c - mmap SIGBUS error and initializing the file

Question

I'm trying to model a basic CPU by mmapping a 1 MiB file, corresponding to the RAM size. I want to read/write this file. Currently I'm getting a SIGBUS error with ram[2] = 1 which I gather is from trying to mmap outside the file range. I've read that perhaps I need to fill the file with zeroes as placeholders, but I'm a bit confused as to why I have to do this, since I thought mmap would automatically set aside a memory chunk for me that would be allocated when I first touch it (as I am trying to do below with my test). What am I missing?

int16_t ramD;
if ( (ramD = open("ramMap.txt", O_RDWR | O_CREAT, 0666)) == -1)
{
    errx(EX_OSERR, "RAM could not be initialized");
}

uint8_t* ram = mmap(0, ram_bytes, PROT_READ | PROT_WRITE, MAP_SHARED, ramD, 0);

ram[2] = 1;
printf("%i", ram[2]);

Answer 1

The SIGBUS means that you're writing outside the file. From Linux man pages mmap(2):

SIGBUS

Attempted access to a portion of the buffer that does not correspond to the file (for example, beyond the end of the file, including the case where another process has truncated the file).

As you create a new file, it is initially empty, i.e. has size of 0 bytes. You need to resize it using ftruncate to be at least big enough to contain the address written to (possibly rounded up to the page size). As you wanted to have a ram disk of size ram_bytes, then:

ftruncate(ramD, ram_bytes);

See this answer for a longer explanation about the same mechanism, using POSIX shared memory objects.

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PS. open returns an int; you should use an int, not int16_t, to store the file descriptor.