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archive_write(3) FreeBSD Library Functions Manual archive_write(3) NAME archive_write_new, archive_write_set_format_cpio, archive_write_set_format_pax, archive_write_set_format_pax_restricted, archive_write_set_format_shar, archive_write_set_format_shar_binary, archive_write_set_format_ustar, archive_write_set_bytes_per_block, archive_write_set_bytes_in_last_block, archive_write_set_compressor_gzip, archive_write_set_compressor_bzip2, archive_write_open, archive_write_open_fd, archive_write_open_file, archive_write_prepare, archive_write_header, archive_write_data, archive_write_close, archive_write_finish -- functions for creating archives SYNOPSIS #include <archive.h> struct archive * archive_write_new(void); int archive_write_set_bytes_per_block(archive *, int bytes_per_block); int archive_write_set_bytes_in_last_block(archive *, int); int archive_write_set_compressor_gzip(struct archive *); int archive_write_set_compressor_bzip2(struct archive *); int archive_write_set_format_cpio(struct archive *); int archive_write_set_format_pax(struct archive *); int archive_write_set_format_pax_restricted(struct archive *); int archive_write_set_format_shar(struct archive *); int archive_write_set_format_shar_binary(struct archive *); int archive_write_set_format_ustar(struct archive *); int archive_write_open(struct archive *, void *client_data, archive_write_archive_callback *, archive_open_archive_callback *, archive_close_archive_callback *); int archive_write_open_fd(struct archive *, int fd); int archive_write_open_file(struct archive *, const char *filename); int archive_write_header(struct archive *); int archive_write_data(struct archive *, const void *, size_t); int archive_write_close(struct archive *); void archive_write_finish(struct archive *); DESCRIPTION These functions provide a complete API for creating streaming archive files. The general process is to first create the struct archive object, set any desired options, initialize the archive, append entries, then close the archive and release all resources. The following summary describes the functions in approximately the order they are ordinarily used: archive_write_new() Allocates and initializes a struct archive object suitable for writing a tar archive. archive_write_set_bytes_per_block() Sets the block size used for writing the archive data. Every call to the write callback function, except possibly the last one, will use this value for the length. The third parameter is a boolean that specifies whether or not the final block written will be padded to the full block size. If it is zero, the last block will not be padded. If it is non-zero, padding will be added both before and after compression. The default is to use a block size of 10240 bytes and to pad the last block. archive_write_set_bytes_in_last_block() Sets the block size used for writing the last block. If this value is zero, the last block will be padded to the same size as the other blocks. Otherwise, the final block will be padded to a multiple of this size. In particular, setting it to 1 will cause the final block to not be padded. For compressed output, any padding generated by this option is applied only after the com- pression. The uncompressed data is always unpadded. The default is to pad the last block to the full block size (note that archive_write_open_file() will set this based on the file type). Unlike the other ``set'' functions, this function can be called after the archive is opened. archive_write_set_format_cpio(), archive_write_set_format_pax(), archive_write_set_format_pax_restricted(), archive_write_set_format_shar(), archive_write_set_format_shar_binary(), archive_write_set_format_ustar() Sets the format that will be used for the archive. The library can write POSIX octet-oriented cpio format archives, POSIX-stan- dard ``pax interchange'' format archives, traditional ``shar'' archives, enhanced ``binary'' shar archives that store a variety of file attributes and handle binary files, and POSIX-standard ``ustar'' archives. The pax interchange format is a backwards- compatible tar format that adds key/value attributes to each entry and supports arbitrary filenames, linknames, uids, sizes, etc. ``Restricted pax interchange format'' is the library default; this is the same as pax format, but suppresses the pax extended header for most normal files. In most cases, this will result in ordinary ustar archives. archive_write_set_compression_gzip(), archive_write_set_compression_bzip2() The resulting archive will be compressed as specified. Note that the compressed output is always properly blocked. archive_write_open() Freeze the settings, open the archive, and prepare for writing entries. This is the most generic form of this function, which accepts pointers to three callback functions which will be invoked by the compression layer to write the constructed ar- chive. In order to support external compression programs, the compression is permitted to fork and invoke the callbacks from a separate process. In particular, clients should not assume that they can communicate between the callbacks and the mainline code using shared variables. (The standard gzip, bzip2, and "none" compression methods do not fork.) archive_write_open_fd() A convenience form of archive_write_open() that accepts a file descriptor. archive_write_open_file() A convenience form of archive_write_open() that accepts a file- name. A NULL argument indicates that the output should be