Manual browser: archive_read(3)

archive_read(3) Library Functions Manual archive_read(3)


archive_read_new, archive_read_set_filter_options, archive_read_set_format_options, archive_read_set_options, archive_read_support_compression_all, archive_read_support_compression_bzip2, archive_read_support_compression_compress, archive_read_support_compression_gzip, archive_read_support_compression_lzma, archive_read_support_compression_none, archive_read_support_compression_xz, archive_read_support_compression_program, archive_read_support_compression_program_signature, archive_read_support_format_all, archive_read_support_format_ar, archive_read_support_format_cpio, archive_read_support_format_empty, archive_read_support_format_iso9660, archive_read_support_format_mtree, archive_read_support_format_raw, archive_read_support_format_tar, archive_read_support_format_zip, archive_read_open, archive_read_open2, archive_read_open_fd, archive_read_open_FILE, archive_read_open_filename, archive_read_open_memory, archive_read_next_header, archive_read_next_header2, archive_read_data, archive_read_data_block, archive_read_data_skip, archive_read_data_into_buffer, archive_read_data_into_fd, archive_read_extract, archive_read_extract2, archive_read_extract_set_progress_callback, archive_read_close, archive_read_finishfunctions for reading streaming archives


#include <archive.h>

struct archive *

archive_read_support_compression_all(struct archive *);

archive_read_support_compression_bzip2(struct archive *);

archive_read_support_compression_compress(struct archive *);

archive_read_support_compression_gzip(struct archive *);

archive_read_support_compression_lzma(struct archive *);

archive_read_support_compression_none(struct archive *);

archive_read_support_compression_xz(struct archive *);

archive_read_support_compression_program(struct archive *, const char *cmd);

archive_read_support_compression_program_signature(struct archive *, const char *cmd, const void *signature, size_t signature_length);

archive_read_support_format_all(struct archive *);

archive_read_support_format_ar(struct archive *);

archive_read_support_format_cpio(struct archive *);

archive_read_support_format_empty(struct archive *);

archive_read_support_format_iso9660(struct archive *);

archive_read_support_format_mtree(struct archive *);

archive_read_support_format_raw(struct archive *);

archive_read_support_format_tar(struct archive *);

archive_read_support_format_zip(struct archive *);

archive_read_set_filter_options(struct archive *, const char *);

archive_read_set_format_options(struct archive *, const char *);

archive_read_set_options(struct archive *, const char *);

archive_read_open(struct archive *, void *client_data, archive_open_callback *, archive_read_callback *, archive_close_callback *);

archive_read_open2(struct archive *, void *client_data, archive_open_callback *, archive_read_callback *, archive_skip_callback *, archive_close_callback *);

archive_read_open_FILE(struct archive *, FILE *file);

archive_read_open_fd(struct archive *, int fd, size_t block_size);

archive_read_open_filename(struct archive *, const char *filename, size_t block_size);

archive_read_open_memory(struct archive *, void *buff, size_t size);

archive_read_next_header(struct archive *, struct archive_entry **);

archive_read_next_header2(struct archive *, struct archive_entry *);

archive_read_data(struct archive *, void *buff, size_t len);

archive_read_data_block(struct archive *, const void **buff, size_t *len, off_t *offset);

archive_read_data_skip(struct archive *);

archive_read_data_into_buffer(struct archive *, void *, ssize_t len);

archive_read_data_into_fd(struct archive *, int fd);

archive_read_extract(struct archive *, struct archive_entry *, int flags);

archive_read_extract2(struct archive *src, struct archive_entry *, struct archive *dest);

archive_read_extract_set_progress_callback(struct archive *, void (*func)(void *), void *user_data);

archive_read_close(struct archive *);

archive_read_finish(struct archive *);


