The String Library

strlen — the length

#include <string.h>
size_t n = strlen("hello");     // 5 — scans to the terminator

Counts characters BEFORE '\0'. On an unterminated array it reads out of bounds — the terminator is your responsibility.

strcpy — the dangerous one

char dst[8];
strcpy(dst, "hello");           // copies 6 bytes (5 + terminator)

strcpy copies until the source's terminator and does NOT know how big dst is. Source longer than dst = buffer overflow. The course rule for graded work and for life: know the destination size, and prefer snprintf or bounded copies when the length is not statically obvious:

char dst[8];
snprintf(dst, sizeof dst, "%s", src);   // copies at most sizeof dst - 1 + '\0'

strcmp — comparison is NOT ==

if (a == b)          // compares POINTERS — almost always wrong
if (strcmp(a, b) == 0)  // compares CONTENTS — correct

strcmp returns <0, 0, or >0 (lexicographic). Equality is exactly == 0.

The habit to build

Before any string operation, answer: is the destination big enough, and is the source terminated? If either answer is "not sure", fix that first.