
+                                    size_t offset, size_t size, void *buf) {
+  mz_zip_archive *pzip = NULL;
+
+  if (!zip) {
+    // zip_t handler is not initialized
+    return (ssize_t)ZIP_ENOINIT;
+  }
+
+  if (offset >= (size_t)zip->entry.uncomp_size) {
+    return (ssize_t)ZIP_EINVAL;
+  }
+
+  if ((offset+size) > (size_t)zip->entry.uncomp_size) {
+    size = (ssize_t)zip->entry.uncomp_size - offset;
+  }
+
+  pzip = &(zip->archive);
+  if (pzip->m_zip_mode != MZ_ZIP_MODE_READING ||
+      zip->entry.index < (ssize_t)0) {
+    // the entry is not found or we do not have read access
+    return (ssize_t)ZIP_ENOENT;
+  }
+
+  mz_zip_reader_extract_iter_state* iter =
+      mz_zip_reader_extract_iter_new(pzip, (mz_uint)zip->entry.index, 0);
+  if (!iter) {
+    return (ssize_t)ZIP_ENORITER;
+  }
+
+  mz_uint8* writebuf = (mz_uint8*)buf;
+  size_t    file_offset = 0;
+  size_t    write_cursor = 0;
+  size_t    to_read = size;
+
+  // iterate until the requested offset is in range
+  while (file_offset < zip->entry.uncomp_size && to_read > 0)
+  {
+    size_t nread = mz_zip_reader_extract_iter_read(iter, (void*)&writebuf[write_cursor], to_read);
+
+    if (nread == 0)
+      break;
+
+    if (offset < (file_offset+nread)) {
+      size_t read_cursor = offset - file_offset;
+      MZ_ASSERT(read_cursor < size);
+      size_t read_size = nread - read_cursor;
+
+      if (to_read < read_size)
+        read_size = to_read;
+      MZ_ASSERT(read_size <= size);
+
+      // If it's an unaligned read (i.e. the first one)
+      if (read_cursor != 0)
+      {
+        memmove(&writebuf[write_cursor], &writebuf[read_cursor], read_size);
+      }
+
+      write_cursor += read_size;
+      offset += read_size;
+      to_read -= read_size;
+    }
+
+    file_offset += nread;
+  }
+
+  mz_zip_reader_extract_iter_free(iter);
+  return (ssize_t)write_cursor;
+}
+
 int zip_entry_fread(struct zip_t *zip, const char *filename) {
   mz_zip_archive *pzip = NULL;
   mz_uint idx;

src/zip.h

@@ -96,6 +96,8 @@ typedef long ssize_t; /* byte count or error */
 #define ZIP_ERINIT -30      // cannot initialize reader
 #define ZIP_EWINIT -31      // cannot initialize writer
 #define ZIP_EWRINIT -32     // cannot initialize writer from reader
+#define ZIP_EINVAL -33      // invalid argument
+#define ZIP_ENORITER -34    // cannot initialize reader iterator
 
 /**
  * Looks up the error message string corresponding to an error number.
@@ -373,6 +375,24 @@ extern ZIP_EXPORT ssize_t zip_entry_read(struct zip_t *zip, void **buf,
 extern ZIP_EXPORT ssize_t zip_entry_noallocread(struct zip_t *zip, void *buf,
                                                 size_t bufsize);
 
+/**
+ * Extracts the part of the current zip entry into a memory buffer using no memory
+ * allocation for the buffer.
+ *
+ * @param zip zip archive handler.
+ * @param offset the offset of the entry (in bytes).
+ * @param size requested number of bytes (in bytes).
+ * @param buf preallocated output buffer.
+ *
+ * @note the iterator api uses an allocation to create its state
+ * @note each call will iterate from the start of the entry
+ *
+ * @return the return code - the number of bytes actually read on success.
+ *         Otherwise a negative number (< 0) on error (e.g. offset is too large).
+ */
+extern ZIP_EXPORT ssize_t zip_entry_noallocreadwithoffset(struct zip_t *zip,
+                                        size_t offset, size_t size, void *buf);
+
 /**
  * Extracts the current zip entry into output file.
  *

test/test_read.c

@@ -129,11 +129,52 @@ MU_TEST(test_noallocread) {
   zip_close(zip);
 }
 
+
+MU_TEST(test_noallocreadwithoffset) {
+  size_t expected_size = strlen(TESTDATA2);
+  char *expected_data = calloc(expected_size, sizeof(char));
+
+  struct zip_t *zip = zip_open(ZIPNAME, 0, 'r');
+  mu_check(zip != NULL);
+  mu_assert_int_eq(1, zip_is64(zip));
+
+  mu_assert_int_eq(0, zip_entry_open(zip, "test/test-2.txt"));
+  zip_entry_noallocread(zip, (void *)expected_data, expected_size);
+
+  // Read the file in different chunk sizes
+  for (size_t i = 1; i <= expected_size; ++i) {
+    size_t buflen = i;
+    char *tmpbuf = calloc(buflen, sizeof(char));
+
+    for (size_t j = 0; j < expected_size; ++j) {
+      // we test starting from different offsets, to make sure we hit the "unaligned" code path
+      size_t offset = j;
+      while (offset < expected_size) {
+
+        ssize_t nread = zip_entry_noallocreadwithoffset(zip, offset, buflen, tmpbuf);
+
+        mu_assert(nread <= buflen, "too many bytes read");
+        mu_assert(0u != nread, "no bytes read");
+
+        // check the data
+        for (ssize_t j = 0; j < nread; ++j) {
+          mu_assert_int_eq(expected_data[offset + j], tmpbuf[j]);
+        }
+
+        offset += nread;
+      }
+    }
+  }
+
+  zip_close(zip);
+}
+
 MU_TEST_SUITE(test_read_suite) {
   MU_SUITE_CONFIGURE(&test_setup, &test_teardown);
 
   MU_RUN_TEST(test_read);
   MU_RUN_TEST(test_noallocread);
+  MU_RUN_TEST(test_noallocreadwithoffset);
 }
 
 #define UNUSED(x) (void)x