How to use C string returned by C function and free?

Help
1

This is the C definition:

// must be freed after use
extern const char *string_at(const typ_t *arr, int idx);

The Zig imports:

pub extern fn string_at(arr: ?*const typ_t, idx: c_int) [*c]u8;

How can I use it as string in zig and free it when deinit?

2

I found that I can use std.mem.span(s) to convert to zig string. But I’m not sure if this can leak memory.

3

I’m not sure what’s the problem, can’t you just call free() to deinitialize it?

4

If it’s getting allocated through malloc, you may try using the c_allocators free in the heap section of the standard library:

/// Supports the full Allocator interface, including alignment, and exploiting
/// `malloc_usable_size` if available. For an allocator that directly calls
/// `malloc`/`free`, see `raw_c_allocator`.
pub const c_allocator = Allocator{
    .ptr = undefined,
    .vtable = &c_allocator_vtable,
};
const c_allocator_vtable = Allocator.VTable{
    .alloc = CAllocator.alloc,
    .resize = CAllocator.resize,
    .free = CAllocator.free,
};

/// Asserts allocations are within `@alignOf(std.c.max_align_t)` and directly calls
/// `malloc`/`free`. Does not attempt to utilize `malloc_usable_size`.
/// This allocator is safe to use as the backing allocator with
/// `ArenaAllocator` for example and is more optimal in such a case
/// than `c_allocator`.
pub const raw_c_allocator = Allocator{
    .ptr = undefined,
    .vtable = &raw_c_allocator_vtable,
};
const raw_c_allocator_vtable = Allocator.VTable{
    .alloc = rawCAlloc,
    .resize = rawCResize,
    .free = rawCFree,
};

According to free - cppreference.com, the C version of free from stdlib.h works for:

  • malloc
  • calloc
  • aligned_alloc
  • realloc

That’s what I’d try first. It would be helpful if you posted the body of the C function that you’re referring to.

5

Thank you! Use the associated free function works :smiley:

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