How to get length of C string in Zig

I built a binary dynamic library as zig build-lib -dynamic string_utils.zig for the below code:

const std = @import("std");

pub fn joinStrings(firstname: []const u8, lastname: []const u8, buf: *[]u8) !void {
    var stream = std.io.fixedBufferStream(buf.*);
    var writer = stream.writer();

    try writer.print("Hello, {s} {s}!", .{ firstname, lastname });

    buf.* = stream.getWritten();
}

Then created the string.utls.h file as:

void joinStrings(const char* firstname, const char* lastname, char** buf);

Then created my main.zig file as below:

const std = @import("std");
const c = @cImport({
    @cInclude("string_ptr.h");
});

pub fn main() !void {
    var buf: [*c]u8 = undefined;
    c.joinStrings("Hasan", "Yousef", &buf);
    const len = // how to read the length of the buf for memory allocation //
    const greetings = std.mem.sliceFromRaw(u8, buf[0..len]);
    std.debug.print("Saved string: {s}\n", .{greetings});
}

1. How can I get the size required for memory allocation?
2. Is my approach in creating this lib and calling it correct, or wromg, or can be enhanced?

To get the length of a C string, you can use the std.mem.len function.