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Master the essential string functions in C including strlen, strcpy, strncpy, strcat, strncat, strcmp, and strncmp with examples, custom implementations, and safety tips.
C provides a powerful set of string manipulation functions through the <string.h> header. These functions handle the most common string operations — measuring length, copying, concatenating, and comparing. Since C strings don't carry size information, understanding how these functions work (and their safety limitations) is critical for writing reliable programs.
Including the String Header
All standard string functions require:
c example
#include <string.h>strlen() - String Length
Returns the number of characters in a string, not counting the null terminator.
Syntax and Usage
c example
size_t strlen(const char *str);c example
#include <stdio.h>
#include <string.h>
int main() {
char greeting[] = "Hello, World!";
size_t len = strlen(greeting);
printf("String: \"%s\"\n", greeting);
printf("Length: %lu\n", len);
printf("sizeof: %lu\n", sizeof(greeting));
return 0;
}Output
String: "Hello, World!" Length: 13 sizeof: 14
Custom Implementation
Example
size_t my_strlen(const char *str) {
size_t count = 0;
while (str[count] != '\0') {
count++;
}
return count;
}
How strlen Works Internally
"Hello\0"
^ ^
| └── strlen stops here (found '\0')
└── starts counting here
Count: H(1) e(2) l(3) l(4) o(5) → returns 5
strcpy() - String Copy
Copies the source string (including null terminator) into the destination buffer.
Syntax and Usage
c example
char *strcpy(char *dest, const char *src);Example
#include <stdio.h>
#include <string.h>
int main() {
char src[] = "Programming";
char dest[20];
strcpy(dest, src);
printf("Source: %s\n", src);
printf("Destination: %s\n", dest);
return 0;
}
Output
Source: Programming Destination: Programming
⚠️ The Danger of strcpy
strcpy does NOT check if the destination is large enough. This causes buffer overflow:
Example
char small[5];
strcpy(small, "This is way too long!"); // BUFFER OVERFLOW!
Safer Alternative: strncpy
c example
char *strncpy(char *dest, const char *src, size_t n);Example
#include <stdio.h>
#include <string.h>
int main() {
char src[] = "Hello, World!";
char dest[8];
strncpy(dest, src, sizeof(dest) - 1);
dest[sizeof(dest) - 1] = '\0'; // MUST null-terminate manually!
printf("Safe copy: %s\n", dest);
return 0;
}
Output
Safe copy: Hello,
Important:strncpydoes NOT guarantee null termination if the source is longer than n. Always manually add'\0'.
Custom strcpy Implementation
Example
char* my_strcpy(char *dest, const char *src) {
char *original = dest;
while ((*dest++ = *src++) != '\0');
return original;
}
strcat() - String Concatenation
Appends the source string to the end of the destination string.
Syntax and Usage
c example
char *strcat(char *dest, const char *src);Example
#include <stdio.h>
#include <string.h>
int main() {
char result[50] = "Hello";
strcat(result, ", ");
strcat(result, "World");
strcat(result, "!");
printf("Concatenated: %s\n", result);
printf("Length: %lu\n", strlen(result));
return 0;
}
Output
Concatenated: Hello, World! Length: 13
How strcat Works
Example
Before: dest = "Hello\0__________"
^ strcat starts appending here
After: dest = "Hello, World!\0__"
Safer Alternative: strncat
c example
char *strncat(char *dest, const char *src, size_t n);Example
#include <stdio.h>
#include <string.h>
int main() {
char dest[15] = "Hello";
// Append at most 7 characters from source
strncat(dest, ", World!!!", 7);
printf("Result: %s\n", dest); // "Hello, World"
return 0;
}
Output
Result: Hello, World
Unlike strncpy, strncat always null-terminates the result.
Custom strcat Implementation
Example
char* my_strcat(char *dest, const char *src) {
char *original = dest;
// Move to end of dest
while (*dest != '\0') {
dest++;
}
// Copy src to end of dest
while ((*dest++ = *src++) != '\0');
return original;
}
strcmp() - String Comparison
Compares two strings character by character (lexicographically).
