C Notes

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C String Handling Library - Complete string.h Reference

Complete reference guide to the C string.h library covering all string and memory functions including strchr, strstr, strtok, memcpy, memset, and more.

The <string.h> header in C provides a comprehensive set of functions for manipulating strings and memory blocks. Beyond the basic strlen, strcpy, strcat, and strcmp, there are powerful searching, tokenizing, and memory manipulation functions that every C programmer should know.

String Searching Functions

strchr() - Find First Occurrence of Character

c example

char *strchr(const char *str, int c);

Returns a pointer to the first occurrence of character c in str, or NULL if not found.

c example

#include <stdio.h>
#include <string.h>

int main() {
    char str[] = "Hello, World!";
    
    char *result = strchr(str, 'W');
    if (result != NULL) {
        printf("Found 'W' at position: %ld\n", result - str);
        printf("Substring from 'W': %s\n", result);
    }
    
    // Find last occurrence
    char *last = strrchr(str, 'l');
    if (last != NULL) {
        printf("Last 'l' at position: %ld\n", last - str);
    }
    
    return 0;
}

Output

Found 'W' at position: 7
Substring from 'W': World!
Last 'l' at position: 10

strrchr() - Find Last Occurrence of Character

c example

char *strrchr(const char *str, int c);

Useful for finding file extensions:

c example

#include <stdio.h>
#include <string.h>

int main() {
    char path[] = "/home/user/document.backup.tar.gz";
    
    char *ext = strrchr(path, '.');
    if (ext != NULL) {
        printf("File extension: %s\n", ext);
    }
    
    return 0;
}

Output

File extension: .gz

strstr() - Find Substring

c example

char *strstr(const char *haystack, const char *needle);

Returns a pointer to the first occurrence of needle in haystack.

c example

#include <stdio.h>
#include <string.h>

int main() {
    char text[] = "The quick brown fox jumps over the lazy dog";
    
    char *found = strstr(text, "brown");
    if (found != NULL) {
        printf("Found \"brown\" at position: %ld\n", found - text);
        printf("From match: %s\n", found);
    }
    
    // Check if substring exists
    if (strstr(text, "cat") == NULL) {
        printf("\"cat\" not found in text\n");
    }
    
    return 0;
}

Output

Found "brown" at position: 10
From match: brown fox jumps over the lazy dog
"cat" not found in text

strpbrk() - Find First of Any Characters

c example

char *strpbrk(const char *str, const char *accept);

Finds the first character in str that matches ANY character in accept:

c example

#include <stdio.h>
#include <string.h>

int main() {
    char str[] = "hello123world";
    
    // Find first digit
    char *result = strpbrk(str, "0123456789");
    if (result != NULL) {
        printf("First digit found: '%c' at position %ld\n", *result, result - str);
    }
    
    return 0;
}

Output

First digit found: '1' at position 5

strspn() and strcspn() - Span Functions

c example

size_t strspn(const char *str, const char *accept);   // Length of initial matching segment
size_t strcspn(const char *str, const char *reject);  // Length of initial non-matching segment

c example

#include <stdio.h>
#include <string.h>

int main() {
    char str[] = "123abc456";
    
    // How many initial chars are digits?
    size_t digits = strspn(str, "0123456789");
    printf("Initial digit span: %lu\n", digits);  // 3
    
    // How many initial chars are NOT vowels?
    char str2[] = "hello";
    size_t nonvowel = strcspn(str2, "aeiou");
    printf("Chars before first vowel: %lu\n", nonvowel);  // 1 ('h')
    
    return 0;
}

Output

Initial digit span: 3
Chars before first vowel: 1

strtok() - String Tokenizer

Splits a string into tokens based on delimiter characters. Note: strtok modifies the original string!

