Baking Data with Serialization

Suppose you want to bake binary data directly into a program’s executable. It could be image pixel data (PNG, BMP, JPEG), a text file, or some sort of complex data structure. Perhaps the purpose is to build a single executable with no extraneous data files — easier to install and manage, though harder to modify. Or maybe you’re lazy and don’t want to worry about handling the various complications and errors that arise when reading external data: Where to find it, and what to do if you can’t find it or can’t read it. This article is about two different approaches I’ve used a number of times for C programs.

The linker approach

The simpler, less portable option is to have the linker do it. Both the GNU linker and the gold linker (ELF only) can create object files from arbitrary files using the --format (-b) option set to binary (raw data). It’s combined with --relocatable (-r) to make it linkable with the rest of the program. MinGW supports all of this, too, so it’s fairly portable so long as you stick to GNU Binutils.

For example, to create an object file, my_msg.o with the contents of the text file my_msg.txt:

$ ld -r -b binary -o my_file.o my_msg.txt

The object file will have three symbols, each named after the input file. Unfortunately there’s no control over the symbol names, section (.data), alignment, or protections (e.g. read-only). You’re completely at the whim of the linker, short of objcopy tricks.

$ nm my_msg.o
000000000000000e D _binary_my_msg_txt_end
000000000000000e A _binary_my_msg_txt_size
0000000000000000 D _binary_my_msg_txt_start

To access these in C, declare them as global variables like so:

extern char _binary_my_msg_txt_start[];
extern char _binary_my_msg_txt_end[];
extern char _binary_my_msg_txt_size;

The size symbol, _binary_my_msg_txt_size, is misleading. The “A” from nm means it’s an absolute symbol, not relocated. It doesn’t refer to an integer that holds the size of the raw data. The value of the symbol itself is the size of the data. That is, take the address of it and cast it to an integer.

size_t size = (size_t)&_binary_my_msg_txt_size;

Alternatively — and this is my own preference — just subtract the other two symbols. It’s cleaner and easier to understand.

size_t size = _binary_my_msg_txt_end - _binary_my_msg_txt_start;

Here’s the “Hello, world” for this approach (hello.c).

#include <stdio.h>

extern char _binary_my_msg_txt_start[];
extern char _binary_my_msg_txt_end[];
extern char _binary_my_msg_txt_size;

int
main(void)
{
    size_t size = _binary_my_msg_txt_end - _binary_my_msg_txt_start;
    fwrite(_binary_my_msg_txt_start, size, 1, stdout);
    return 0;
}

The program has to use fwrite() rather than fputs() because the data won’t necessarily be null-terminated. That is, unless a null is intentionally put at the end of the text file itself.

And for the build:

$ cat my_msg.txt
Hello, world!
$ ld -r -b binary -o my_msg.o my_msg.txt
$ gcc -o hello hello.c my_msg.o
$ ./hello
Hello, world!

If this was binary data, such as an image file, the program would instead read the array as if it were a memory mapped file. In fact, that’s what it really is: the raw data memory mapped by the loader before the program started.

How about a data structure dump?

This could be taken further to dump out some kinds of data structures. For example, this program (table_gen.c) fills out a table of the first 90 Fibonacci numbers and dumps it to standard output.

#include <stdio.h>

#define TABLE_SIZE 90

long long table[TABLE_SIZE] = {1, 1};

int
main(void)
{
    for (int i = 2; i < TABLE_SIZE; i++)
        table[i] = table[i - 1] + table[i - 2];
    fwrite(table, sizeof(table), 1, stdout);
    return 0;
}

Build and run this intermediate helper program as part of the overall build.

$ gcc -std=c99 -o table_gen table_gen.c
$ ./table_gen > table.bin
$ ld -r -b binary -o table.o table.bin

And then the main program (print_fib.c) might look like:

#include <stdio.h>

extern long long _binary_table_bin_start[];
extern long long _binary_table_bin_end[];

int
main(void)
{
    long long *start = _binary_table_bin_start;
    long long *end   = _binary_table_bin_end;
    for (long long *x = start; x < end; x++)
        printf("%lld\n", *x);
    return 0;
}

However, there are some good reasons not to use this feature in this way:

  1. The format of table.bin is specific to the host architecture (byte order, size, padding, etc.). If the host is the same as the target then this isn’t a problem, but it will prohibit cross-compilation.

  2. The linker has no information about the alignment requirements of the data. To the linker it’s just a byte buffer. In the final program the long long array will not necessarily aligned properly for its type, meaning the above program might crash. The Right Way is to never dereference the data directly but rather memcpy() it into a properly-aligned variable, just as if the data was an unaligned buffer.

  3. The data structure cannot use any pointers. Pointer values are meaningless to other processes and will be no different than garbage.

Towards a more portable approach

There’s an easy way to address all three of these problems and eliminate the reliance on GNU linkers: serialize the data into C code. It’s metaprogramming, baby.

