Weird problems with Zig Array List(s)

Question

In the below code, I have the function "get_specific_files". The objective of this function is to check if certain filenames contain some word, and then return all the files that contain that word in a list of strings (hopefully some kind of ArrayList of type []const u8, which will get converted to a slice).

However, if I stick anything whatsoever in that for loop other than just a simple function call, or a print("{s}\n", .{elems});, the data goes completely buggy.

For example, if I have the following code where the for loop is in the "get_specific_files" function, the data is printed out completely wrong:

var count: u8 = 0;

for (input) |elems| {
    print("{d}: {s}\n", .{count, elems});
    count += 1;
}

The result is something along the lines of:

1: file1.txt
2: file2.txt
3: weird unknown symbols
(repeat for however long the list is)

I am quite new to Zig, but I have been using C for a little while, so I am not new to pointers. I am confused about Zig pointers and allocation, so maybe that's the problem? But in the above code segment, if I just remove the count, everything prints correctly??

(The program thus far)

pub fn main() !void {
    var retval = try get_filenames();
    retval = try get_specific_files(retval);
}

fn get_specific_files(input: [][]const u8) ![][]const u8 {
    try out.print("Finding specific files . . .\n", .{});

    for (input) |elems| {

        if (std.mem.indexOf(u8, elems, "some_word") != null) {
            try foo(elems);
        }
    }
    return input;
}
 

fn get_filenames() ![][]const u8 {
    try out.print("Finding files . . .\n", .{});
    var dir = try std.fs.cwd().openDir("some/dir/foo/", .{.iterate = true});
    defer dir.close();

    var file_list = std.ArrayList([]const u8).init(allocator);

    var dir_iterator = dir.iterate();
    while (try dir_iterator.next()) |dir_content| {
        try file_list.append(dir_content.name);
    }

    const owned_slice = try file_list.toOwnedSlice();
    return owned_slice;
}

I was expecting the program to spit out the right files, but the unknown characters and everything is what actually happened.

sigod · Accepted Answer · 2025-08-07 23:58:27Z

2

try file_list.append(dir_content.name);

You do not own the memory that dir_content.name points to. You're saving a pointer to the iterator's internal memory. This pointer becomes invalid at the next step of the iteration. First, you need to save a copy in a memory that you control. This can be done via the allocator.dupe function.

The fixed code could look like this: try file_list.append(try allocator.dupe(u8, dir_content.name));

edited Aug 7 at 23:58

answered Aug 7 at 16:27

sigod

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2 Comments

fwamingo 273 Aug 7 at 17:24

Worked! Thank you! If you wouldn't mind, could you offer a little bit of an explanation on how editing the loop changed the data?

sigod Aug 7 at 23:59

I've updated the answer to improve the explanation. I also recommend reading the Memory section in Zig's documentation.

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