c example

char *strtok(char *str, const char *delimiters);

c example

#include <stdio.h>
#include <string.h>

int main() {
    char sentence[] = "Hello,World,How,Are,You";
    
    printf("Tokens:\n");
    char *token = strtok(sentence, ",");
    
    while (token != NULL) {
        printf("  \"%s\"\n", token);
        token = strtok(NULL, ",");  // Pass NULL for subsequent calls
    }
    
    // WARNING: Original string is modified!
    printf("\nOriginal after strtok: \"%s\"\n", sentence);
    
    return 0;
}

Output

Tokens:
  "Hello"
  "World"
  "How"
  "Are"
  "You"

Original after strtok: "Hello"

How strtok Works Internally

Input	"Hello,World,How"
Call 1	strtok(str, ",")
Call 2	strtok(NULL, ",")
Call 3	strtok(NULL, ",")
Call 4	strtok(NULL, ",")

Parsing CSV-Like Data

c example

#include <stdio.h>
#include <string.h>

int main() {
    char data[] = "John:25:Engineer:New York";
    
    char *name = strtok(data, ":");
    char *age = strtok(NULL, ":");
    char *job = strtok(NULL, ":");
    char *city = strtok(NULL, ":");
    
    printf("Name: %s\n", name);
    printf("Age: %s\n", age);
    printf("Job: %s\n", job);
    printf("City: %s\n", city);
    
    return 0;
}

Output

Name: John
Age: 25
Job: Engineer
City: New York

Memory Functions (Also in string.h)

These work on raw memory blocks — not limited to strings.

memcpy() - Copy Memory Block

c example

void *memcpy(void *dest, const void *src, size_t n);

Example

#include <stdio.h> #include <string.h> int main() { int src[] = {1, 2, 3, 4, 5}; int dest[5]; memcpy(dest, src, 5 * sizeof(int)); printf("Copied: "); for (int i = 0; i < 5; i++) printf("%d ", dest[i]); printf("\n"); return 0; }

Output

Copied: 1 2 3 4 5

memmove() - Safe Copy for Overlapping Memory

Unlike memcpy, memmove handles overlapping source and destination:

Example

#include <stdio.h> #include <string.h> int main() { char str[] = "Hello, World!"; // Shift "World!" to the beginning (overlapping regions) memmove(str, str + 7, 6); // Safe with overlap str[6] = '\0'; printf("Result: %s\n", str); return 0; }

Output

Result: World!

memset() - Fill Memory with a Byte Value

c example

void *memset(void *ptr, int value, size_t n);

Example

#include <stdio.h> #include <string.h> int main() { char buffer[20]; // Fill with asterisks memset(buffer, '*', 10); buffer[10] = '\0'; printf("Filled: %s\n", buffer); // Zero-out an integer array int arr[5]; memset(arr, 0, sizeof(arr)); printf("Zeroed: "); for (int i = 0; i < 5; i++) printf("%d ", arr[i]); printf("\n"); return 0; }

Output

Filled: **********
Zeroed: 0 0 0 0 0

memcmp() - Compare Memory Blocks

c example

#include <stdio.h>
#include <string.h>

int main() {
    int a[] = {1, 2, 3, 4, 5};
    int b[] = {1, 2, 3, 4, 5};
    int c[] = {1, 2, 3, 4, 6};
    
    printf("a vs b: %d\n", memcmp(a, b, sizeof(a)));  // 0 (equal)
    printf("a vs c: %d\n", memcmp(a, c, sizeof(a)));  // negative
    
