Question

I’ve been pouring over the format description and source code for the 7z archive format, but I’m still having trouble writing a valid container. I assume I can create an empty container… anyway here’s my start:

std::ofstream ofs(archivename.c_str(), std::ios::binary|std::ios::trunc);  Byte signature[6] = {'7', 'z', 0xBC, 0xAF, 0x27, 0x1C}; Byte major = 0; Byte minor = 3;  ofs.write((const char*)signature, 6); ofs.write((const char*)major, 1); ofs.write((const char*)minor, 1);  UInt64 offset = 0; UInt64 size = 0; UInt32 crc = 0;  ofs.write((const char*)offset, 4); ofs.write((const char*)size, 8); ofs.write((const char*)crc, 8); ofs.write((const char*)CrcCalc(0, 0), 8);  ofs.close(); 

I think my main problem is a lack of understanding of std::ofstream::write(). Byte is an ‘unsigned char’, UInt64 & UInt32 are both ‘unsigned long’.

UPDATE0: As everyone points out, it’d be a problem if I ran this on a big-endian machine. That’s not the case here. Per Fredrik Janssen, I should be casting the address of the non-arrays. I should also mention that CrcCalc() is a function in the LZMA SDK. Adding & helps a bit, it’s that first unsigned char[6] that’s having some problems.

UPDATE1: Working code to get an empty archive file below.

static void SetUInt32(Byte *p, UInt32 d) {   for (int i = 0; i < 4; i++, d >>= 8)     p[i] = (Byte)d; }  static void SetUInt64(Byte *p, UInt64 d) {   for (int i = 0; i < 8; i++, d >>= 8)     p[i] = (Byte)d; }  void make_7z_archive() {   CrcGenerateTable();    std::ofstream ofs(archivename.c_str(), std::ios::binary|std::ios::trunc);    Byte signature[6] = {'7', 'z', 0xBC, 0xAF, 0x27, 0x1C};   Byte major = 0;   Byte minor = 3;    ofs.write((const char*)signature, 6);   ofs.write((const char*)&major, 1);   ofs.write((const char*)&minor, 1);    UInt64 offset = 0;   UInt64 size = 0;   UInt32 crc = 0;    Byte buf[24];   SetUInt64(buf + 4, offset);   SetUInt64(buf + 12, size);   SetUInt32(buf + 20, crc);   SetUInt32(buf, CrcCalc(buf + 4, 20));    ofs.write((const char*)buf, 24);    ofs.close(); } 

NOTE: CrcGenerateTable() and CrcCalc() are from the LZMA SDK.

Answer 1

don’t know the format of 7z, but I notice when you write down offset, size and crc that these will be written to the file in little-endian format (I assume you have a little-endian CPU).

Edit: An probably worse, you are missing the & before major, minor, offset, size and crc, i.e. you are casting the actual values to a pointer.