tarw.h

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#ifndef TARW_H_
#define TARW_H_

#include <stdio.h>
#include <stdint.h>
#include <stdbool.h>
#include <string.h>
#include <assert.h>

#ifdef __cplusplus
extern "C" {
#endif // __cplusplus

typedef struct {
  FILE *sink;

  const char *owner;
  const char *group;
  size_t owner_id;
  size_t group_id;
} TarWriter;

bool tarw_write_directory(TarWriter w, const char *path, uint16_t mode);
bool tarw_write_file(TarWriter w, const char *path, uint16_t mode,
                                  const char *buff, size_t buff_size);

#define TARW_CHUNK_SIZE 512

/* UStar TAR file header */
typedef struct {
  char file_path[100];

  // all encoded as null-terminated octal strings
  char file_mode[8];
  char owner_id[8];
  char group_id[8];
  char file_size[12];
  char last_modification_time[12]; // seconds since unix epoch

  char    checksum[8]; // null-terminated octal string
  uint8_t type_flag;

  char name_of_linked_file[100];

  char ustarw_indicator[6];
  char ustarw_version[2];

  char owner_name[32];
  char group_name[32];

  char device_major_number[8];
  char device_minor_number[8];
  char file_name_prefix[155];

  char padding[12]; // padding to make the header 512 bytes
} TarHeader;

static_assert(sizeof(TarHeader) == TARW_CHUNK_SIZE,
              "TarHeader struct has the wrong size");

enum {
  TARW_NORMAL           = '0',
  TARW_HARD_LINK        = '1', // TODO: unsupported
  TARW_SYMLINK          = '2', // TODO: unsupported
  TARW_CHARACTER_DEVIDE = '3', // TODO: unsupported
  TARW_BLOCK_DEVICE     = '4', // TODO: unsupported
  TARW_DIRECTORY        = '5',
  TARW_PIPE             = '6', // TODO: unsupported
};

#ifdef TARW_IMPLEMENTATION
#define TARW_USTARW_INDICATOR "ustar"

void tarw_header_set_checksum(TarHeader *h)
{
  uint8_t *data = (uint8_t*)h;

  size_t checksum = 0;
  // the checksum field should be set to "  ...  " for computing the checksum
  memset(&h->checksum, ' ', sizeof(h->checksum));
  for (size_t i = 0; i < sizeof(*h); i++) checksum += (size_t)data[i];

  snprintf((char*)&h->checksum, sizeof(h->checksum), "%zo", checksum);
}

TarHeader tarw_make_header(const char *path,  uint16_t mode,
                           const char *owner, const char *group,
                           size_t owner_id,   size_t group_id,
                           uint8_t type_flag, size_t file_size)
{
  TarHeader h = {0};

  snprintf((char*)&h.file_path, sizeof(h.file_path), "%s",     path);
  snprintf((char*)&h.file_mode, sizeof(h.file_mode), "%07o",   mode & 0x7fff);
  snprintf((char*)&h.owner_id,  sizeof(h.owner_id),  "%07zo",  owner_id);
  snprintf((char*)&h.group_id,  sizeof(h.group_id),  "%07zo",  group_id);
  snprintf((char*)&h.file_size, sizeof(h.file_size), "%011zo", file_size);
  // set h.last_modification_time to the UNIX epoch
  memset((char*)&h.last_modification_time, '0',
         sizeof(h.last_modification_time)-1);

  h.type_flag = type_flag;

  snprintf((char*)&h.ustarw_indicator, sizeof(h.ustarw_indicator), TARW_USTARW_INDICATOR);

  snprintf((char*)&h.owner_name, sizeof(h.owner_name), "%s", owner);
  snprintf((char*)&h.group_name, sizeof(h.group_name), "%s", group);
  tarw_header_set_checksum(&h);

  return h;
}

bool tarw_write_directory(TarWriter w, const char *path, uint16_t mode)
{
  TarHeader h = tarw_make_header(path, mode,
                                 w.owner, w.group, w.owner_id, w.group_id,
                                 TARW_DIRECTORY, 0);

  if (fwrite((char*)&h, 1, sizeof(h), w.sink) != sizeof(h)) return false;
  return true;
}

bool tarw_write_file(TarWriter w, const char *path, uint16_t mode,
                                  const char *buff, size_t buff_size)
{
  TarHeader h = tarw_make_header(path, mode,
                                 w.owner, w.group, w.owner_id, w.group_id,
                                 TARW_NORMAL, buff_size);

  if (fwrite((char*)&h, 1, sizeof(h), w.sink) != sizeof(h)) return false;
  if (fwrite(buff, 1, buff_size, w.sink) != buff_size) return false;

  static const char zeros[TARW_CHUNK_SIZE] = {0};
  size_t chunks_ceil = (buff_size + (TARW_CHUNK_SIZE-1))/TARW_CHUNK_SIZE;
  size_t padding = chunks_ceil*TARW_CHUNK_SIZE - buff_size;
  if (fwrite(zeros, 1, padding, w.sink) != padding) return false;
  return true;
}

#endif // TARW_IMPLEMENTATION
#ifdef __cplusplus
}
#endif // __cplusplus
#endif // TARW_H_