Implement strncpy() function in C

Write an efficient function to implement strncpy() like function in C, which copies the given n characters from source C-string to another string.

The prototype of the strncpy() is:

char* strncpy(char* destination, const char* source, size_t num);

The C99 standard adds the restrict qualifiers to the prototype:

char* strncpy(char* restrict destination, const char* restrict source, size_t num);

The strncpy() function copies num characters from the null-terminated string pointed to by source to the memory pointed to by destination and finally returns the pointer destination.

C

#include <stdio.h>

#include <stddef.h>

// Function to implement `strncpy()` function

char* strncpy(char* destination, const char* source, size_t num)

{

// return if no memory is allocated to the destination

if (destination == NULL) {

return NULL;

}

// take a pointer pointing to the beginning of the destination string

char* ptr = destination;

// copy first `num` characters of C-string pointed by source

// into the array pointed by destination

while (*source && num--)

{

*destination = *source;

destination++;

source++;

}

// null terminate destination string

*destination = '\0';

// the destination is returned by standard `strncpy()`

return ptr;

}

// Implement `strncpy()` function in C

int main(void)

{

char* source = "Techie Delight";

char destination[20];

size_t num = 6;

// Copies the first `num` characters of the source to destination

printf("%s", strncpy(destination, source, num));

return 0;

}

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Output:

Techie

The time complexity of the above solution is O(num).

Shorter Version:

while (*source && num--)

{

*destination = *source;

destination++;

source++;

}

*destination = '\0';

We can replace the above lines of code with a single line of code. The following code will copy the first num characters of the source to the array pointed by destination, including the terminating null character.

1	while (num-- && (destination++ = source++));

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That’s all about strncpy() implementation in C.