Syntax and Return Values
c example
int strcmp(const char *s1, const char *s2);| Return Value | Meaning |
|---|---|
| 0 | Strings are identical |
| < 0 (negative) | s1 comes before s2 lexicographically |
| > 0 (positive) | s1 comes after s2 lexicographically |
c example
#include <stdio.h>
#include <string.h>
int main() {
printf("strcmp(\"apple\", \"apple\") = %d\n", strcmp("apple", "apple"));
printf("strcmp(\"apple\", \"banana\") = %d\n", strcmp("apple", "banana"));
printf("strcmp(\"banana\", \"apple\") = %d\n", strcmp("banana", "apple"));
printf("strcmp(\"Hello\", \"hello\") = %d\n", strcmp("Hello", "hello"));
return 0;
}Output
strcmp("apple", "apple") = 0
strcmp("apple", "banana") = -1
strcmp("banana", "apple") = 1
strcmp("Hello", "hello") = -32Using strcmp for Equality Check
Example
#include <stdio.h>
#include <string.h>
int main() {
char password[] = "secret123";
char input[50];
printf("Enter password: ");
scanf("%s", input);
if (strcmp(input, password) == 0) {
printf("Access granted!\n");
} else {
printf("Access denied!\n");
}
return 0;
}
Output
Enter password: secret123 Access granted!
strncmp - Compare First N Characters
c example
int strncmp(const char *s1, const char *s2, size_t n);c example
#include <stdio.h>
#include <string.h>
int main() {
char str1[] = "Hello, World!";
char str2[] = "Hello, C!";
// Compare only first 5 characters
if (strncmp(str1, str2, 5) == 0) {
printf("First 5 characters match!\n");
}
// Compare all characters
if (strcmp(str1, str2) != 0) {
printf("Full strings are different.\n");
}
return 0;
}Output
First 5 characters match! Full strings are different.
Custom strcmp Implementation
Example
int my_strcmp(const char *s1, const char *s2) {
while (*s1 != '\0' && *s1 == *s2) {
s1++;
s2++;
}
return *(unsigned char*)s1 - *(unsigned char*)s2;
}
Quick Reference Table
| Function | Purpose | Null-safe? | Buffer-safe? |
|---|---|---|---|
strlen(s) | Get length | No | N/A |
strcpy(d, s) | Copy string | No | ❌ No |
strncpy(d, s, n) | Copy up to n chars | No | ⚠️ May not null-terminate |
strcat(d, s) | Append string | No | ❌ No |
strncat(d, s, n) | Append up to n chars | No | ✅ Always null-terminates |
strcmp(s1, s2) | Compare strings | No | N/A |
strncmp(s1, s2, n) | Compare first n chars | No | N/A |
Common Mistakes
Example
// WRONG: Cannot assign strings with =
char str[20];
str = "Hello"; // ERROR: array is not assignable
// CORRECT: Use strcpy
strcpy(str, "Hello"); // OK
// WRONG: Cannot compare strings with ==
if (str == "Hello") // Compares ADDRESSES, not content!
// CORRECT: Use strcmp
if (strcmp(str, "Hello") == 0) // Compares characters
Interview Questions
Q1: What's the difference between strcpy and strncpy?
strcpy copies the entire source string regardless of destination size (dangerous). strncpy copies at most n characters, providing buffer overflow protection. However, strncpy may not null-terminate if the source length exceeds n.
Q2: Why does strcmp return an integer instead of a boolean?
The return value indicates the ordering relationship: negative means first string is "smaller" (comes first alphabetically), zero means equal, and positive means first string is "larger". This makes it useful for sorting algorithms.
Q3: What happens if you use == to compare strings in C?
== compares the memory addresses (pointer values), not the string content. Two identical strings at different memory locations will compare as not equal. Always use strcmp() for content comparison.
Q4: Is strlen O(1) or O(n)?
O(n) — strlen must traverse the entire string character by character until it finds '\0'. Unlike some languages where string length is stored separately, C strings have no cached length.
Q5: Can you use strcat on an uninitialized array?
No. strcat looks for the null terminator in the destination to know where to start appending. An uninitialized array has garbage values, so strcat will write to an unpredictable location.
Summary
strlencounts characters until'\0'— returns length excluding the terminatorstrcpycopies strings but is unsafe — preferstrncpywith manual null terminationstrcatappends strings but requires sufficient destination spacestrcmpreturns 0 for equality, negative/positive for ordering- Always use
strcmp()to compare strings — never use== - The
n-variants (strncpy,strncat,strncmp) add length bounds for safety - None of these functions check for NULL pointers — validate inputs first
Exam Focus
Revise definitions, diagrams, examples, and short-answer points for String Functions in C - strlen, strcpy, strcat, strcmp.
Interview Use
Prepare one clear explanation, one practical example, and one common mistake for this C Programming topic.
Search Terms
c-programming, c programming, programming, strings, string, functions, string functions in c - strlen, strcpy, strcat, strcmp
Related C Programming Topics
Continue learning this concept
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