CODEWITHSUNDEEP
cembeddedbinary-datamicrochip
balmerjd
balmerjd

Reputation: 176

Is there a way to load a binary file as a const variable in C at compile time

I was wondering if there is a way to load an external binary file as a variable in C through an include or a header file or something of the like.

For example, in a project I am currently working on I am working with an embedded system that has a graphic display that will do text and minor graphics (boxes, lines, etc.) using ASCII data and commands. But, it will also display monochrome bitmaps. So I have a series of static displays that I use for a user interface, and a couple of bitmaps for splash screens.

Now the reason I mention that it is an embedded system is that there is no file system to load data from, only RAM and program memory, so any "pre-fab" data or tables I wish to use must but loaded at compile time either through a source file or through an object file using the linker. Unfortunately, the IDE does not provide any means to load a binary, in any form really, into program memory for use as a data cache in this fashion in any readily recognizable fashion.

So short of doing what I already have to get around this, (use a hex editor to read the binary as ASCII encoded hex and copy and paste the raw data into a header file as a variable), is there a way to "link" to a file or "include" a file that could be loaded as a const variable at compile time?

The system I am working with is MPLAB X for the Microchip series of processors and the compiler is GNC. I am mainly looking to know if there is a way to do this through some C command or function before I go trying to look for a way specifically through their specific compiler/linker software.

Upvotes: 10

Views: 6200

Answers (4)

Bo R
Bo R

Reputation: 2371

With C23, you have the option of using the #embed preprocessor directive.

With it, you can write something like this:

#include <stdio.h>

constexpr unsigned char graphdata[] = {
#embed "somefile.bin" if_empty("somefile.bin is empty")
};

int main()
{
    if(sizeof graphdata == 22)
        puts(graphdata); // Prints "somefile.bin is empty\n"
    else
        printf("graphdata is `%s'\n", graphdata);       
}

This will compile if the preprocessor can find the "somefile.bin" file during compilation. I've also included a directive to detect if the file is empty, which provides some fallback content in that case.

If the file is not found at all, you get a compile error to that effect.

Upvotes: 3

John Zwinck
John Zwinck

Reputation: 249093

The usual way to do this on Unix-like systems is using ld -r binary. Here's a tutorial for Linux: http://www.burtonini.com/blog/computers/ld-blobs-2007-07-13-15-50

In summary, the command:

ld -r -b binary -o foo.o foo.txt

will produce an object file foo.o that contains the symbols _binary_foo_txt_start, _binary_foo_txt_size, and _binary_foo_txt_end, which can be used like:

extern char _binary_foo_txt_start[];

And one for Mac OS X, which is a little more complex: Compile a binary file for linking OSX

The idea is to have the linker create an object file with known symbol names which point to the beginning and end of a binary blob which it copies into the resulting object file. You then link that object file into your application, and reference the blob via extern char* pointers or so.

I suppose you know how to link objects on your system, so the remaining question is whether your linker supports something like -r binary.

Upvotes: 5

mfro
mfro

Reputation: 3335

Of course you could invent your own format, but before you do so: have a look into the XPM bitmap format which is exactly meant for that purpose: integrating pixmaps into C source code.

Upvotes: 0

Jabberwocky
Jabberwocky

Reputation: 50768

No you can't do this directly. One way is to convert your binary files into C source code and include these pieces into your project. The conversion can be done by a simple program written by you or by some third party program.

For example:

Binaray1.c (generated automatically)

unsigned char binaray_file1[] =
{
  1,2,3,10,20,
  ....
} ;

Binaray2.c (generated automatically)

unsigned char binaray_file2[] =
{
  0,0,10,20,45,32,212,
  ....
} ;

SomeSourceFile.c

extern unsigned char binary_file1[] ;
extern unsigned char binary_file2[] ;

// use binary_file1 and binary_file2 here.

Upvotes: 5

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