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String Manipulation Functions in C

C provides built-in library functions to work with strings — declared in <string.h>. These let you copy, concatenate, compare, measure, and reverse strings without writing the logic yourself. 
#include <string.h>

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1. strlen() — Get the Length

Returns the number of characters in a string, excluding the null terminator '\0'. 
#include <stdio.h>
#include <string.h>

int main() {
    char name[20] = "Vishal";
    int len = strlen(name);
    printf("Length = %d", len);   // Output: 6
    return 0;
}

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2. strcpy() — Copy a String

Copies the content of one string into another. The destination must be large enough. 
#include <stdio.h>
#include <string.h>

int main() {
    char str1[20] = "Hello";
    char str2[20];
    strcpy(str2, str1);
    printf("Copied: %s", str2);   // Output: Hello
    return 0;
}

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3. strcat() — Concatenate Strings

Appends one string to the end of another. The destination must have enough space. 
#include <stdio.h>
#include <string.h>

int main() {
    char str1[30] = "Hello ";
    char str2[10] = "World!";
    strcat(str1, str2);
    printf("%s", str1);   // Output: Hello World!
    return 0;
}

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4. strcmp() — Compare Strings

Compares two strings character by character:
  • Returns 0 if both strings are equal
  • Returns > 0 if the first string is greater
  • Returns < 0 if the second string is greater
#include <stdio.h>
#include <string.h>

int main() {
    char str1[20] = "abc";
    char str2[20] = "abd";
    int result = strcmp(str1, str2);
    if (result == 0)
        printf("Strings are equal");
    else
        printf("Strings are not equal");
    return 0;
}
// Output: Strings are not equal

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5. strrev() — Reverse a String

Reverses the string in place. Available in Turbo C / MinGW compilers — not in all compilers (not part of the C standard). 
#include <stdio.h>
#include <string.h>

int main() {
    char str[20] = "Future";
    printf("Reversed: %s", strrev(str));
    return 0;
}
// Output: Reversed: erutuF
For portability, write your own reverse: 
#include <stdio.h>
#include <string.h>

void reverseStr(char str[]) {
    int n = strlen(str);
    for (int i = 0; i < n / 2; i++) {
        char temp = str[i];
        str[i] = str[n - 1 - i];
        str[n - 1 - i] = temp;
    }
}

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Quick Reference

All functions below need #include <string.h>.
FunctionDescription
strlen(s)Length of string (excludes '\0')
strcpy(d, s)Copy s into d
strcat(d, s)Append s to end of d
strcmp(s1, s2)Compare two strings
strrev(s)Reverse string (compiler-specific)
strupr(s)Convert to uppercase (Turbo C)
strlwr(s)Convert to lowercase (Turbo C)