Mount an APFS snapshot using the mount() function, not the mount tool

I want to mount an APFS snapshot using code running on macOS, using the equivalent of this tool invocation:

mount_apfs -o ro,nobrowse -s snapshot_name snapshot_volume destpath

As there's no mount_apfs() function, I'm stuck with mount(). I also cannot use fs_snapshot_mount() because that requires root permissions, and my app doesn't have those.

man 2 mount states:

int mount(const char *type, const char *dir, int flags, void *data);

Data is a pointer to a structure that contains the type specific
arguments to mount.  The format for these argument structures is
described in the manual page for each filesystem.

The problem is that I am unable to find a definition for this data structure for APFS. E.g., where's that "manual page"?

In short, how do I pass the root path of the volume and its snapshot name to the mount() function?

The structure can be found at: mount_args.h from palera1n/jbinit jailbreak repository

struct apfs_mount_args {
#ifndef KERNEL
    char* fspec; /* path to device to mount from */
#endif
    uint64_t apfs_flags; /* The standard mount flags, OR'd with apfs-specific flags (APFS_FLAGS_* above) */
    uint32_t mount_mode; /* APFS_MOUNT_* */
    uint32_t pad1; /* padding */
    uint32_t unk_flags; /* yet another type some sort of flags (bitfield), possibly volume role related */
    union {
        char snapshot[256]; /* snapshot name */
        struct {
            char tier1_dev[128]; /* Tier 1 device (Fusion mount) */
            char tier2_dev[128]; /* Tier 2 device (Fusion mount) */
        };
    };
    void* im4p_ptr;
    uint32_t im4p_size;
    uint32_t pad2; /* padding */
    void* im4m_ptr;
    uint32_t im4m_size;
    uint32_t pad3; /* padding */
    uint32_t cryptex_type; /* APFS_CRYPTEX_TYPE_* */
    int32_t auth_mode; /* APFS_AUTH_ENV_* */
    uid_t uid;
    gid_t gid;
}__attribute__((packed, aligned(4)));

With that the call to mount looks like this:

int main(int argc, char *argv[]) {
    if (argc < 2) {
    printf("call with snapshot name and mount-point\n");
    return 0;
    }
    char* snapshotName = argv[1];
    char* mountPath = argv[2];
    struct apfs_mount_args mnt_args = {};
    mnt_args.apfs_flags = (MNT_RDONLY | MNT_DONTBROWSE);
    mnt_args.mount_mode = APFS_MOUNT_SNAPSHOT;
    mnt_args.unk_flags = 0x0100003F; // no idea what that is
    mnt_args.uid = getuid();
    mnt_args.gid = getgid();
    strcpy(mnt_args.snapshot, snapshotName);
    int result = mount("apfs", mountPath, MNT_RDONLY | MNT_DONTBROWSE, &mnt_args);
    if (result == 0)
        printf("success!\n");
    else
        printf("error: %i - %s\n",errno,strerror(errno));
    return 1;
}

The snapshot name is part of the structure, there're only 256 bytes reserved for it.

The unk_flags are unknown flags and i don't know why these specific flags are required, but that's what's being passed when you use the arguments that you provided in your question.

What surprises me a bit is the lack of source-volume. In theory that's the first member of the struct apfs_mount_args but in practice it's a null-pointer.