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Detailed Explanation of Common Character and String Functions in C

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In C programming, character and string functions are indispensable tools for handling text data. This article will provide a detailed introduction to the definitions, functions, usage methods of these two types of functions, and help readers better understand and apply them through example code.

1. Character Functions

Character functions are typically located in the ctype.h header file. They operate on individual characters, determining the character type or transforming the character.

1.1 Character Classification Functions

FunctionReturns True if the Argument Meets the Condition
iscntrlControl characters: page break , newline , carriage return , tab , vertical tab , etc.
isspaceWhitespace characters: space ' ', page break , newline , carriage return , tab , vertical tab , etc.
isdigitDecimal digits 0~9
isxdigitHexadecimal digits, including decimal digits, lowercase letters a~f, uppercase letters A~F
islowerLowercase letters a~z
isupperUppercase letters A~Z
isalphaLetters a~z or A~Z
isalnumLetters or digits a~z, A~Z, 0~9
ispunctPunctuation characters, any graphical character that is not a letter or digit (printable)
isgraphAny graphical character
isprintAny printable character, including graphical characters and whitespace characters

Usage Example:

The usage of these functions is very similar. Here is an example for one function, the rest follow similarly:

int islower(int c); // Checks if the character is a lowercase letter
// Returns a non-zero integer if it's a lowercase letter, otherwise returns 0.

Code Example: Convert lowercase letters in a string to uppercase:

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <ctype.h>

int main() {
    int i = 0;
    char str[] = "i love China
";
    char c;
    while (str[i]) {
        c = str[i];
        if (islower(c))
            c -= 32;
        putchar(c);
        i++;
    }
    return 0;
}

1.2 Character Conversion Functions

  • tolower(int ch); // Converts uppercase letters to lowercase.

  • toupper(int ch); // Converts lowercase letters to uppercase.

Example:

In the code above, we used -32 to convert to uppercase. With these conversion functions, you can directly use toupper:

#include <stdio.h>
#include <ctype.h>

int main() {
    int i = 0;
    char str[] = "i love China
";
    char c;
    while (str[i]) {
        c = str[i];
        c = toupper(c); // Use function to convert
        putchar(c);
        i++;
    }
    return 0;
}

2. String Functions

String functions are typically located in the <string.h> header file. They operate on null-terminated strings.

2.1 gets and puts

Both gets and puts are defined in <stdio.h>, whereas the others are defined in <string.h>.

gets Function Prototype:

char *gets(char *str);

Parameters:

  • str: A pointer to a character array where the string will be stored.

Functionality:

  • The gets function reads a line of input until it encounters a newline character, replacing the newline character with a null terminator �.

puts Function Prototype:

int puts(const char *str);

Parameters:

  • str: A pointer to a null-terminated string.

Functionality:

  • The puts function outputs the string pointed to by str and appends a newline character  at the end. It does not check or output embedded newline characters in the string.

Return Value:

  • Returns a non-negative value on success.

  • Returns EOF (typically -1) on failure.

Example:

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

int main() {
    char str[20];
    printf("Input:
");
    gets(str);
    printf("Output:
");
    puts(str);
    return 0;
}

2.2 strlen

Function Prototype:

size_t strlen(const char *s);

Parameters:

  • s: A pointer to the string whose length is to be computed (must be null-terminated).

Functionality:

  • The strlen function counts the number of characters in the string, excluding the null terminator �.

Return Value:

  • Returns the number of characters in the string (excluding �).

Example:

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

int main() {
    char* str = "Hello World";
    printf("%d", strlen(str));
    return 0;
}

2.3 strcpy

Function Prototype:

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

Parameters:

  • dest: A pointer to the destination string, which should have enough space to hold the source string and the null terminator.

  • src: A pointer to the source string, which must be null-terminated.

Functionality:

  • The strcpy function copies the source string to the destination string and adds a null terminator to the destination.

Return Value:

  • Returns the dest pointer.

Example:

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

int main() {
    char src[] = "Hello, World!";
    char dest[20] = {0};
    strcpy(dest, src);
    printf("Copied string: %s
", dest);
    return 0;
}

2.4 strncpy

Function Prototype:

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

Parameters:

  • dest: A pointer to the destination string, which should have enough space to hold a part of the source string and the null terminator.

  • src: A pointer to the source string, which must be null-terminated.

  • n: The maximum number of characters to copy.

Functionality:

  • The strncpy function copies up to n characters from the source string to the destination string. If the source string is shorter than n, the remaining characters in the destination string are filled with null terminators.

Return Value:

  • Returns the dest pointer.

Example:

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

int main() {
    char src[] = "Hello, World!";
    char dest[20] = {0};
    strncpy(dest, src, 5);
    printf("Copied string: %s
", dest);
    return 0;
}

2.5 strcat

Function Prototype:

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

Parameters:

  • dest: A pointer to the destination string, which should have enough space to hold the source string and the null terminator.

  • src: A pointer to the source string.

