String compare when using Serial

Question

I try to detect commands given by user in Serial Monitor, but strcmp fails to flag as identical.

What do I do wrong ?

Is there a better way not reading as String and convert to char for comparison ?

Guy

char* additional_callbacks [] = {"command_1", "command_2", "command_3"};
void setup() {
 Serial.begin(9600);
}
void loop() {
 if (Serial.available() > 0) {
 char tmpchar[50];
 Serial.print("entered_text:");
 String str = Serial.readString();
 str.toCharArray(tmpchar, 50);
 Serial.println(tmpchar);
 for (int i = 0; i < sizeof(additional_callbacks) / sizeof(char*); i++) {
 if (strcmp(tmpchar, additional_callbacks[i]) == 0) {
 Serial.println("match");
 }
 }
 Serial.flush();
 }
 delay(50);
}

• Jaromanda X: are you sure the input string is exactly one of the "command"s ... i.e. is there a end-of-line character being sent for example
• Jot: We don't use the Serial.readString because it sometimes waits for data, although it has a timeout. The Serial.flush can not be used to clear the input buffer: arduino.cc/en/Serial/Flush At this moment it is best to write your own code to read data from the serial port.
• Juraj: you can use readBytesUntil to read text from Serial to char array
• guyd: @JaromandaX I send ‘command_1’ as input and get no match
• Jot: Guy D. we know that you send that command and we know that the sketch does not work. The serial monitor might add extra charaters after the 'command_1', a line feed or carriage return character. Even if you fix that, there is still a synchronization problem, the sketch does not know the start or the end of the characters of a command.

Answer 1

In your for() loop, use Serial.println() to display both strings before the compare. You'll be able to see why the compare is failing.

Update:

it seemed identical.

Something has to be different for the test to fail. If the strings look identical then there must be some non-printing characters that differ: \r, \n, , , missing 0円 ), etc.

Try printing your buffers byte-by-byte in hex for a few more bytes than the string length, to be sure there is an where it should be.

Or print the buffers with this hexdump function which lists a specified memory range in both ASCII and hex:

/***
Name: hexdump
Function: Dump memory contents to terminal
Description:Dump a specified range of memory in hex and ASCII
Parameters: byte *mem - starting address
 uint16_t len - # of bytes
Returns: void
Notes: Based on: http://grapsus.net/blog/post/Hexadecimal-dump-in-C
 Formats with sprintf(); outputs with Serial.print().
 Modified to print ...0 - ...F of each hex-block containing
 any of the requested memory range. Data not in the range are
 not shown.
***/
void hexdump(byte *mem, uint16_t len)
{
 byte *p, *pfirst, *plast; // -> curr byte, first & last hex-blocks to print from
 char buf[10+1]; // sprintf o/p buffer
 // Print entire hex-blocks that contain the requested mem range,
 // except only show data within the range.
 pfirst = (byte *)((uint16_t)mem & 0xFFF0); // beg of 1st hex-block
 plast = (byte *)((uint16_t)(mem + len) | 0xF); // end of last hex-block
 for( p = pfirst; p <= plast; ++p ){
 /* Print block addr */
 if( ((byte *)((uint16_t)p & 0xF)) == 0 ){
 sprintf_P(buf, PSTR("%06X: "), (unsigned int)p);
 Serial.print(buf);
 }
 // Print hex data, or if outside mem range, print spaces
 if( mem <= p && p < (mem + len) ){
 sprintf_P(buf, PSTR("%02hhX "), *p);
 Serial.print(buf);
 }
 else
 Serial.print(" "); // outside requested range - just spaces
 // Maybe print gutters and/or ASCII data
 if( ((uint16_t)p & 0xF) == 0x7 )
 Serial.print(" "); // narrow gutter after 8 bytes
 // If at end of hex block, print a gutter & re-print the block as ASCII
 if( ((uint16_t)p & 0xF) == 0xF ){
 Serial.print(" "); // wide gutter after 16 bytes
 // Print as ASCII.
 // Note: In this loop, we re-use the outer loop's index, 'p', to rescan
 // the hex block. We must leave 'p' as we found it!
 for( p = (byte *)((uint16_t)p & 0xFFF0); ; ++p ){
 if( !(mem <= p && p < mem+len) )
 Serial.print(' '); // not in requested mem range
 else if( !isprint(*p) )
 Serial.print('.'); // not printable
 else
 Serial.print(*(char *)p); // print as ASCII
 if( ((uint16_t)p & 0xF) == 0xF ){
 break; // end of hex-block
 }
 }
 Serial.print('\n');
 }
 }
}

• guyd: Already test, and it seemed identical. Perhaps what Martin say has something to do with it ( will try it later )

Answer 2

You try to compare Serial input

command_1\r\n

with

command_1

and these two are not equal.

• MichaelT: Can you elaborate, to make this a proper answer rather than a comment?
• Martin Dittrich: Thanks Michael, i will regard this next time.