For large files, converting to text and then making the compiler parse it all over again is inefficient and unnecessary. Use objcopy instead:

objcopy -I binary -O elf32-i386 stuff stuff.o

(Adjust the output architecture as necessary for non-x86 platforms.) Then once you link it into your program, you can access it like so:

extern char _binary_stuff_start[], _binary_stuff_end[];
#define SIZE_OF_STUFF (_binary_stuff_end - _binary_stuff_start)

...

foo(_binary_stuff_start[i]);

In case you want to use caps or not:

def bytes_to_c_arr(data, lowercase=True):
    return [format(b, '#04x' if lowercase else '#04X') for b in data]

x = b'abc123'
print("unsigned char x[] = {{{}}}".format(", ".join(bytes_to_c_arr(x))))
print("unsigned char x[] = {{{}}}".format(", ".join(bytes_to_c_arr(x, False))))

# output: unsigned char x[] = {0x61, 0x62, 0x63, 0x31, 0x32, 0x33}
#         unsigned char x[] = {0X61, 0X62, 0X63, 0X31, 0X32, 0X33}

You can almost do what you want with hexdump, but I can't figure out how to get quotes & single backslashes into the format string. So I do a little post-processing with sed. As a bonus, I've also indented each line by 4 spaces. :)

hexdump -e '16/1 "_x%02X" "\n"' filename | sed 's/_/\\/g; s/.*/    "&"/'

Edit

As Cengiz Can pointed out, the above command line doesn't cope well with short data lines. So here's a new improved version:

hexdump -e '16/1 "_x%02X" "\n"' filename | sed 's/_/\\/g; s/\\x  //g; s/.*/    "&"/'

As Malvineous mentions in the comments, we also need to pass the -v verbose option to hexdump to prevent it from abbreviating long runs of identical bytes to *.

hexdump -v -e '16/1 "_x%02X" "\n"' filename | sed 's/_/\\/g; s/\\x  //g; s/.*/    "&"/'