lib/nolibc: Add README.md

Add a brief description of nolibc, providing an overview
of its purpose, usage and limitations.

This commit also shows a relevant example of how to configure
an application to use Unikraft-specific assertions or the ones
implemented in nolibc.

Signed-off-by: Alexandru Calciu <alexandru.calciu@stud.acs.upb.ro>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Razvan Deaconescu <razvand@unikraft.io>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #778

diff --git a/lib/nolibc/README.md b/lib/nolibc/README.md
new file mode 100644 (file)
index 0000000..4c18223
--- /dev/null
+++ b/lib/nolibc/README.md
@@ -0,0 +1,70 @@
+# nolibc, Unikraft's Lightweight Subset of the C Standard Library
+
+The `nolibc` library provides the basic core functionalities of libc in a lighter and more compact package, fulfilling the needs of most simple applications without occupying a large amount of memory, while also remaining POSIX-compliant.
+Complex applications that require a more comprehensive set of standard C functions are better suited with Unikraft's port of [Musl](https://github.com/unikraft/lib-musl).
+
+## Configuring Applications to Use `nolibc`
+
+The `nolibc` library is an internal library of Unikraft and is enabled by default when configuring a new project.
+Opting for a different implementation of the C standard library will automatically override `nolibc` and utilize the one provided by the user.
+This behavior can be observed in the configuration menu under the `Library Configuration` section.
+
+An example of a simple application that uses the `nolibc` library is the following:
+
+```c
+#include <stdio.h>
+#include <string.h>
+
+int main(int argc, char *argv[])
+{
+    char str[1024], *p;
+
+    strcpy(str, "There's the way it ought to be");
+    printf("%s\n", str);
+
+    strcat(str, "and there's the way it is");
+    p = strchr(str, 'b');
+    p += 2;
+    fprintf(stderr, "%s\n", p);
+
+    return 0;
+}
+```
+
+The steps for configuring the application to use `nolibc` are as follows:
+
+1. Enter the configuration interface with:
+
+```console
+$ make menuconfig
+```
+
+1. Enter the `Library Configuration` submenu
+1. Select `nolibc: Only necessary subset of libc functionality`
+1. This submenu allows you to choose between using [Unikraft-specific assertions](https://github.com/unikraft/unikraft/blob/staging/lib/ukdebug/include/uk/assert.h) or [`nolibc` assertions](https://github.com/unikraft/unikraft/blob/staging/lib/nolibc/include/assert.h) by toggling the `Implement assertions with libukdebug` option
+
+Building and running the program above results in output similar to the following:
+
+```console
+SeaBIOS (version 1.13.0-1ubuntu1.1)
+Booting from ROM...
+Powered by
+o.   .o       _ _               __ _
+Oo   Oo  ___ (_) | __ __  __ _ ' _) :_
+oO   oO ' _ `| | |/ /  _)' _` | |_|  _)
+oOo oOO| | | | |   (| | | (_) |  _) :_
+ OoOoO ._, ._:_:_,\_._,  .__,_:_, \___)
+             Epimetheus 0.12.0~4c7352c0
+There's the way it ought to be
+and there's the way it is
+Console terminated, terminating guest (PID: 10490)...
+```
+
+## Limitations and Caveats
+
+The small size and simple interface of `nolibc` came at the cost of reducing the scope of some functions.
+Notable limitations include:
+
+-   `fprintf()` being suitable only for `stdout` and `stderr`, not other files or devices
+-   No internal buffering for `stdio` functions
+-   No implementation of `fscanf` and its derivatives (input should be received only from `stdin`)