    return 0;
}

Output

a vs b: 0
a vs c: -1

Complete Function Reference Table

Function	Purpose	Returns
`strlen(s)`	String length	`size_t`
`strcpy(d, s)`	Copy string	`char*` to dest
`strncpy(d, s, n)`	Copy up to n chars	`char*` to dest
`strcat(d, s)`	Concatenate	`char*` to dest
`strncat(d, s, n)`	Concat up to n	`char*` to dest
`strcmp(s1, s2)`	Compare	`int`
`strncmp(s1, s2, n)`	Compare first n	`int`
`strchr(s, c)`	Find char (first)	`char*` or NULL
`strrchr(s, c)`	Find char (last)	`char*` or NULL
`strstr(s1, s2)`	Find substring	`char*` or NULL
`strpbrk(s, accept)`	Find any of chars	`char*` or NULL
`strspn(s, accept)`	Initial match span	`size_t`
`strcspn(s, reject)`	Initial reject span	`size_t`
`strtok(s, delim)`	Tokenize	`char*` or NULL
`memcpy(d, s, n)`	Copy bytes	`void*`
`memmove(d, s, n)`	Copy (overlap safe)	`void*`
`memset(s, c, n)`	Fill bytes	`void*`
`memcmp(s1, s2, n)`	Compare bytes	`int`
`memchr(s, c, n)`	Find byte	`void*` or NULL

Interview Questions

Q1: What's the difference between memcpy and memmove?

memcpy is faster but has undefined behavior when source and destination overlap. memmove handles overlapping regions correctly by using a temporary buffer or copying in the appropriate direction. Use memmove when overlap is possible.

Q2: Why does strtok use NULL for subsequent calls?

strtok maintains a static internal pointer that remembers where it left off. Passing NULL tells it to continue from the last position. This also makes strtok non-reentrant (not thread-safe). Use strtok_r for thread-safe tokenization.

Q3: Can memset be used to initialize an int array to a value like 5?

No, memset sets each byte to the value. For integers, memset(arr, 5, sizeof(arr)) sets each byte to 5, resulting in 0x05050505 per integer (84215045 in decimal). Only 0 and -1 produce expected results for int arrays.

Q4: How would you implement strstr (substring search)?

The naive approach checks each position in the haystack: for each position, compare character-by-character with the needle. Time complexity: O(n×m). More efficient algorithms include KMP (O(n+m)) and Boyer-Moore.

Summary

strchr/strrchr find single characters; strstr finds substrings
strtok splits strings but modifies the original — not thread-safe
memcpy is fast but unsafe with overlapping memory; use memmove when unsure
memset fills bytes, not multi-byte values — only use for 0 or character fills
strspn/strcspn measure span lengths based on character sets
All search functions return NULL on failure — always check before using the result

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for C String Handling Library - Complete string.h Reference.

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, handling, library, c string handling library - complete string.h reference

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StringsString Comparison in C - strcmp, strncmp, Case-InsensitiveComplete guide to comparing strings in C using strcmp, strncmp, case-insensitive comparison, and custom comparison functions with practical examples.StringsString Functions in C - strlen, strcpy, strcat, strcmpMaster the essential string functions in C including strlen, strcpy, strncpy, strcat, strncat, strcmp, and strncmp with examples, custom implementations, and safety tips.StringsString Manipulation in C - Reverse, Uppercase, PalindromeLearn string manipulation techniques in C including reversing strings, converting case, checking palindromes, removing characters, and extracting substrings.StringsString Programs in C - Anagram, Word Count, Vowels, FrequencyPractice essential string programs in C including anagram check, word count, vowel count, character frequency, remove duplicates, and more coding problems.StringsStrings in C Programming - Complete IntroductionLearn about strings in C programming including character arrays, null terminator, string declaration, initialization methods, and how strings are stored in memory.Advanced CC Standard Library Overview – All Headers and Common FunctionsComplete guide to the C Standard Library covering all header files, commonly used functions from stdio, stdlib, string, math, and more with practical examples.

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C Notes

C String Handling Library - Complete string.h Reference

Complete reference guide to the C string.h library covering all string and memory functions including strchr, strstr, strtok, memcpy, memset, and more.

