C strlwr() Function

Beginner
5 min read
Updated: Nov 2026
3 examples
string.h
Decorative image with the word STRLWR

Definition

In C, strlwr() changes uppercase letters in a writable C-string to lowercase, in the same buffer. It is a common extension and is not part of ISO C. On Microsoft compilers the name is often _strlwr().

You pass one writable string. It is required. The result is the same pointer. Prefer a portable loop with tolower when you need standard C.

Syntax

Pass a writable char array. Start with this call.

C
strlwr(word);
CallWhat it does
strlwr(word)Turns uppercase letters in word into lowercase, in place.

Not ISO C. Never pass a string literal. Use a portable loop when you need standard C.

Library declarations
C
char *strlwr(char *str);
char *_strlwr(char *str);

Parameters

ParameterDescription
strRequired. A writable C-string. Do not pass a string literal.

Return Value

Return ValueDescription
str pointerThe same pointer you passed in.

C Examples

Example 1 — Portable Lowercase with tolower

ISO C approach: walk the string and apply tolower. Cast to unsigned char before calling tolower.

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

void str_to_lower(char *s) {
    int i;
    for (i = 0; s[i] != '\0'; i++) {
        s[i] = (char)tolower((unsigned char)s[i]);
    }
}

int main(void) {
    char s[] = "Hello!";
    str_to_lower(s);
    printf("%s\n", s);
    return 0;
}

How It Works

  1. Writable array. char s[] can be modified; a literal could not.
  2. Loop to '\0'. Every character is visited once.
  3. tolower each byte. 'H' becomes 'h'; '!' stays '!'.
  4. Cast first. (unsigned char) avoids undefined behavior on platforms where char is signed.

Example 2 — Using strlwr (Where Available)

On MSVC and some older compilers, call strlwr / _strlwr directly. Skip this on strict ISO C toolchains.

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

int main(void) {
    char s[] = "CODE";

    /* MSVC: prefer _strlwr(s); some compilers offer strlwr(s) */
    strlwr(s);
    printf("%s\n", s);
    return 0;
}

How It Works

  1. Fill a buffer. "CODE" lives in a mutable array.
  2. Call strlwr. Each uppercase letter is rewritten as lowercase.
  3. Same pointer. The function returns s; printing s shows "code".
  4. Toolchain check. If strlwr is missing, use Example 1 instead.

Example 3 — Lowercase a Copy (Keep Original)

When you must keep the original text, copy first, then lower the copy.

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

void str_to_lower(char *s) {
    int i;
    for (i = 0; s[i] != '\0'; i++) {
        s[i] = (char)tolower((unsigned char)s[i]);
    }
}

int main(void) {
    const char *src = "Hi";
    char dest[8];

    strcpy(dest, src);
    str_to_lower(dest);

    printf("src  = %s\n", src);
    printf("dest = %s\n", dest);
    return 0;
}

How It Works

  1. Copy first. strcpy fills dest with "Hi".
  2. Lower only dest. str_to_lower changes dest to "hi".
  3. src unchanged. The original pointer still shows "Hi".
  4. Size dest safely. dest[8] is large enough for "Hi" plus '\0'.

Frequently Asked Questions

strlwr() converts uppercase letters in a writable C-string to lowercase, in place.
str is required. It must be a writable C-string, not a string literal.
The same str pointer you passed in.
No. It is an extension. Use a portable tolower loop for ISO C.
No. String literals must not be modified.

Next: strupr()

Convert a writable C-string to uppercase in place.

strupr() tutorial →

About the author

Mari Selvan M P
Mari Selvan M P 🔗

Developer, cloud engineer, and technical writer

  • Experience 12 years building web and cloud systems
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I write practical tutorials so students and working developers can learn by doing—from databases and APIs to deployment on AWS.

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