How to Get the Architecture of a '.A' File

How can I get the architecture of a '.a' file?

You can also skip the ar command and use readelf, via something like:

readelf -h <archive>.a | grep 'Class\|File\|Machine'

[00:32:15] /usr/lib $ readelf -h libxslt.a | grep 'Class\|File\|Machine'
File: libxslt.a(attrvt.o)
  Class:                             ELF32
  Machine:                           Intel 80386
File: libxslt.a(xslt.o)
  Class:                             ELF32
  Machine:                           Intel 80386
... #Trimmed this, it goes on a bit
File: libxslt.a(transform.o)
  Class:                             ELF32
  Machine:                           Intel 80386
File: libxslt.a(security.o)
  Class:                             ELF32
  Machine:                           Intel 80386
[00:32:24] /usr/lib $

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In case it's relevant, here's the other information that you can get from readelf -h. I just trimmed the above with grep, obviously:

File: libxslt.a(security.o)
ELF Header:
  Magic:   7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00 
  Class:                             ELF32
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              REL (Relocatable file)
  Machine:                           Intel 80386
  Version:                           0x1
  Entry point address:               0x0
  Start of program headers:          0 (bytes into file)
  Start of section headers:          2548 (bytes into file)
  Flags:                             0x0
  Size of this header:               52 (bytes)
  Size of program headers:           0 (bytes)
  Number of program headers:         0
  Size of section headers:           40 (bytes)
  Number of section headers:         16
  Section header string table index: 13

That output is for one of the object files in libxslt.a, but it gives the same information for each file.

How do I determine the architecture of an executable binary on Windows 10

The architecture of the executable is written in the Machine field of the COFF header. You can retrieve it programatically or manually with a hex editor:

• Go to offset 0x3C in the file. The four bytes there hold the offset of the COFF header (from the beginning of the file).
• Go to the COFF header pointed by the above field, and advance by four (4) bytes.
• The following two (2) bytes are the Machine field.

You can see PE structure here. The valid Machine field values are listed here.

EDIT: Here's a C code that does that, untested:

int main(int argc, char *argv[]) {
    FILE *f = fopen(argv[1], "rb");
    uint32_t offset = 0;
    fseek(f, 0x3c, SEEK_SET);
    fread(&offset, sizeof(offset), 1, f);
    fseek(f, offset + 4, SEEK_SET);
    uint16_t machine = 0;
    fread(&machine, sizeof(machine), 1, f);
    printf("Machine: 0x%.4x\n", machine);
}

How can I know which architecture an *.a file is built for?

You can use otool to find out the architecture(s) of a file: otool man page

otool -hv mylibrary.a

Determining the CPU architecture of a static library (LIB) on Windows

Use dumpbin /headers

The machine type is almost the first line you'll get.

It will be 14c for x86 and 8664 for x64

n:>dumpbin lib642.lib /headers

Microsoft (R) COFF/PE Dumper Version

10.00.30319.01 Copyright (C) Microsoft Corporation. All rights reserved.

Dump of file lib642.lib

File Type: LIBRARY

FILE HEADER VALUES
8664 machine (x64

Or

n:>dumpbin Lib32.lib /headers

Microsoft (R) COFF/PE Dumper Version

10.00.30319.01 Copyright (C) Microsoft Corporation. All rights reserved.

Dump of file Lib32.lib

File Type: LIBRARY

FILE HEADER VALUES
14C machine (x86)

Where should I put uploaded files on a Clean Architecture project?

I believe the best place to put an upload folder is in the project root:

 - src
   - <project files> 
 - uploads # put your files here

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