String Searching Functions

strchr() - Find First Occurrence of Character

c example

char *strchr(const char *str, int c);

Returns a pointer to the first occurrence of character c in str, or NULL if not found.

c example

#include <stdio.h>
#include <string.h>

int main() {
    char str[] = "Hello, World!";
    
    char *result = strchr(str, 'W');
    if (result != NULL) {
        printf("Found 'W' at position: %ld\n", result - str);
        printf("Substring from 'W': %s\n", result);
    }
    
    // Find last occurrence
    char *last = strrchr(str, 'l');
    if (last != NULL) {
        printf("Last 'l' at position: %ld\n", last - str);
    }
    
    return 0;
}

Output

Found 'W' at position: 7
Substring from 'W': World!
Last 'l' at position: 10

strrchr() - Find Last Occurrence of Character

c example

char *strrchr(const char *str, int c);

Useful for finding file extensions:

c example

#include <stdio.h>
#include <string.h>

int main() {
    char path[] = "/home/user/document.backup.tar.gz";
    
    char *ext = strrchr(path, '.');
    if (ext != NULL) {
        printf("File extension: %s\n", ext);
    }
    
    return 0;
}

Output

File extension: .gz

strstr() - Find Substring

c example

char *strstr(const char *haystack, const char *needle);

Returns a pointer to the first occurrence of needle in haystack.

c example

#include <stdio.h>
#include <string.h>

int main() {
    char text[] = "The quick brown fox jumps over the lazy dog";
    
    char *found = strstr(text, "brown");
    if (found != NULL) {
        printf("Found \"brown\" at position: %ld\n", found - text);
        printf("From match: %s\n", found);
    }
    
    // Check if substring exists
    if (strstr(text, "cat") == NULL) {
        printf("\"cat\" not found in text\n");
    }
    
    return 0;
}

Output

Found "brown" at position: 10
From match: brown fox jumps over the lazy dog
"cat" not found in text

strpbrk() - Find First of Any Characters

c example

char *strpbrk(const char *str, const char *accept);

Finds the first character in str that matches ANY character in accept:

c example

#include <stdio.h>
#include <string.h>

int main() {
    char str[] = "hello123world";
    
    // Find first digit
    char *result = strpbrk(str, "0123456789");
    if (result != NULL) {
        printf("First digit found: '%c' at position %ld\n", *result, result - str);
    }
    
    return 0;
}

Output

First digit found: '1' at position 5

strspn() and strcspn() - Span Functions

c example

size_t strspn(const char *str, const char *accept);   // Length of initial matching segment
size_t strcspn(const char *str, const char *reject);  // Length of initial non-matching segment

c example

#include <stdio.h>
#include <string.h>

int main() {
    char str[] = "123abc456";
    
    // How many initial chars are digits?
    size_t digits = strspn(str, "0123456789");
    printf("Initial digit span: %lu\n", digits);  // 3
    
    // How many initial chars are NOT vowels?
    char str2[] = "hello";
    size_t nonvowel = strcspn(str2, "aeiou");
    printf("Chars before first vowel: %lu\n", nonvowel);  // 1 ('h')
    
    return 0;
}

Output

Initial digit span: 3
Chars before first vowel: 1

strtok() - String Tokenizer

Splits a string into tokens based on delimiter characters. Note: strtok modifies the original string!

c example

char *strtok(char *str, const char *delimiters);

c example

#include <stdio.h>
#include <string.h>

int main() {
    char sentence[] = "Hello,World,How,Are,You";
    
    printf("Tokens:\n");
    char *token = strtok(sentence, ",");
    
    while (token != NULL) {
        printf("  \"%s\"\n", token);
        token = strtok(NULL, ",");  // Pass NULL for subsequent calls
    }
    
    // WARNING: Original string is modified!
    printf("\nOriginal after strtok: \"%s\"\n", sentence);
    
    return 0;
}

Output

Tokens:
  "Hello"
  "World"
  "How"
  "Are"
  "You"

Original after strtok: "Hello"

How strtok Works Internally

Input	"Hello,World,How"
Call 1	strtok(str, ",")
Call 2	strtok(NULL, ",")
Call 3	strtok(NULL, ",")
Call 4	strtok(NULL, ",")

