docs: call_v_from_c example (#15844)

examples/call_v_from_c/.gitignore

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+# Ignore the generated C files in this directory
+*.c

examples/call_v_from_c/README.md

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+## A simple example to show how to call a function written in v from c
+
+### Compile as a shared library
+
+#### On Linux:
+
+Step 1: Compile the v code to a shared library using `v -cc gcc -shared v_test_print.v` or
+`v -cc gcc -shared v_test_math.v`.
+
+Step 2: Compile the c file using `gcc test_print.c v_test_print.so -o test_print` or
+`gcc test_math.c v_test_math.so -o test_math`.
+
+Step 3: Run the compiled c file using `LD_LIBRARY_PATH=. ./test_print` or
+`LD_LIBRARY_PATH=. ./test_math`.
+
+#### On Windows:
+
+Step 1: Compile the v code to a shared library using `v -cc gcc -shared v_test_print.v` or
+`v -cc gcc -shared v_test_math.v`.
+
+Step 2: Compile the c file using `gcc test_print.c v_test_print.dll -o test_print.exe` or
+`gcc test_math.c v_test_math.dll -o test_math.exe`.
+
+Step 3: Run the compiled c file using `test_print.exe` or `test_math.exe`.
+
+### Compile as a c file
+
+***Requirements: `libgc` must be installed***
+
+Step 1: Compile the v code to a shared library using
+`v -shared -cc gcc -o v_test_print.c v_test_print.v` or
+`v -shared -cc gcc -o v_test_math.c v_test_math.v`.
+
+*Specifying the output with a `.c` extension will generate the corresponding C source file.*
+
+Step 2: Compile the c file using `gcc test_print.c v_test_print.c -o test_print -lgc` or
+`gcc test_math.c v_test_math.c -o test_math -lgc -lm`.
+
+Step 3: Run the compiled c file using `./test_print` or `./test_math`.

examples/call_v_from_c/test_math.c

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+#include <stdio.h>
+
+extern int square(int i);
+
+extern double sqrt_of_sum_of_squares(double x, double y);
+
+int main()
+{
+	int i = 10;
+	printf("square(%d) = %d\n", i, square(i));
+	double x=0.9;
+	double y=1.2;
+	printf("sqrt_of_sum_of_squares(%f, %f) = %f\n", x, y, sqrt_of_sum_of_squares(x, y));
+}

examples/call_v_from_c/test_print.c

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+extern void foo(const char* s);
+
+int main()
+{
+	foo("hello");
+	foo("bye");
+}

examples/call_v_from_c/v_test_math.v

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+module test_math
+
+import math
+
+[export: 'square']
+fn calculate_square(i int) int {
+	return i * i
+}
+
+[export: 'sqrt_of_sum_of_squares']
+fn calculate_sqrt_of_sum_of_squares(x f64, y f64) f64 {
+	return math.sqrt(x * x + y * y)
+}

examples/call_v_from_c/v_test_print.v

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+module test_print
+
+[export: 'foo']
+fn show_foo(s &char) {
+	println(unsafe { cstring_to_vstring(s) })
+}