These functions provide a complete API for reading streaming archives. The general process is to first create the struct archive object, set options, initialize the reader, iterate over the archive headers and associated data, then close the archive and release all resources. The following summary describes the functions in approximately the order they would be used:
Allocates and initializes a struct archive object suitable for reading from an archive.
archive_read_support_compression_bzip2(), archive_read_support_compression_compress(), archive_read_support_compression_gzip(), archive_read_support_compression_lzma(), archive_read_support_compression_none(), archive_read_support_compression_xz()
Enables auto-detection code and decompression support for the specified compression. Returns ARCHIVE_OK if the compression is fully supported, or ARCHIVE_WARN if the compression is supported only through an external program. Note that decompression using an external program is usually slower than decompression through built-in libraries. Note that “none” is always enabled by default.
Enables all available decompression filters.
Data is fed through the specified external program before being dearchived. Note that this disables automatic detection of the compression format, so it makes no sense to specify this in conjunction with any other decompression option.
This feeds data through the specified external program but only if the initial bytes of the data match the specified signature value.
archive_read_support_format_all(), archive_read_support_format_ar(), archive_read_support_format_cpio(), archive_read_support_format_empty(), archive_read_support_format_iso9660(), archive_read_support_format_mtree(), archive_read_support_format_tar(), archive_read_support_format_zip()
Enables support---including auto-detection code---for the specified archive format. For example, archive_read_support_format_tar() enables support for a variety of standard tar formats, old-style tar, ustar, pax interchange format, and many common variants. For convenience, archive_read_support_format_all() enables support for all available formats. Only empty archives are supported by default.
The “raw” format handler allows libarchive to be used to read arbitrary data. It treats any data stream as an archive with a single entry. The pathname of this entry is “data”; all other entry fields are unset. This is not enabled by archive_read_support_format_all() in order to avoid erroneous handling of damaged archives.
archive_read_set_filter_options(), archive_read_set_format_options(), archive_read_set_options()
Specifies options that will be passed to currently-registered filters (including decompression filters) and/or format readers. The argument is a comma-separated list of individual options. Individual options have one of the following forms:
The option/value pair will be provided to every module. Modules that do not accept an option with this name will ignore it.
The option will be provided to every module with a value of “1”.
The option will be provided to every module with a NULL value.
module:option=value, module:option, module:!option
As above, but the corresponding option and value will be provided only to modules whose name matches module.
The return value will be ARCHIVE_OK if any module accepts the option, or ARCHIVE_WARN if no module accepted the option, or ARCHIVE_FATAL if there was a fatal error while attempting to process the option.

The currently supported options are:

Format iso9660
Support Joliet extensions. Defaults to enabled, use !joliet to disable.
The same as archive_read_open2(), except that the skip callback is assumed to be NULL.
Freeze the settings, open the archive, and prepare for reading entries. This is the most generic version of this call, which accepts four callback functions. Most clients will want to use archive_read_open_filename(), archive_read_open_FILE(), archive_read_open_fd(), or archive_read_open_memory() instead. The library invokes the client-provided functions to obtain raw bytes from the archive.
Like archive_read_open(), except that it accepts a FILE * pointer. This function should not be used with tape drives or other devices that require strict I/O blocking.
Like archive_read_open(), except that it accepts a file descriptor and block size rather than a set of function pointers. Note that the file descriptor will not be automatically closed at end-of-archive. This function is safe for use with tape drives or other blocked devices.
This is a deprecated synonym for archive_read_open_filename().
Like archive_read_open(), except that it accepts a simple filename and a block size. A NULL filename represents standard input. This function is safe for use with tape drives or other blocked devices.
Like archive_read_open(), except that it accepts a pointer and size of a block of memory containing the archive data.
Read the header for the next entry and return a pointer to a struct archive_entry. This is a convenience wrapper around archive_read_next_header2() that reuses an internal struct archive_entry object for each request.
Read the header for the next entry and populate the provided struct archive_entry.
Read data associated with the header just read. Internally, this is a convenience function that calls archive_read_data_block() and fills any gaps with nulls so that callers see a single continuous stream of data.
Return the next available block of data for this entry. Unlike archive_read_data(), the archive_read_data_block() function avoids copying data and allows you to correctly handle sparse files, as supported by some archive formats. The library guarantees that offsets will increase and that blocks will not overlap. Note that the blocks returned from this function can be much larger than the block size read from disk, due to compression and internal buffer optimizations.
A convenience function that repeatedly calls archive_read_data_block() to skip all of the data for this archive entry.
This function is deprecated and will be removed. Use archive_read_data() instead.
A convenience function that repeatedly calls archive_read_data_block() to copy the entire entry to the provided file descriptor.
archive_read_extract(), archive_read_extract_set_skip_file()
A convenience function that wraps the corresponding archive_write_disk(3) interfaces. The first call to archive_read_extract() creates a restore object using archive_write_disk_new(3) and archive_write_disk_set_standard_lookup(3), then transparently invokes archive_write_disk_set_options(3), archive_write_header(3), archive_write_data(3), and archive_write_finish_entry(3) to create the entry on disk and copy data into it. The flags argument is passed unmodified to archive_write_disk_set_options(3).
This is another version of archive_read_extract() that allows you to provide your own restore object. In particular, this allows you to override the standard lookup functions using archive_write_disk_set_group_lookup(3), and archive_write_disk_set_user_lookup(3). Note that archive_read_extract2() does not accept a flags argument; you should use archive_write_disk_set_options() to set the restore options yourself.
Sets a pointer to a user-defined callback that can be used for updating progress displays during extraction. The progress function will be invoked during the extraction of large regular files. The progress function will be invoked with the pointer provided to this call. Generally, the data pointed to should include a reference to the archive object and the archive_entry object so that various statistics can be retrieved for the progress display.
Complete the archive and invoke the close callback.
Invokes archive_read_close() if it was not invoked manually, then release all resources. Note: In libarchive 1.x, this function was declared to return void, which made it impossible to detect certain errors when archive_read_close() was invoked implicitly from this function. The declaration is corrected beginning with libarchive 2.0.