Parsing CSV-Like Data

c example

#include <stdio.h>
#include <string.h>

int main() {
    char data[] = "John:25:Engineer:New York";
    
    char *name = strtok(data, ":");
    char *age = strtok(NULL, ":");
    char *job = strtok(NULL, ":");
    char *city = strtok(NULL, ":");
    
    printf("Name: %s\n", name);
    printf("Age: %s\n", age);
    printf("Job: %s\n", job);
    printf("City: %s\n", city);
    
    return 0;
}

Output

Name: John
Age: 25
Job: Engineer
City: New York

Memory Functions (Also in string.h)

These work on raw memory blocks — not limited to strings.

memcpy() - Copy Memory Block

c example

void *memcpy(void *dest, const void *src, size_t n);

Example

Output

Copied: 1 2 3 4 5

memmove() - Safe Copy for Overlapping Memory

Unlike memcpy, memmove handles overlapping source and destination:

Example

Output

Result: World!

memset() - Fill Memory with a Byte Value

c example

void *memset(void *ptr, int value, size_t n);

Example

Output

Filled: **********
Zeroed: 0 0 0 0 0

memcmp() - Compare Memory Blocks

c example

#include <stdio.h>
#include <string.h>

int main() {
    int a[] = {1, 2, 3, 4, 5};
    int b[] = {1, 2, 3, 4, 5};
    int c[] = {1, 2, 3, 4, 6};
    
    printf("a vs b: %d\n", memcmp(a, b, sizeof(a)));  // 0 (equal)
    printf("a vs c: %d\n", memcmp(a, c, sizeof(a)));  // negative
    
    return 0;
}

Output

a vs b: 0
a vs c: -1

Complete Function Reference Table

Function	Purpose	Returns
`strlen(s)`	String length	`size_t`
`strcpy(d, s)`	Copy string	`char*` to dest
`strncpy(d, s, n)`	Copy up to n chars	`char*` to dest
`strcat(d, s)`	Concatenate	`char*` to dest
`strncat(d, s, n)`	Concat up to n	`char*` to dest
`strcmp(s1, s2)`	Compare	`int`
`strncmp(s1, s2, n)`	Compare first n	`int`
`strchr(s, c)`	Find char (first)	`char*` or NULL
`strrchr(s, c)`	Find char (last)	`char*` or NULL
`strstr(s1, s2)`	Find substring	`char*` or NULL
`strpbrk(s, accept)`	Find any of chars	`char*` or NULL
`strspn(s, accept)`	Initial match span	`size_t`
`strcspn(s, reject)`	Initial reject span	`size_t`
`strtok(s, delim)`	Tokenize	`char*` or NULL
`memcpy(d, s, n)`	Copy bytes	`void*`
`memmove(d, s, n)`	Copy (overlap safe)	`void*`
`memset(s, c, n)`	Fill bytes	`void*`
`memcmp(s1, s2, n)`	Compare bytes	`int`
`memchr(s, c, n)`	Find byte	`void*` or NULL

Interview Questions

Q1: What's the difference between memcpy and memmove?

Q2: Why does strtok use NULL for subsequent calls?

Q3: Can memset be used to initialize an int array to a value like 5?

Q4: How would you implement strstr (substring search)?

Summary

strchr/strrchr find single characters; strstr finds substrings
strtok splits strings but modifies the original — not thread-safe
memcpy is fast but unsafe with overlapping memory; use memmove when unsure
memset fills bytes, not multi-byte values — only use for 0 or character fills
strspn/strcspn measure span lengths based on character sets
All search functions return NULL on failure — always check before using the result

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for C String Handling Library - Complete string.h Reference.

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, handling, library, c string handling library - complete string.h reference

Continue learning this concept

PreviousString Functions in C - strlen, strcpy, strcat, strcmp NextString Manipulation in C - Reverse, Uppercase, Palindrome