String safety

Learn safe string handling with strncpy and strncat.

Safe copying with strncpy

strncpy(dest, src, n) copies at most n bytes from src to dest. This prevents overflow but has a catch: if src is longer than n, dest isn't null-terminated.

Example: char dest[4]; strncpy(dest, "Merhaba", 4);. Here only "Merh" is copied (4 bytes), no null is added. For safe use you must terminate with dest[n-1] = '\0';. This pattern is standard for untrusted sources like user input and network data. Unlike strcpy, strncpy checks size.

Safe concatenation with strncat

strncat(dest, src, n) appends at most n bytes from src to the end of dest. Unlike strncpy, it always adds a null terminator.

Example: char dest[20] = "Merhaba"; strncat(dest, " dünya", 6);. Here at most 6 bytes are appended; the result is null-terminated. n is the maximum number of bytes to append; not the total size of dest. So computing n as sizeof(dest) - strlen(dest) - 1 is the safe pattern. strncat is the safe version of strcat.

Exercise Safe copy with strncpy

Declare a four-byte array char dest[4];. Copy at most three bytes with strncpy(dest, "Merhaba", 3);. Null-terminate with dest[3] = '\0';. Print with printf("%s\n", dest);. Include <string.h>.

Show example solution

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

int main(void) {
    char dest[4];
    strncpy(dest, "Merhaba", 3);
    dest[3] = '\0';
    printf("%s\n", dest);
    return 0;
}

Test yourself

Match the safe functions

strncpy

Bounded copy
strncat

Bounded append
snprintf

Bounded format
strcpy

Unbounded (unsafe)

Complete the strncpy call

char dest[4];
____(dest, "Merhaba", 3);
dest[3] = '\0';

Show answer

char dest[4];
strncpy(dest, "Merhaba", 3);
dest[3] = '\0';

What does strncpy do?
- Copies without bound
- Only returns a pointer
- Returns the string length
- Copies at most n bytes
Show answer

Copies at most n bytes — strncpy copies at most n bytes; this prevents overflow.

Next lesson: Summary - Advanced strings

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