Note that the library determines most of the relevant information about the archive by inspection. In particular, it automatically detects gzip(1) or bzip2(1) compression and transparently performs the appropriate decompression. It also automatically detects the archive format.

A complete description of the struct archive and struct archive_entry objects can be found in the overview manual page for libarchive(3).


The callback functions must match the following prototypes:
  • typedef ssize_t archive_read_callback(struct archive *, void *client_data, const void **buffer);
  • typedef int archive_skip_callback(struct archive *, void *client_data, size_t request);
  • typedef int archive_open_callback(struct archive *, void *client_data)
  • typedef int archive_close_callback(struct archive *, void *client_data)

The open callback is invoked by archive_open(). It should return ARCHIVE_OK if the underlying file or data source is successfully opened. If the open fails, it should call archive_set_error() to register an error code and message and return ARCHIVE_FATAL.

The read callback is invoked whenever the library requires raw bytes from the archive. The read callback should read data into a buffer, set the const void **buffer argument to point to the available data, and return a count of the number of bytes available. The library will invoke the read callback again only after it has consumed this data. The library imposes no constraints on the size of the data blocks returned. On end-of-file, the read callback should return zero. On error, the read callback should invoke archive_set_error() to register an error code and message and return -1.

The skip callback is invoked when the library wants to ignore a block of data. The return value is the number of bytes actually skipped, which may differ from the request. If the callback cannot skip data, it should return zero. If the skip callback is not provided (the function pointer is NULL ), the library will invoke the read function instead and simply discard the result. A skip callback can provide significant performance gains when reading uncompressed archives from slow disk drives or other media that can skip quickly.

The close callback is invoked by archive_close when the archive processing is complete. The callback should return ARCHIVE_OK on success. On failure, the callback should invoke archive_set_error() to register an error code and message and return ARCHIVE_FATAL.


The following illustrates basic usage of the library. In this example, the callback functions are simply wrappers around the standard open(2), read(2), and close(2) system calls.

list_archive(const char *name) 
  struct mydata *mydata; 
  struct archive *a; 
  struct archive_entry *entry; 
  mydata = malloc(sizeof(struct mydata)); 
  a = archive_read_new(); 
  mydata->name = name; 
  archive_read_open(a, mydata, myopen, myread, myclose); 
  while (archive_read_next_header(a, &entry) == ARCHIVE_OK) { 
myread(struct archive *a, void *client_data, const void **buff) 
  struct mydata *mydata = client_data; 
  *buff = mydata->buff; 
  return (read(mydata->fd, mydata->buff, 10240)); 
myopen(struct archive *a, void *client_data) 
  struct mydata *mydata = client_data; 
  mydata->fd = open(mydata->name, O_RDONLY); 
  return (mydata->fd >= 0 ? ARCHIVE_OK : ARCHIVE_FATAL); 
myclose(struct archive *a, void *client_data) 
  struct mydata *mydata = client_data; 
  if (mydata->fd > 0) 
  return (ARCHIVE_OK); 


Most functions return zero on success, non-zero on error. The possible return codes include: ARCHIVE_OK (the operation succeeded), ARCHIVE_WARN (the operation succeeded but a non-critical error was encountered), ARCHIVE_EOF (end-of-archive was encountered), ARCHIVE_RETRY (the operation failed but can be retried), and ARCHIVE_FATAL (there was a fatal error; the archive should be closed immediately). Detailed error codes and textual descriptions are available from the archive_errno() and archive_error_string() functions.

archive_read_new() returns a pointer to a freshly allocated struct archive object. It returns NULL on error.

archive_read_data() returns a count of bytes actually read or zero at the end of the entry. On error, a value of ARCHIVE_FATAL, ARCHIVE_WARN, or ARCHIVE_RETRY is returned and an error code and textual description can be retrieved from the archive_errno() and archive_error_string() functions.

The library expects the client callbacks to behave similarly. If there is an error, you can use archive_set_error() to set an appropriate error code and description, then return one of the non-zero values above. (Note that the value eventually returned to the client may not be the same; many errors that are not critical at the level of basic I/O can prevent the archive from being properly read, thus most I/O errors eventually cause ARCHIVE_FATAL to be returned.)


The libarchive library first appeared in FreeBSD 5.3.


The libarchive library was written by Tim Kientzle <>.


Many traditional archiver programs treat empty files as valid empty archives. For example, many implementations of tar(1) allow you to append entries to an empty file. Of course, it is impossible to determine the format of an empty file by inspecting the contents, so this library treats empty files as having a special “empty” format.
April 13, 2009 NetBSD 7.0