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“C Language Beginner’s Must-Learn Knowledge Points Note Series 100 Articles“

The following notes finally enter the practical series, which is also the most important and difficult part of the C language. Keep it up! Youngsters!

35. Comprehensive Summary of C Language String Handling Functions with Specific Usage Examples

1. String Length Function: strlen

1. Function Prototype

size_t strlen(const char *str);

2. Function Characteristics

• • Calculates the actual length of the string (excluding ‘\0’)
• • Stops counting when ‘\0’ is encountered
• • Time complexity O(n)

3. Usage Example

char str[] = "Hello";
printf("%zu", strlen(str));  // Outputs 5

4. Notes

char s[10] = {'a','b','c'}; 
printf("%zu", strlen(s));  // Result is uncertain because the string in the array is not terminated, missing '\0'

2. String Copy Function

1. strcpy Function

char* strcpy(char *dest, const char *src);

• • Copies src to dest (including ‘\0’)
• • Does not check the size of the destination buffer dest

2. strncpy Function (Safe Version, Specify Copy Size Limit)

char* strncpy(char *dest, const char *src, size_t n);

• • Copies at most n characters
• • Does not automatically add the terminator ‘\0’, must be handled manually

3. Usage Comparison

char dest1[10];
strcpy(dest1, "Hello");  // Unsafe

char dest2[10];
strncpy(dest2, "Hello", sizeof(dest2));
dest2[sizeof(dest2)-1] = '\0';  // Ensure termination

3. String Concatenation Function

1. strcat Function

char* strcat(char *dest, const char *src);

• • Appends src to the end of dest
• • Requires dest to have enough space

2. strncat Function (Safe Version)

char* strncat(char *dest, const char *src, size_t n);

• • Appends at most n characters
• • Automatically adds ‘\0’

3. Usage Example

char str[20] = "Hello";
strcat(str, " World");  // str becomes "Hello World"

char safe_str[20] = "Hello";
strncat(safe_str, " World!", sizeof(safe_str)-strlen(safe_str)-1);

4. String Comparison Function

1. strcmp Function

int strcmp(const char *s1, const char *s2);

• • Return value:
• • 0: strings are equal
• •
  

  0: s1 > s2

• • <0: s1 < s2

2. strncmp Function (Safe Version)

int strncmp(const char *s1, const char *s2, size_t n);

• • Compares only the first n characters

3. Comparison Rules

• • Compares in dictionary order
• • Case-sensitive

if (strcmp("apple", "banana") < 0) {
    printf("apple comes first\n");
}

5. String Search Functions

1. strchr Function

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

• • Finds the first occurrence of character c
• • Returns pointer or NULL

2. strrchr Function

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

• • Finds the last occurrence of character c

3. strstr Function

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

• • Finds the first occurrence of substring needle

4. Usage Example

char *p = strchr("Hello", 'l');
printf("%s\n", p);  // Outputs "llo"

p = strstr("Hello world", "wor");
printf("%s\n", p);  // Outputs "world"

6. String Tokenization Function: strtok

1. Function Prototype

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

2. Usage Characteristics

• • First call passes the string to be tokenized
• • Subsequent calls pass NULL
• • Modifies the original string (replaces delimiters with ‘\0’)

3. Usage Example

char str[] = "apple,orange,banana";
char *token = strtok(str, ",");
while (token) {
    printf("%s\n", token);
    token = strtok(NULL, ",");
}

7. Memory Operation Functions

1. memcpy Function

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

• • Copies n bytes (does not handle ‘\0’)
• • Memory areas must not overlap

2. memmove Function

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

• • Safely handles overlapping memory situations

3. memset Function

void* memset(void *str, int c, size_t n);

• • Sets the first n bytes to c

8. Common Errors

1. Buffer Overflow

char buf[5];
strcpy(buf, "Hello");  // Overflow!

Correction:

strncpy(buf, "Hello", sizeof(buf));
buf[sizeof(buf)-1] = '\0';

2. Forgetting to Check NULL

char *p = strchr(str, 'X');
*p = 'Y';  // Crashes if p is NULL

Correction:

if (p) *p = 'Y';

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