Filling and comparing char* inside a function - c++

Question

I wrote the function readChar() which is designed to read the characters send by my WiFi module one by one(function works has advertised) and pass them to a char buffer of increasing size. The function should stop when char *endChar (multiple characters) have been detected or the number of character returned by timedRead() has exceeded size_t length.
I have several issues:
1/. I don't understand the syntax (found inside the Arduino Stream library) :
*buffer++ = (char)c;
Can you explain how the array buffer gets filled?
And why buffer[index] = (char)c; doesn't work here?
2/. I would like to compare buffer and endChar in the loop, possibly by using strcmp(buffer,endChar) (maybe there is a better way). But that doesn't seem to work. In fact when printing the ASCII values of my char *buffer then seem to increment from the end of the buffer. E.G.:
So what is the best way to do that comparison?
The code, inserted in the loop:
_dbgSerial->println("buffer");
for (int i = 0; i < 32; i++){
_dbgSerial->print(buffer[i], DEC);
_dbgSerial->print(",");
}
_dbgSerial->println("");
prints:
buffer
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,13,
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,13,10,
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,13,10,13,
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,13,10,13,10,
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,13,10,13,10,0,
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,13,10,13,10,0,0,
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,13,10,13,10,0,0,0,
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,13,10,13,10,0,0,0,0,
Here is the function readChar():
size_t Debugwifi::readChar(char *endChar, char *buffer, size_t length) {
if (length < 1) return 0;
size_t index = 0;
while (index < length) {
int c = timedRead();
if (c < 0 ) break;
//buffer[index] = (char)c;
*buffer++ = (char)c;
_dbgSerial->println("buffer");
for (int i = 0; i < 32; i++){
_dbgSerial->print(buffer[i], DEC);
_dbgSerial->print(",");
}
_dbgSerial->println("");
if (strcmp(buffer,endChar)==0) {
break;
_dbgSerial->println("brk");}
index++;
}
return index;
}

Answer 1

As Rickard has explained, *buffer++ = (char)c; is how you assign a character to the memory a pointer points at, and then increment the pointer.
However, your function has a lot of problems - you keep comparing unset memory with *endChar. I suggest:
size_t Debugwifi::readChar(const char * const endStr, // const pointer to const.
char * const buffer, const size_t length) {
if (length < 1) return 0;
const size_t endLen = strlen(endStr);
for (size_t index = 0; index < length; index++) {
const int c = timedRead();
if (c < 0 ) break;
buffer[index] = (char)c;
// Debug
_dbgSerial->println("buffer");
for (size_t i = 0; i < length; i++){ // Better to use size_t here,
// and compare against length not 32
_dbgSerial->print(buffer[i], DEC);
_dbgSerial->print(",");
}
_dbgSerial->println("");
// Finished?
if (index >= endLen) {
if (memcmp(&buffer[index-endLen], endStr, endLen)==0) {
_dbgSerial->println("brk"); // Must do this *before* "break"!
break;
}
}
}
return index;
}
I have added a lot of consts. It's hard to have too many.
The important point is that once you have read enough characters, to start comparing the last characters you have read to the end marker.
Note that this function does not remove the end marker, and if you pass a 32-byte zero-filled array and it reads 32 characters, the result will NOT be zero terminated.
Finally, I changed the argument name to endStr because I had expected endChar to be a pointer to a single character - not a NUL-terminated string.

Answer 2

*buffer++ = (char) c;
First writes the value of c to what buffer is currently pointing to.
Then it increments the buffer
This is also why your loop to print buffer doesn't work.
You start printing from the position after what was just filled.
This is also why your strcmp doesn't work. It doesn't actually compare what you have filled your buffer with. It compares the content beyond what have been filled.
If you want your printing code to work you should save the initial value of buffer before the loop;
const char *buffer_start = buffer;
Then use that in your printing code instead of buffer.