strncpy() in C: Copy Part of a String

strncpy() copies at most n characters from a source string to a destination array. If the source is at least n characters long, the destination is not automatically null-terminated.

Syntax Top ↑

strncpy(destination, source, n);

Copy the First Five Characters Top ↑

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

int main(void) {
    char source[] = "First one";
    char destination[6];

    strncpy(destination, source, 5);
    destination[5] = '\0';

    printf("%s\n", destination);
    return 0;
}

Output

First

Limit Copy to the Destination Size Top ↑

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

int main(void) {
    char source[] = "Very long input string";
    char destination[10];

    strncpy(destination, source, sizeof(destination) - 1);
    destination[sizeof(destination) - 1] = '\0';

    printf("%s\n", destination);
    return 0;
}

Output

Very long

Copy into a Structure Member Top ↑

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

struct Student {
    char name[20];
    int age;
};

int main(void) {
    struct Student student;
    char input[] = "Alice Wonderland";

    strncpy(student.name, input, sizeof(student.name) - 1);
    student.name[sizeof(student.name) - 1] = '\0';
    student.age = 20;

    printf("Name: %s, Age: %d\n", student.name, student.age);
    return 0;
}

strncpy() Is Not Simply a Safer strcpy() Top ↑

• It may leave the destination without a terminating \0 when truncation occurs.
• If the source is shorter than n, the remaining destination positions are padded with null bytes.
• Always design the destination size and termination behavior deliberately.

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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