Strings

Introduction

In C, a string is just an array of characters terminated with a null character '\0'. That’s it — no fancy object, just memory with a contract.

char name[] = "Alice";
// = {'A', 'l', 'i', 'c', 'e', '\0'};

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Declaring Strings

1. String Literal

Stored in Flash/ROM, typically read-only. You must not modify this.

char* s = "Hello";
// Stored in .rodata (Flash), pointer is in RAM
// s[0] = 'h'; // Undefined behavior

Use const char* s = "Hello"; — makes intent clear.

2. Character Array

Stored in RAM, modifiable.

char s[] = "Hello";
// Copied from Flash to RAM at runtime
// s[0] = 'h'; // Allowed

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Memory Mapping of Strings

Declaration	Location	Modifiable
char* s = "abc";	Flash (read-only)	❌
char s[] = "abc";	RAM	✅
const char* s = "abc"	Flash (safe)	❌

String literals are stored in Flash, but if copied to char[], they occupy both Flash and RAM.

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Common String Functions

Refer Commonly Used String‑Handling Functions for detailed explanation.

Function	Description
strlen(s)	Get string length (excludes \0)
strcpy(dest, src)	Copy string
strncpy(dest, src, n)	Copy at most n chars
strcat(dest, src)	Append string
strcmp(a, b)	Compare two strings (0 if equal)
strncmp(a, b, n)	Compare first n chars
strchr(s, ch)	First occurrence of char ch in s
strrchr(s, ch)	Last occurrence
strstr(a, b)	First occurrence of string b in a
strtok(s, delim)	Tokenize using delimiter
strcspn(s, set)	Index of first match in set

Embedded Use Cases

1. UART Buffers

Strings are used as receive buffers for UART/SPI/USB.

volatile char rx_buf[64]; // DMA-compatible buffer

2. Command Parsing

For example, parsing "SET LED ON" via sscanf() or strtok().

3. Flash Lookup Tables (LUT)

const char* modes[] = {"ON", "OFF", "AUTO"};

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Tips & Best Practices

• Use const for string literals to avoid accidental modification.

• Always ensure enough space for \0.

• Avoid dynamic memory unless absolutely required (malloc).

• In embedded, avoid strtok() in ISRs or multitasking environments.

• When using DMA or ISR with buffers, mark as volatile.