Unexpected variable values reading from file (ESP32) - c++

Question

I am still learning Cpp, so please advise if I am misunderstanding here.
Using an ESP32, I am trying to read / write files to Flash / FFat. This is the method I have created which should read a file from flash and load it into PSRAM:
unsigned char* storage_read(char* path) {
File file = FFat.open(path);
if(!file) {
Serial.println("no file");
return 0x00;
}
int count = file.size();
unsigned char* buffer = (unsigned char*)ps_malloc(count);
Serial.printf("Bytes: %d\n", count);
Serial.printf("Count: %d\n", sizeof(buffer));
for (int i = 0; i < count; i++) {
buffer[i] = (unsigned char)file.read();
}
file.close();
return buffer;
}
The problem is that I get the contents of my b64 data file, with the addition of several extra bytes of data globbed on the end.
Calling the method with:
Serial.printf("Got: %s", storage_read("/frame/testframe-000.b64"));
I get the output:
Bytes: 684
Count: 4
Got: <myb64string> + <68B of garbage>
Why would sizeof not be returning the proper size?
What would be the proper way of loading this string into a buffer?

Answer 1

Why would sizeof not be returning the proper size?
That's because sizeof() has a very specific function (not very intuitive). It is used - compile time - to query the size of the data type passed to it. Calling sizeof(buffer) returns the size, in bytes, of the type of variable buffer. It's an unsigned char*, so a 4-byte memory address. So that's what you get.
What would be the proper way of loading this string into a buffer?
What I noticed is that you're expecting to load string data from your file, but you don't explicitly terminate it with a zero byte. As you probably know, all C strings must be terminated with a zero byte. Data that you load from the file most likely doesn't have one (unless you took extra care to add it while saving). So when you read a string from a file sized N bytes, allocate a buffer of N+1 bytes, load the file into it and terminate it with a zero. Something like this:
unsigned char* storage_read(char* path) {
File file = FFat.open(path);
if(!file) {
Serial.println("no file");
return 0x00;
}
int count = file.size();
unsigned char* buffer = (unsigned char*)ps_malloc(count + 1); //< Updated
Serial.printf("Bytes: %d\n", count);
Serial.printf("Count: %d\n", sizeof(buffer));
for (int i = 0; i < count; i++) {
buffer[i] = (unsigned char)file.read();
}
buffer[count] = 0; //< Added
file.close();
return buffer;
}
And since you're returning a heap-allocated buffer from your function, take extra care to remember to delete it in caller when finished. This line in your code will leak the memory:
Serial.printf("Got: %s", storage_read("/frame/testframe-000.b64"));