strnset() and Portable Partial String Replacement

strnset() is a non-standard extension that replaces up to the first n characters of a string. Portable C can implement the same operation with a bounded loop.

Legacy Syntax Top ↑

strnset(string, '*', n);  // non-standard

Portable Replacement Top ↑

#include <stdio.h>

void set_first_n(char text[], char replacement, size_t n) {
    size_t i = 0;

    while (i < n && text[i] != '\0') {
        text[i] = replacement;
        i++;
    }
}

int main(void) {
    char text[] = "plus2net";

    set_first_n(text, '*', 4);
    printf("%s\n", text);

    return 0;
}

Output

****2net

When n Is Longer Than the String Top ↑

#include <stdio.h>

void set_first_n(char text[], char replacement, size_t n) {
    size_t i = 0;

    while (i < n && text[i] != '\0') {
        text[i] = replacement;
        i++;
    }
}

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

    set_first_n(text, '#', 5);
    printf("%s\n", text);

    return 0;
}

The loop stops at the null terminator, so it does not write beyond the string.

Subhendu Mohapatra

Author & Instructor at plus2net

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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