Rust: How to fix 'Could not find openssl via pkg-config:'

Problem

While compiling your Rust project, you see an error block like this:

example.sh
error: failed to run custom build command for `openssl-sys v0.9.111`

Caused by:
  process didn't exit successfully: `/home/uli/NoxecoLog/LogMerger/target/release/build/openssl-sys-33138a8caaaab305/build-script-main` (exit status: 101)
  --- stdout
  cargo:rustc-check-cfg=cfg(osslconf, values("OPENSSL_NO_OCB", "OPENSSL_NO_SM4", "OPENSSL_NO_SEED", "OPENSSL_NO_CHACHA", "OPENSSL_NO_CAST", "OPENSSL_NO_IDEA", "OPENSSL_NO_CAMELLIA", "OPENSSL_NO_RC4", "OPENSSL_NO_BF", "OPENSSL_NO_PSK", "OPENSSL_NO_DEPRECATED_3_0", "OPENSSL_NO_SCRYPT", "OPENSSL_NO_SM3", "OPENSSL_NO_RMD160", "OPENSSL_NO_EC2M", "OPENSSL_NO_OCSP", "OPENSSL_NO_CMS", "OPENSSL_NO_COMP", "OPENSSL_NO_SOCK", "OPENSSL_NO_STDIO", "OPENSSL_NO_EC", "OPENSSL_NO_SSL3_METHOD", "OPENSSL_NO_KRB5", "OPENSSL_NO_TLSEXT", "OPENSSL_NO_SRP", "OPENSSL_NO_SRTP", "OPENSSL_NO_RFC3779", "OPENSSL_NO_SHA", "OPENSSL_NO_NEXTPROTONEG", "OPENSSL_NO_ENGINE", "OPENSSL_NO_BUF_FREELISTS", "OPENSSL_NO_RC2"))

  [...]

  Could not find openssl via pkg-config:

  pkg-config exited with status code 1
  > PKG_CONFIG_ALLOW_SYSTEM_CFLAGS=1 pkg-config --libs --cflags openssl

  The system library `openssl` required by crate `openssl-sys` was not found.
  The file `openssl.pc` needs to be installed and the PKG_CONFIG_PATH environment variable must contain its parent directory.
  The PKG_CONFIG_PATH environment variable is not set.

  HINT: if you have installed the library, try setting PKG_CONFIG_PATH to the directory containing `openssl.pc`.


  cargo:warning=Could not find directory of OpenSSL installation, and this `-sys` crate cannot proceed without this knowledge. If OpenSSL is installed and this crate had trouble finding it,  you can set the `OPENSSL_DIR` environment variable for the compilation process. See stderr section below for further information.

  --- stderr


  Could not find directory of OpenSSL installation, and this `-sys` crate cannot
  proceed without this knowledge. If OpenSSL is installed and this crate had
  trouble finding it,  you can set the `OPENSSL_DIR` environment variable for the
  compilation process.

  Make sure you also have the development packages of openssl installed.
  For example, `libssl-dev` on Ubuntu or `openssl-devel` on Fedora.

  If you're in a situation where you think the directory *should* be found
  automatically, please open a bug at https://github.com/rust-openssl/rust-openssl
  and include information about your system as well as this message.

  $HOST = x86_64-unknown-linux-gnu
  $TARGET = x86_64-unknown-linux-gnu
  openssl-sys = 0.9.111

Solution

This error occurs just because the OpenSSL library and its development files are not installed on your system. To fix this, you need to install the OpenSSL development package. On Ubuntu-like systems:

example.sh
sudo apt-get install libssl-dev

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