An In-Depth Look at the Arduino strcpy() Function

The strcpy() function is an essential tool for manipulating strings and character arrays in Arduino C/C++. This definitive guide will explore how strcpy() works, proper usage, performance considerations, alternatives, and best practices for safe string copying in Arduino.

How Arduino‘s strcpy() Function Works

Under the hood, strcpy() simply loops through the source string, copying each byte sequentially into the destination buffer, until it encounters the null terminating character ‘\0‘. This includes:

The strcpy() Copying Algorithm

Behind the scenes, strcpy() relies on a simple iterating loop algorithm:

Set pointer SRC to start address of source string
Set pointer DEST to start address of destination buffer

WHILE (*SRC != ‘\0‘)
  *DEST = *SRC   // Copy byte from source to dest
  SRC = SRC + 1  // Increment source pointer 
  DEST = DEST + 1 // Increment dest pointer
END WHILE

*DEST = ‘\0‘ // Add null terminator

This shows that strcpy() does a byte-for-byte shallow copy based purely on the C-string data itself. It has no inherent bounds checking or size limitation.

Memory Allocation

The source string resides wherever in memory it was initially stored – strcpy() does not create or allocate additional storage for it. The destination char array should however be a fixed allocation large enough to contain the entire source contents plus terminating null.

Failing to pre-allocate adequate space leads to undefined behavior, usually manifesting as random crashes or blank strings as content overwrites past the end of the buffer.

Performance Considerations

Since strcpy() simply loops over content byte-by-byte until hitting the terminating null, performance is generally optimal. Exact speed however depends on:

• Processor architecture – 8-bit AVR cores copy data slower than 32-bit ARM chips for instance.
• Bus speeds – some Arduinos have dedicated RAM buses allowing faster data transfers.
• Quantity of copied data – longer strings take more iterations to move.

As a baseline, copying a 100 char string on 16MHz ATmega328P hardware can complete in around 12 microseconds. But this can vary widely across scenarios.

When to Use Arduino‘s strcpy() Function

Arrays and C-strings are ubiquitous in embedded programming, meaning strcpy() can assist with many string manipulation tasks:

Combining String Literals

Often fixed string data like labels, tags, outputs, etc need to be combined:

char label[20];

strcpy(label, "Sensor: "); 
strcat(label, "A1"); // Concatenate sensor number 

This builds a new string without altering the original array or literal.

Buffering Input From Sensors

Incoming data streams often need buffered locally:

const int BUF_SIZE = 64;
char input_buffer[BUF_SIZE]; // Declare buffer

void getInput() {
  // Fetch & copy data
  char data[32];
  sensor.fetch(data, 32); 
  strcpy(input_buffer, data);  
}

Now analysis can be done directly on input_buffer without concern for incoming stream modifications.

Passing Copies of Strings

As arguments are pass-by-value in C++, strcpy() lets function params alter new string data:

void process(char* str); 

void loop() {

  char data[32];
  // Populate ‘data‘ 

  char copy[32]; 

  strcpy(copy, data); // Pass copy to function
  process(copy);

}

This prevents process() from changing the original data accidentally.

Arduino Alternatives to strcpy()

While strcpy() is ubiquitous, other string functions have distinct advantages in select use cases:

strncpy(dest, src, length)

This variant accepts an explicit max length limiting copy size, helpful when sizes aren‘t fixed:

char dyn_buff[64];
char var_data[MAX_SIZE]; // Some dynamic size 

strncpy(dyn_buff, var_data, sizeof dyn_buff); // Safer copy

Without a length, dynamically allocated src data could lead to dyn_buff overflow.

strcat(dest, src)

Concatenates src onto the end of dest rather than overwriting, allowing string chaining:

char concat[64];
strcpy(concat, "Data");
strcat(concat, "A"); 
strcat(concat, "B"); // concat = "DataAB"

memcpy(dest, src, len)

Lower-level byte copying without assuming ‘\0‘ terminated strings. Use when manipulating pure binary buffers.

sprintf(buffer, format, args)

Powerful method for building formatted strings similar to printf, with specifier placeholders %s, %d etc:

char report[128];
int errors = 2;

sprintf(report, "There were %d errors detected", errors); // "There were 2 errors detected"

Safe Usage of Arduino‘s strcpy() Function

While strcpy() is relatively straightforward, problems arise from invalid assumptions or unchecked errors. Some tips for safe usage:

Validate Array Sizes

Ensure the destination buffer accounts for both present and future growth by a healthy margin:

src_len = strlen(src) + 1;  // +1 for \0
dst_size = sizeof dst; 

if (src_len >= dst_size) {
  // Error - overflow!  
  abort(); 
} 

This avoids corruption from overflow scenarios.

Bounds Check Inputs

If supporting dynamic or externally-provided input strings, check it early:

void setName(const char* external_name) {

  if (strlen(external_name) >= MAX_NAME_LEN) {
    // Invalid input size
    return; 
  }

  // Copy safe
} 

Use Fixed Buffer Sizes

When possible, allocate static buffers matched exactly to the necessary capacity. Avoid using dynamically sized buffers as overflow detection becomes more complex.

Only Copy From Valid Strings

Never try copying from an uninitialized char array as behavior is undefined:

char bad_data[32]; // Uninitialized
strcpy(good_data, bad_data); // Undefined!

Instead always start by explicitly initializing all string buffers before writing.

Optimizing Arduino Code with strcpy()

Understanding Arduino‘s memory architecture helps optimize string manipulation performance.

Storing Literal Strings in PROGMEM

Literal strings like labels and messages consume precious RAM. Use PROGMEM qualifier to store them in flash instead:

const char msg[] PROGMEM = "Sensor limit exceeded!"; 

// Copy from flash into RAM buffer
strcpy_P(buffer, msg); 

Implementing Fixed vs Dynamic String Buffers

Fixed string buffers have maximum simplicity and safety:

char buf[8]; // Allocate 8 bytes 

strcpy(buf, "01234567"); // Error caught immediately  

But dynamic buffers add flexibility:

char *buf = malloc(MAX_EXPECTED);
if (!buf) {
  // Allocation failed
  return; 
}

// Ensure length < MAX_EXPECTED
strcpy(buf, large_string); 

Choose static or dynamic based on the specific use case.

Memory Usage Analysis

Minimizing RAM consumption is always key. As a baseline, copying an 8 char string requires:

• Source string = 9 bytes (8 char + ‘\0‘)
• Dest buffer = 9 bytes again
• Total = 18 bytes

So string memory grows linearly in direct proportion to content size. Benchmark your application and optimize most frequently used string buffers first.

Conclusion

While a basic string function, mastering usage of strcpy() is vital for building robust Arduino firmware that manipulates text efficiently. Pay particular attention to the destination buffer size, preventing overflows through input validation and bounds checks. Also leverage alternatives like strncpy() or strcat() when appropriate.

With the safety guidelines, code examples, and performance best practices covered here however, developers can confidently leverage strcpy() to transfer null-terminated C-strings between arrays – a common need across nearly all embedded systems work.