Functionality:

  • The strcat function appends the source string to the destination string and adds a null terminator at the end.

Return Value:

  • Returns the dest pointer.

Example:

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

int main() {
    char dest[20] = "Hello, ";
    char src[] = "World!";
    strcat(dest, src);
    printf("Concatenated string: %s
", dest);
    return 0;
}



2.6 strncat
The usage of strncat is similar to the strcat function, but it specifies the maximum number of characters to concatenate, thus avoiding the risk of buffer overflow.

Function prototype:

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

Parameters:

  • dest: A pointer to the destination string, which must be large enough to hold the concatenated string, including the terminating null character �.

  • src: A pointer to the source string, i.e., the string to append.

  • n: The maximum number of characters to copy from src to dest.

Functionality: The strncat function appends the first n characters (excluding the null character �) of the string pointed to by src to the end of the string pointed to by dest, and automatically adds a null character � at the end of the result. If the number of characters in src is less than n, strncat copies the entire src string.

Return value: The function returns a pointer to the destination string dest.

Usage example:

#define  _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <string.h>
int main() {
    char dest[20] = "Hello, ";
    const char src[] = "World!!!!";
    // Use strncat to append the string
    strncat(dest, src, 5); // Append only "World" (5 characters) and the null character
    // Print result
    printf("The concatenated string is: '%s'
", dest);
    return 0;
}

2.7 strcmp
Function prototype:

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

Parameters:

  • s1: A pointer to the first string to compare.

  • s2: A pointer to the second string to compare.

Functionality: The strcmp function compares two null-terminated strings s1 and s2. The comparison is based on the ASCII values of the characters, comparing them one by one until a difference is found or the end of a string is reached (�).

Return value:

  • If s1 is equal to s2, it returns 0.

  • If s1 is less than s2, it returns a negative value.

  • If s1 is greater than s2, it returns a positive value.

Usage example:

#define  _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <string.h>
int main() {
    const char* str1 = "Hello";
    const char* str2 = "World";
    int result = strcmp(str1, str2);
    if (result < 0) {
        printf("'%s' is less than '%s'
", str1, str2);
    }
    else if (result > 0) {
        printf("'%s' is greater than '%s'
", str1, str2);
    }
    else {
        printf("'%s' is equal to '%s'
", str1, str2);
    }
    return 0;
}

2.8 strncmp
strncmp works similarly to the strcmp function, but it specifies the maximum number of characters to compare, thus avoiding the risk of buffer overflow.

Function prototype:

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

Parameters:

  • s1: A pointer to the first string to compare.

  • s2: A pointer to the second string to compare.

  • n: Specifies the number of characters to compare. The comparison is limited to the first n characters of s1 and s2.

Functionality: Compares the first n characters of s1 and s2. If they are equal, the comparison continues with the next characters. If any difference is found, the comparison ends. The string with the larger character is considered greater. If all n characters are equal, it returns 0.

Return value:

  • If the first n characters of s1 and s2 are equal, it returns 0.

  • If a character in s1 is less than the corresponding character in s2 before comparing n characters, it returns a negative value.

  • If a character in s1 is greater than the corresponding character in s2 before comparing n characters, it returns a positive value.

Usage example:

#define  _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <string.h>
int main() {
    const char* str1 = "Hello";
    const char* str2 = "World";
    int result = strncmp(str1, str2, 1); // Compare only the first character, i.e., H and W
    if (result < 0) {
        printf("'%s' is less than '%s'
", str1, str2);
    }
    else if (result > 0) {
        printf("'%s' is greater than '%s'
", str1, str2);
    }
    else {
        printf("'%s' is equal to '%s'
", str1, str2);
    }
    return 0;
}

2.9 strstr
Function prototype:

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

Parameters:

  • haystack: A pointer to the string to search.

  • needle: A pointer to the substring to find.

Functionality: The strstr function finds the first occurrence of the substring needle in the string haystack. The search is case-sensitive.

Return value:

  • If needle is a substring of haystack, strstr returns a pointer to the first occurrence of needle in haystack.

  • If needle is not a substring of haystack, strstr returns NULL.

Usage example:

#define  _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <string.h>
int main() {
    const char* str = "Hello, world!";
    const char* substr = "world";
    char* pos = strstr(str, substr);
    if (pos) {
        printf("Found '%s' in '%s' at position: %ld
", substr, str, (long)(pos - str));
    }
    else {
        printf("'%s' not found in '%s'
", substr, str);
    }
    return 0;
}

2.10 strchr
Function prototype:

char *strchr(const char *s, int c);

Parameters:

  • s: A pointer to the string to search.

  • c: The ASCII value of the character to find.

Functionality: The strchr function finds the first occurrence of the specified character c in the string s. The search does not include the terminating null character �.

Return value:

  • If the specified character is found, strchr returns a pointer to that character.

  • If the character is not found, strchr returns NULL.

