determine whether a given archive is solid

What
Is it possible to check if an archive is solid without reading all of it?

Why
We are using the 7z_C sources, to load a sort of extensions for our software. these are packaged with LZMA. each extension has a special file with meta info + some MB of data and code. we often have to make an index of all extensions, and so have to scan all the available extension archives, looking for the meta-info file. there might be hundreds of extensions present. to keep the scan fast, we want to enforce non-solid archives by showing an error if an extension is packed as solid archive.

You can check number of "folders" - number of solid blocks.

I did some testing, but i do not understand how i could find out whether an archive is solid or not, using hte properties you suggested (hope i got it right).

using this stuct from 7z.h:

typedef struct
{
  UInt64 *PackSizes;
  Byte *PackCRCsDefined;
  UInt32 *PackCRCs;
  CSzFolder *Folders;
  CSzFileItem *Files;
  UInt32 NumPackStreams;
  UInt32 NumFolders;
  UInt32 NumFiles;
} CSzAr;

i get these results for 2 small test content packs:

archiveA_solid.sd7 ("-ms=on")
NumPackStreams: 1
NumFolders:     1
NumFiles:       9

archiveA_non_solid.sd7 ("-ms=off")
NumPackStreams: 3
NumFolders:     3
NumFiles:       9

archiveB_solid.sd7 ("-ms=on")
NumPackStreams: 2
NumFolders:     2
NumFiles:       15

archiveB_non_solid.sd7 ("-ms=off")
NumPackStreams: 8
NumFolders:     8
NumFiles:       15

I see that the solid archives have less NumPackStreams/NumFolders then the non-solid ones. As in practise, i will only have one of the two, i can not see how this woudl help me to evaluate whether an archive is solid or not.
Did i do it wrong? :D

Side-Note:
It seems like solid is the default for "7z -t7z", which is quite unintuitive, as the documentation only tells you how to enable solid ("-ms=on"), which suggests that off is the default. I may be wrong though; i just saw that "-ms=on" and "" spit out the same values for NumPackStreams/NumFolders/NumFiles, while "-ms=off" gave different results. Might be because i had quite small test archives.

… as an alternative solution, i saw that partly solid archives are possible (eg, blocks or 512KB solid data).
when archiving, is it possible to have a filter for files to be excluded from solid, or be placed in a separate solid block, and have all the rest in one big solid block?
that would actually be an even better solution then to disallow solid archives.

1) at decompression time:
You find meta-info file, detect what folder it's placed, check size of that folder, and if it's small (< 500 KB), you decompress it.
2) at compression time you can use 2 commands:
7z a a.7z meta-info
7z a a.7z other-files
So meta-info file will be in separate solid block.

There is a property named kpidSolid (PropID.h), you can query it with IInArchive::GetArchiveProperty

thanks you two! :-)
We will use the two things explained by ipavlov, as they are more flexible and suite us better, and cause the kpidSolid (using CArchiveDatabase::IsSolid() internally), is in the C++ lib only, and we use the C one.

The current (beta) rule we use to check if an extension is well packed, is checking the cost of extracting a meta-file:

bool costTooHigh = (to-be-unpacked-data-size > 32KB) && (to-be-unpacked-data-size > file-size);

this should work well for non-solid archives, tiny meta-files in small solid blocks, and big meta-files in a separate solid block.

thanks again… i quite like this solution :-)