writ- ten to standard output; an argument of ``-'' will open a file with that name. If you have not invoked archive_write_set_bytes_in_last_block(), then archive_write_open_file() will adjust the last-block padding depending on the file: it will enable padding when writing to standard output or to a character or block device node, it will disable padding otherwise. You can override this by manually invoking archive_write_set_bytes_in_last_block() either before or after calling archive_write_open(). archive_write_header() Build and write a header using the data in the provided struct archive_entry structure. archive_write_data() Write data corresponding to the header just written. archive_write_close() Complete the archive and invoke the close callback. archive_write_finish() Invokes archive_write_close() if it wasn't invoked manually, then release all resources. The callback functions are defined as follows: typedef ssize_t archive_write_archive_callback(struct archive *, void *client_data, void *buffer, size_t length) typedef int archive_open_archive_callback(struct archive *, void *client_data) typedef int archive_close_archive_callback(struct archive *, void *client_data) For correct blocking, each call to the write callback function should translate into a single write(2) system call. This is especially criti- cal when writing tar archives to tape drives. More information about tar archive formats and blocking can be found in the tar(5) manual page. More information about the struct archive object and the overall design of the library can be found in the libarchive(3) overview. IMPLEMENTATION Compression support is built-in to libarchive, which uses zlib and bzlib to handle gzip and bzip2 compression, respectively. EXAMPLE The following sketch illustrates basic usage of the library. In this example, the callback functions are simply wrappers around the standard open(2), write(2), and close(2) system calls. void write_archive(const char **filename) { struct mydata *mydata = malloc(sizeof(struct mydata)); struct archive *a; struct archive_entry *entry; struct stat st; char buff[8192]; int len; a = archive_write_new(); mydata->name = name; archive_write_set_compression_gzip(a); archive_write_set_format_ustar(a); archive_write_open(a, mydata, myopen, mywrite, myclose); while (*filename) { stat(*filename, &st); entry = archive_entry_new(); archive_entry_copy_stat(entry, &st); archive_entry_set_pathname(entry, *filename); archive_write_header(a, entry); fd = open(*filename, O_RDONLY); len = read(fd, buff, sizeof(buff)); while ( len >= 0 ) { archive_write_data(a, buff, len); len = read(fd, buff, sizeof(buff)); } archive_entry_free(entry); filename++; } archive_write_finish(a); } int myopen(struct archive *a, void *client_data) { struct mydata *mydata = client_data; mydata->fd = open(mydata->name, O_WRONLY | O_CREAT, 0644); return (mydata->fd >= 0); } ssize_t mywrite(struct archive *a, void *client_data, void *buff, size_t n) { struct mydata *mydata = client_data; return (write(mydata->fd, buff, n)); } int myclose(struct archive *a, void *client_data) { struct mydata *mydata = client_data; if (mydata->fd > 0) close(mydata->fd); return (0); } RETURN VALUES Most functions return zero on success, non-zero on error. The archive_errno() and archive_error_string() functions can be used to retrieve an appropriate error code and a textual error message. archive_write_new() returns a pointer to a newly-allocated struct archive object. archive_write_data() returns a count of the number of bytes actually written. On error, -1 is returned and the archive_errno() and archive_error_string() functions will return appropriate values. Note that if the client-provided write callback function returns -1, that error will be propagated back to the caller through whatever API function resulted in that call, which may include archive_write_header(), archive_write_data(), or archive_write_close(). In such a case, the archive_errno() or archive_error_string() fields will not return useful information; you should use client-private data to return error informa- tion back to your mainline code. SEE ALSO tar(1), libarchive(3), tar(5) HISTORY The libarchive library first appeared in FreeBSD 5.3. AUTHORS The libarchive library was written by Tim Kientzle <kientzle@acm.org>. BUGS There are many peculiar bugs in historic tar implementations that may cause certain programs to reject archives written by this library. For example, several historic implementations calculated header checksums incorrectly and will thus reject valid archives; GNU tar does not fully support pax interchange format; some old tar implementations required specific field terminations. The default pax interchange format eliminates most of the historic tar limitations and provides a generic key/value attribute facility for ven- dor-defined extensions. One oversight in POSIX is the failure to provide a standard attribute for large device numbers. This library uses ``SCHILY.devminor'' and ``SCHILY.devmajor'' for device numbers that exceed the range supported by the backwards-compatible ustar header. These keys are compatible with Joerg Schilling's star archiver. Other implementations may not recognize these keys and will thus be unable to correctly restore large device numbers archived by this library. FreeBSD 11.1 October 1, 2003 FreeBSD 11.1
NAME | SYNOPSIS | DESCRIPTION | IMPLEMENTATION | EXAMPLE | RETURN VALUES | SEE ALSO | HISTORY | AUTHORS | BUGS
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