Usage example:

#define  _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <string.h>
int main() {
    const char* str = "Hello, world!";
    char ch = 'o';
    // strchr returns a pointer to the first 'o' character in the string
    char* pos = strchr(str, ch);
    if (pos) {
        printf("Found '%c' in '%s' at position: %ld
", ch, str, (long)(pos - str));
    }
    else {
        printf("'%c' not found in '%s'
", ch, str);
    }
    return 0;
}

2.11. strtok

Function Prototype:

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

Parameters:

  • str: A pointer to the string to be tokenized. On the first call, this pointer should point to the string to be tokenized. In subsequent calls, this pointer should be set to NULL so the function continues processing from where it stopped previously.

  • delim: A pointer to a string containing delimiter characters.

Functionality:The strtok function breaks the string str into a series of tokens separated by the delimiters specified in the delim parameter. On the first call, strtok replaces the first delimiter in the string str with a null character ('�') and returns a pointer to the token. In subsequent calls, the first parameter should be set to NULL, so the function continues from the last token found.

Return Value:

  • On success, strtok returns a pointer to the next token.

  • If no more tokens are found, strtok returns NULL.

Example Usage:

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <string.h>
int main() {
    char str[] = "This is a sample string";
    const char* delimiters = " ,.";
    char* token;
    // First call to provide the string to tokenize
    token = strtok(str, delimiters);
    while (token != NULL) {
        printf("%s
", token); // Print the current token
        token = strtok(NULL, delimiters); // Subsequent calls use NULL
    }
    return 0;
}

2.12 strlwr

Function Prototype:

char *strlwr(char *str);

Parameters:

  • str: A pointer to the string to be converted to lowercase.

Functionality:The strlwr function converts all uppercase letters in the string str to their corresponding lowercase letters, while other characters remain unchanged.

Return Value:The strlwr function returns a pointer to the converted string, which is the same as the input string str.

Example Usage:

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <string.h>
int main() {
    char str[] = "HELLO WORLD!";
    char* lower_str = strlwr(str);
    printf("%s
", lower_str);
    return 0;
}

2.13 strupr

Function Prototype:

char *strupr(char *str);

Parameters:

  • str: A pointer to the string to be converted to uppercase.

Functionality:The strupr function converts all lowercase letters in the string str to their corresponding uppercase letters, while other characters remain unchanged.

Return Value:The strupr function returns a pointer to the converted string, which is the same as the input string str.

Example Usage:

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <string.h>
int main() {
    char str[] = "hello world!";
    char* upper_str = strupr(str);
    printf("%s
", upper_str);
    return 0;
}

2.14 strerror

Function Prototype:

char *strerror(int errnum);

Parameters:

  • errnum: The error code, which is an integer typically obtained from system calls or library functions (e.g., from the errno variable).

Functionality:Returns a pointer to a string describing the error. This string corresponds to the error message for the error code specified by errnum.

Return Value:

  • Returns a pointer to a string in a static buffer that describes the error. If errnum is not a valid error code, it returns NULL.

Different systems and implementations of the C standard library define error codes, which are usually documented in the errno.h header file. At the start of a C program, the global variable errno is initialized to 0, meaning no errors. When a standard library function encounters an error, the corresponding error code is stored in errno. Since error codes are integers and may be difficult to interpret, each code is associated with a descriptive error message. The strerror function provides the address of this descriptive error message as a string.

Example Usage:

#define _CRT_SECURE_NO_WARNINGS
#include <errno.h>
#include <string.h>
#include <stdio.h>
int main()
{
    int i = 0;
    for (i = 0; i <= 10; i++) {
        printf("%s
", strerror(i));
    }
    return 0;
}

Output on Windows 11 + VS2022

Another Example:

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <string.h>
#include <errno.h>
int main()
{
    FILE* pFile;
    // Attempt to open a non-existent file
    pFile = fopen("unexist.ent", "r");
    if (pFile == NULL)
        // Print error message
        printf("Error opening file unexist.ent: %s
", strerror(errno));
    return 0;
}

2.15 perror

Function Prototype:

void perror(const char *s);

Parameters:

  • s: A custom prefix string used to add context before the error message. This helps clarify the error's context.

Functionality:Prints a string containing an error message. The output consists of the prefix string s followed by the error message corresponding to the current value of errno. The message is sent to the standard error stream (stderr).

Return Value:The perror function does not return a value. It directly outputs the error message to stderr.

Example Usage:

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <string.h>
#include <errno.h>
int main()
{
    FILE* pFile;
    // Attempt to open a non-existent file
    pFile = fopen("unexist.ent", "r");
    if (pFile == NULL)
        // Print error message
        perror("Error opening file unexist.ent:");
    return 0;
}

The perror function essentially combines the tasks of handling and printing the error message, as done in the previous example with strerror. It automatically appends a colon and a space to the user-provided prefix string, followed by the error message.

Summary:

  • When using strerror, you need to handle and print the error message manually.

  • When using perror, the error message is automatically output to stderr, and you only need to provide a meaningful prefix.


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