Beginner allocator questions

Help
1

Hi all.

I’m trying some simple examples to try get a better grasp on allocators.
In my code below, I’m trying to read a file line by line and return a slice of strings.
2 things are not clear to me:
1: my ArrayList always reports that it’s leaking memory
2: Is there a better way to free the items of the ArrayList with a defer instead of a for loop

fn readInput(in_file: std.fs.File, allocator: Allocator) ![][]u8 {
    const reader = in_file.reader();
    var list = std.ArrayList([]u8).init(allocator);
    defer list.deinit();
    const line = try reader.readUntilDelimiterOrEofAlloc(allocator, '\n', 256);
    try list.append(line.?);
    return try list.toOwnedSlice();
}

pub fn main() !void {
    const in_file = try std.fs.cwd().openFile("input.txt", .{});
    defer in_file.close();
    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
    defer _ = gpa.deinit();
    const allocator = gpa.allocator();

    const strings = try readInput(in_file, allocator);
    std.debug.print("Length of strings is {d}\n", .{strings.len});
    for (strings) |s| {
        allocator.free(s);
    }
}

(Yes, I’m only reading a single line from the file at the moment, will extend as soon as I can understand the problems I’m having)

Does anyone have some tips for me on what I’m doing wrong?
Thanks in advance.

2

The leak is because you’re not freeing the strings slice itself. defer and errdefer can use blocks, so I’d do:

// in readInput
    var list = std.ArrayList([]u8).init(allocator);
    errdefer {
        for (list.items) |s| {
            allocator.free(s);
        }
        list.deinit();
    }

// in main
    const strings = try readInput(in_file, allocator);
    defer {
        for (strings) |s| {
            allocator.free(s);
        }
        allocator.free(strings);
    }
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