Manpages - archive_write.3

Streaming Archive Library (libarchive, -larchive)

These functions provide a complete API for creating streaming archive files. The general process is to first create the

object, set any desired options, initialize the archive, append entries, then close the archive and release all resources.

See

To write an archive, you must first obtain an initialized

object from

See

and

You can then modify this object for the desired operations with the various

functions. In particular, you will need to invoke appropriate

and

functions to enable the corresponding compression and format support.

See

See

Once you have prepared the

object, you call

to actually open the archive and prepare it for writing. There are several variants of this function; the most basic expects you to provide pointers to several functions that can provide blocks of bytes from the archive. There are convenience forms that allow you to specify a filename, file descriptor,

object, or a block of memory from which to write the archive data.

See

and

Individual archive entries are written in a three-step process: You first initialize a

structure with information about the new entry. At a minimum, you should set the pathname of the entry and provide a

with a valid

field, which specifies the type of object and

field, which specifies the size of the data portion of the object.

See

After all entries have been written, use the

function to release all resources.

The following sketch illustrates basic usage of the library. In this example, the callback functions are simply wrappers around the standard

and

system calls.

#ifdef linux #define _FILE_OFFSET_BITS 64 #endif #include <sys/stat.h> #include <archive.h> #include <archive_entry.h> #include <fcntl.h> #include <stdlib.h> #include <unistd.h>

struct mydata { const char *name; int fd; };

int myopen(struct archive *a, void *client_data) { struct mydata *mydata = client_data;

mydata->fd = open(mydata->name, O_WRONLY | O_CREAT, 0644); if (mydata->fd >= 0) return (ARCHIVE_OK); else return (ARCHIVE_FATAL); }

la_ssize_t mywrite(struct archive *a, void *client_data, const 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); }

void write_archive(const char *outname, 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; int fd;

a = archive_write_new(); mydata->name = outname; * Set archive format and filter according to output file extension. * If it fails, set default format. Platform depended function. * See supported formats in archive_write_set_format_filter_by_ext.c * if (archive_write_set_format_filter_by_ext(a, outname) != ARCHIVE_OK) { archive_write_add_filter_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); if ((fd = open(*filename, O_RDONLY)) != -1) { len = read(fd, buff, sizeof(buff)); while (len > 0) { archive_write_data(a, buff, len); len = read(fd, buff, sizeof(buff)); } close(fd); } archive_entry_free(entry); filename++; } archive_write_free(a); }

int main(int argc, const char **argv) { const char *outname; argv++; outname = *argv++; write_archive(outname, argv); return 0; }

The

library first appeared in

The

library was written by

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 vendor-defined extensions. One oversight in POSIX is the failure to provide a standard attribute for large device numbers. This library uses

and

for device numbers that exceed the range supported by the backwards-compatible ustar header. These keys are compatible with Joerg Schilling’s

archiver. Other implementations may not recognize these keys and will thus be unable to correctly restore device nodes with large device numbers from archives created by this library.