In the Fibonacci example, change the fwrite() in table_gen.c to this:

    printf("int table_size = %d;\n", TABLE_SIZE);
    printf("long long table[] = {\n");
    for (int i = 0; i < TABLE_SIZE; i++)
        printf("    %lldLL,\n", table[i]);
    printf("};\n");

The output of the program becomes text:

int table_size = 90;
long long table[] = {
    1LL,
    1LL,
    2LL,
    3LL,
    /* ... */
    1779979416004714189LL,
    2880067194370816120LL,
}

And print_fib.c is changed to:

#include <stdio.h>

extern int table_size;
extern long long table[];

int
main(void)
{
    for (int i = 0; i < table_size; i++)
        printf("%lld\n", table[i]);
    return 0;
}

Putting it all together:

$ gcc -std=c99 -o table_gen table_gen.c
$ ./table_gen > table.c
$ gcc -std=c99 -o print_fib print_fib.c table.c

Any C compiler and linker could do all of this, no problem, making it more portable. The intermediate metaprogram isn’t a barrier to cross compilation. It would be compiled for the host (typically identified through HOST_CC) and the rest is compiled for the target (e.g. CC).

The output of table_gen.c isn’t dependent on any architecture, making it cross-compiler friendly. There are also no alignment problems because it’s all visible to compiler. The type system isn’t being undermined.

Dealing with pointers

The Fibonacci example doesn’t address the pointer problem — it has no pointers to speak of. So let’s step it up to a trie using the trie from the previous article. As a reminder, here it is:

#define TRIE_ALPHABET_SIZE  4
#define TRIE_TERMINAL_FLAG  (1U << 0)

struct trie {
    struct trie *next[TRIE_ALPHABET_SIZE];
    struct trie *p;
    int i;
    unsigned flags;
};

Dumping these structures out raw would definitely be useless since they’re almost entirely pointer data. So instead, fill out an array of these structures, referencing the array itself to build up the pointers (later filled in by either the linker or the loader). This code uses the in-place breadth-first traversal technique from the previous article.

void
trie_serialize(struct trie *t, const char *name)
{
    printf("struct trie %s[] = {\n", name);
    struct trie *head = t;
    struct trie *tail = t;
    t->p = NULL;
    size_t count = 0;
    while (head) {
        printf("    {​{");
        for (int i = 0; i < TRIE_ALPHABET_SIZE; i++) {
            struct trie *next = head->next[i];
            const char *comma = i ? ", " : "";
            if (next) {
                /* Add child to the queue. */
                tail->p = next;
                tail = next;
                next->p = NULL;
                /* Print the pointer to the child. */
                printf("%s%s + %zu", comma, name, ++count);
            } else {
                printf("%s0", comma);
            }
        }
        printf("}, 0, 0, %u},\n", head->flags & TRIE_TERMINAL_FLAG);
        head = head->p;
    }
    printf("};\n");
}

Remember that list of strings from before?

AAAAA
ABCD
CAA
CAD
CDBD

Which looks like this?

That serializes to this C code:

struct trie root[] = {
    {{root + 1, 0, root + 2, 0}, 0, 0, 0},
    {{root + 3, root + 4, 0, 0}, 0, 0, 0},
    {{root + 5, 0, 0, root + 6}, 0, 0, 0},
    {{root + 7, 0, 0, 0}, 0, 0, 0},
    {{0, 0, root + 8, 0}, 0, 0, 0},
    {{root + 9, 0, 0, root + 10}, 0, 0, 0},
    {{0, root + 11, 0, 0}, 0, 0, 0},
    {{root + 12, 0, 0, 0}, 0, 0, 0},
    {{0, 0, 0, root + 13}, 0, 0, 0},
    {{0, 0, 0, 0}, 0, 0, 1},
    {{0, 0, 0, 0}, 0, 0, 1},
    {{0, 0, 0, root + 14}, 0, 0, 0},
    {{root + 15, 0, 0, 0}, 0, 0, 0},
    {{0, 0, 0, 0}, 0, 0, 1},
    {{0, 0, 0, 0}, 0, 0, 1},
    {{0, 0, 0, 0}, 0, 0, 1},
};

This trie can be immediately used at program startup without initialization, and it can even have new nodes inserted into it. It’s not without its downsides, particularly because it’s a trie:

  1. It’s really going to blow up the size of the binary, especially when it holds lots of strings. These nodes are anything but compact.

  2. If the code is compiled to be position-independent (-fPIC), each of those nodes is going to hold multiple dynamic relocations, further exploding the size of the binary and preventing the trie from being shared between processes. It’s 24 bytes per relocation on x86-64. This will also slow down program start up time. With just a few thousand strings, the simple test program was taking 5x longer to start (25ms instead of 5ms) than with an empty trie.

  3. Even without being position-independent, the linker will have to resolve all the compile-time relocations. I was able to overwhelm linker and run it out of memory with just some tens of thousands of strings. This would make for a decent linker stress test.

This technique obviously doesn’t scale well with trie data. You’re better off baking in the flat string list and building the trie at run time — though you could compute the exact number of needed nodes at compile time and statically allocate them (in .bss). I’ve personally had much better luck with other sorts of lookup tables. It’s a useful tool for the C programmer’s toolbelt.

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null program

Chris Wellons

(PGP)