Defined in header <string.h>
char *strndup( const char *src, size_t size );
(since C23)

Returns a pointer to a null-terminated byte string, which contains copies of at most size bytes from the string pointed to by src. The space for the new string is obtained as if malloc was called. If the null terminator is not encountered in the first size bytes, it is appended to the duplicated string.

The returned pointer must be passed to free to avoid a memory leak.

If an error occurs, a null pointer is returned and errno might be set.


src - pointer to the null-terminated byte string to duplicate
size - max number of bytes to copy from src

Return value

A pointer to the newly allocated string, or a null pointer if an error occurred.


The function is identical to the POSIX strndup except that it is allowed, but not required to set errno on error.


#include <string.h>
#include <stdio.h>
#include <stdlib.h>
int main(void)
    const size_t n = 3;
    const char *src = "Replica";
    char *dup = strndup(src, n);
    printf("strndup(\"%s\", %lu) == \"%s\"\n", src, n, dup);
    src = "Hi";
    dup = strndup(src, n);
    printf("strndup(\"%s\", %lu) == \"%s\"\n", src, n, dup);
    const char arr[] = {'A','B','C','D'}; // NB: no trailing '\0'
    dup = strndup(arr, n);
    printf("strndup({'A','B','C','D'}, %lu) == \"%s\"\n", n, dup);


strndup("Replica", 3) == "Rep"
strndup("Hi", 3) == "Hi"
strndup({'A','B','C','D'}, 3) == "ABC"

See also

allocates a copy of a string
copies one string to another
allocates memory
deallocates previously allocated memory

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