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Commonly Used String‑Handling Functions

2 min read

Prerequisite: Strings

Quick reference

  • All functions are in <string.h>.
  • All n variants of string functions work on only first n characters of the string.
  • String must always be of size n+1 where n is the number of characters. This is to add the null terminator \0 at the end.

1. strlen() – Length

size_t len = strlen("hello");   // 5

  • Scans until it meets '\0'.

  • O(n); avoid inside tight loops on big buffers.

2. strcpy() / strncpy() – Copy

char dst[10];
strcpy(dst, "hi");          // copies 'h','i','\0'
strncpy(dst, "hello", 3);   // "hel\0..."

Rule: Always guarantee dst ≥ src + 1 to prevent buffer overflow.

3. strcat() / strncat() – Append

char msg[16] = "Hi ";
strcat(msg, "Bob");       // "Hi Bob"

  • strcat first walks to the end (O(n)) then appends.

  • Ensure room for new bytes and terminating \0.

4. strcmp() / strncmp() – Compare

strcmp("abc","abc");   // 0 → equal
strcmp("abc","abd");   // <0
strncmp("abc","abd",2);// 0 (first 2 equal)

Return sign indicates lexical order.

5. strchr() & strrchr() – Find a Char

char *p = strchr(cmd, ' ');   // first space
p = strrchr(path, '/');       // last slash

6. strstr() – Find a Sub‑string

if (strstr(buf, "OK\r\n")) ...

Uses a tuned two‑way search in glibc / newlib → near‑linear on average.

7. strpbrk() – First of Any Set

char *p = strpbrk(str, ",;:");   // first delimiter

8. strcspn() / strspn() – Span Length

size_t n = strcspn("abc,def", ",");

Output

3       (As ',' appears after abc)

  • strcspn = count bytes until a char from set appears.

  • strspn = count bytes while only chars from set appear.

9. memcpy() / memmove() – Raw Copy

memcpy(dst, src, n);      // fastest, undefined if regions overlap
memmove(dst, src, n);     // safe when overlap

Use when data may contain '\0' or isn’t a true C‑string.

10. memset() – Fill

This sets all bytes in the specified buffer to a single value.

memset(buf, 0, sizeof(buf));   // Sets all bytes in buf to 0

11. strtok() / strtok_r() – Tokenise

It performs tokenization of string elements based on the specified delimiter. It carries this out by modifying original string by replacing delimiter with \0.

char s[] = "red,green,blue";
char* tok = strtok(s, ",");
while (tok) {
    printf("%s\n", tok);
    tok = strtok(NULL, ",");
}

Output

red
green
blue

  • Writes '\0' separators into the original buffer.

  • Not thread‑safe → use strtok_r (POSIX) or DIY FSM in bare‑metal.

12. sscanf() / sprintf() – Formatted In/Out

int x; char y[8];
sscanf("12,OK", "%d,%7s", &x, y);   // x=12, y="OK"
sprintf(msg, "V=%u mV", adc_mv);    // to RAM

Embedded tip: prefer snprintf to cap output length.

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