strlwr() and Portable Lowercase Conversion in C

strlwr() converts letters in a string to lowercase on implementations that provide it, but it is not an ISO C standard-library function. Portable C code can use tolower() from <ctype.h>.

Legacy Syntax Top ↑

strlwr(string);  // non-standard extension

Portable Lowercase Function Top ↑

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

void to_lowercase(char text[]) {
    for (size_t i = 0; text[i] != '\0'; i++) {
        text[i] = (char)tolower((unsigned char)text[i]);
    }
}

int main(void) {
    char text[] = "PLUS2NET.COM";

    to_lowercase(text);
    printf("%s\n", text);

    return 0;
}

Output

plus2net.com

The cast to unsigned char avoids undefined behavior for negative plain-char values passed to tolower().

Lowercase User Input Top ↑

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

void to_lowercase(char text[]) {
    for (size_t i = 0; text[i] != '\0'; i++) {
        text[i] = (char)tolower((unsigned char)text[i]);
    }
}

int main(void) {
    char text[80];

    printf("Enter text: ");
    if (fgets(text, sizeof text, stdin) == NULL) {
        return 1;
    }
    text[strcspn(text, "\n")] = '\0';

    to_lowercase(text);
    printf("Lowercase: %s\n", text);

    return 0;
}

Preserved Example: Convert an Array of Strings to Lowercase Top ↑

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

void to_lowercase(char text[]) {
    for (size_t i = 0; text[i] != '\\0'; i++) {
        text[i] = (char)tolower((unsigned char)text[i]);
    }
}

int main(void) {
    char words[][20] = {"HELLO", "WORLD", "PLUS2NET"};
    size_t count = sizeof words / sizeof words[0];

    for (size_t i = 0; i < count; i++) {
        to_lowercase(words[i]);
        printf("Lowercase: %s\n", words[i]);
    }

    return 0;
}

Subhendu Mohapatra

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I write and maintain practical tutorials on Python, PHP, SQL, JavaScript, HTML, jQuery, and web development at plus2net. The tutorials focus on clear explanations, working examples, and code that readers can test and adapt while learning.

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