[apache/incubator-teaclave-sgx-sdk] Intuition behind feature flags (#345)

[clangenb <no...@github.com.INVALID>]

I have a question about the feature flags use in the crates. I just can't understand them.

I thought the below means that we use these dependencies, when we are not in sgx environment. But is this not exactly contrary to what the intuition is?

```
[target.'cfg(not(target_env = "sgx"))'.dependencies]
```

Further, we have this:

```
#![cfg_attr(not(target_env = "sgx"), no_std)]
#![cfg_attr(target_env = "sgx", feature(rustc_private))]
```

If we are not in environment sgx, we are no std. But I thought sgx is exactly the no_std environment. And I don't understand why the line below is the exact opposite.

Last one:

```
#[cfg(not(target_env = "sgx"))]
extern crate sgx_tstd as std;
```
which to me is also: `import extern crate sgx_tstd as std` if we are not in sgx.

I have been using sgx for over a year now and never understood this. I thought it will be clear one day, but the more time passes, the more that the flags are used wrongly here.

So I would finally like to know, do I lack a fundamental understanding here, or are we somewhat working against the cargo system?


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Re: [apache/incubator-teaclave-sgx-sdk] Intuition behind cargo config flags and cargo attributes. (#345)

[clangenb <no...@github.com.INVALID>]

Closed #345.

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Re: [apache/incubator-teaclave-sgx-sdk] Intuition behind cargo config flags and cargo attributes. (#345)

[clangenb <no...@github.com.INVALID>]

Thank you very much. Your explanation and links made it much more clear for me. 👍 

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Re: [apache/incubator-teaclave-sgx-sdk] Intuition behind cargo config flags and cargo attributes. (#345)

[volcano <no...@github.com.INVALID>]

`target_env = "sgx"`
It does not mean that you are currently in the SGX environment.
It is defined in the target specification file of the custom SGX target and is used by xargo to build the sysroot of the SGX target.
[https://github.com/apache/incubator-teaclave-sgx-sdk/blob/master/xargo/x86_64-unknown-linux-sgx.json#L6](url)
[https://github.com/japaric/xargo#compiling-the-sysroot-for-a-custom-target](url)

`#[cfg(not(target_env = "sgx"))]`
Use `cfg` to check if your enclave is being compiled with the sysroot of the SGX target.

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