C String strset() Function

What You’ll Learn

The strset() function replaces every character in a C-string with a single byte value, leaving the null terminator untouched. It is not ISO C standard—this tutorial shows the portable loop that works on every compiler.

Definition and Usage

strset stands for string set. Walking from index 0 until '\0', it writes the same character into each position. The string keeps the same length—only the visible symbols change.

Typical uses include masking an entire buffer for display, stamping a row of identical characters in legacy text formats, or quickly resetting working text before reuse (when you do not need to clear to empty).

📝 Syntax

Typical extension declaration:

char *strset(char *str, int c);

Portable alternative

char *str_set(char *s, char c) {
    char *start = s;
    while (*s != '\0') {
        *s++ = c;
    }
    return start;
}

Parameters

• str — writable null-terminated string to modify.
• c — fill byte (stored as char).

Return Value

• Returns str (pointer to the modified buffer).

Examples Gallery

Examples use the portable str_set helper so they compile on Linux, macOS, and Windows.

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

char *str_set(char *s, char c) {
    char *start = s;
    while (*s != '\0') {
        *s++ = c;
    }
    return start;
}

int main(void) {
    char text[] = "Hello, C Programming!";

    printf("Before: %s (len=%zu)\n", text, strlen(text));
    str_set(text, '*');
    printf("After:  %s (len=%zu)\n", text, strlen(text));

    return 0;
}

Before: Hello, C Programming! (len=21)
After:  ********************* (len=21)

#include <stdio.h>

char *str_set(char *s, char c) {
    char *start = s;
    while (*s) {
        *s++ = c;
    }
    return start;
}

int main(void) {
    char code[] = "XY12";

    str_set(code, '0');

    printf("Code: %s\n", code);

    return 0;
}

Code: 0000

#include <stdio.h>

char *str_set(char *s, char c) {
    char *start = s;
    while (*s) {
        *s++ = c;
    }
    return start;
}

int main(void) {
    char secret[] = "MyPassword";

    str_set(secret, 'x');

    printf("Log: user entered [%s]\n", secret);

    return 0;
}

Log: user entered [xxxxxxxxxx]

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

char *str_set(char *s, char c) {
    char *start = s;
    while (*s) { *s++ = c; }
    return start;
}

void str_nset(char *s, char c, size_t n) {
    for (size_t i = 0; i < n && s[i] != '\0'; i++) {
        s[i] = c;
    }
}

int main(void) {
    char full[] = "Hello";
    char part[] = "Hello";

    str_set(full, '-');
    str_nset(part, '-', 3);

    printf("strset:  %s\n", full);
    printf("strnset: %s\n", part);

    return 0;
}

strset:  -----
strnset: ---lo

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

char *str_set(char *s, char c) {
    char *start = s;
    while (*s) {
        *s++ = c;
    }
    return start;
}

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

    str_set(empty, 'Z');

    printf("Empty after strset: [%s] len=%zu\n", empty, strlen(empty));

    return 0;
}

Empty after strset: [] len=0

🚀 Common Use Cases

• Visual masking — hide entire fields in debug output.
• Placeholder rows — stamp repeated symbols in text UI prototypes.
• Legacy record prep — uniform character fields before editing.
• Teaching loops — walk a C-string until '\0'.
• Reading old code — recognize Borland/MSVC extension calls.

📝 Notes

• Not part of ISO C—use the portable loop on GCC/Linux.
• Does not change string length or the null terminator.
• Requires a writable buffer; never pass string literals.
• Not a secure way to erase secrets—use explicit clearing practices.
• Pair with strnset() when only a prefix should change.

⚡ Optimization

Filling a string is O(n) in its length. For very long buffers, memset(str, c, strlen(str)) may be faster on some platforms because libraries optimize memset heavily—but only when you truly want every byte before the terminator replaced and the null left at the same index.

💡 Best Practices