C String strnset() Function

What You’ll Learn

The strnset() function overwrites the first n characters of a string with a single character. It is a non-standard extension on some compilers; this tutorial also shows a portable loop you can use on every platform.

Definition and Usage

strnset stands for string n set. Given a writable C-string, it replaces up to n characters starting at index 0 with the byte (char)c. Characters after the limit or after an existing '\0' (when the string is shorter than n) are left unchanged.

Typical uses include masking the start of a password for display, overwriting a prefix before reuse, or quickly stamping a run of identical characters in a buffer you already treat as a string.

📝 Syntax

Typical extension declaration:

char *strnset(char *str, int c, size_t n);

Portable alternative

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

Parameters

• str — writable null-terminated string to modify.
• c — fill value (stored as char).
• n — maximum number of characters to overwrite.

Return Value

• Returns str (pointer to the modified buffer).

Examples Gallery

Examples use the portable str_nset helper so they compile on Linux, macOS, and Windows. Behavior matches typical strnset extensions.

#include <stdio.h>

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 str[] = "Hello, World!";

    str_nset(str, '*', 5);

    printf("Modified String: %s\n", str);

    return 0;
}

Modified String: *****, World!

#include <stdio.h>

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 code[] = "42AB";

    str_nset(code, '0', 2);

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

    return 0;
}

Code: 00AB

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

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 word[] = "Hi";

    str_nset(word, 'X', 10);

    printf("Result: [%s] (len=%zu)\n", word, strlen(word));

    return 0;
}

Result: [XX] (len=2)

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

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 a[] = "Test";
    char b[8] = "Test";

    str_nset(a, '#', 6);
    memset(b, '#', 6);
    b[6] = '\0';

    printf("str_nset: [%s]\n", a);
    printf("memset:   [%s]\n", b);

    return 0;
}

str_nset: [####]
memset:   [######]

#include <stdio.h>

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

int main(void) {
    char label[] = "Secret-123";

    printf("%s\n", str_nset(label, '?', 6));

    return 0;
}

??????-123

🚀 Common Use Cases

• Display masking — hide the first characters of a token or PIN in logs.
• Prefix reset — stamp a run of placeholder chars before reusing a buffer.
• Legacy text files — pad or overwrite fixed-width field beginnings.
• Teaching loops — simple in-place string mutation exercise.
• Reading old code — recognize Borland/MSVC extension calls.

📝 Notes

• Not part of ISO C—verify availability or use the portable loop.
• Requires a writable char array; never pass string literals.
• Does not extend string length—cannot add characters past the existing '\0'.
• c is an int but only the low byte is stored (like memset).
• Pair conceptually with strset() (fill entire string) and memset() (raw memory).

⚡ Optimization

Filling a small prefix is O(n) and trivial. For large buffers where you do not need null-aware behavior, memset is often faster because libraries optimize it heavily. For clearing an entire C-string to empty, set buf[0] = '\0' instead of filling every byte.

💡 Best Practices