stpncpy(3) Library Functions Manual stpncpy(3)

stpncpy, strncpy - fill a fixed-size buffer with non-null bytes from a string, padding with null bytes as needed

Standard C library (libc, -lc)

#include <string.h>
char *strncpy(char dst[restrict .dsize], const char *restrict src,
              size_t dsize);
char *stpncpy(char dst[restrict .dsize], const char *restrict src,
              size_t dsize);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):


    Since glibc 2.10:
        _POSIX_C_SOURCE >= 200809L
    Before glibc 2.10:

These functions copy non-null bytes from the string pointed to by src into the array pointed to by dst. If the source has too few non-null bytes to fill the destination, the functions pad the destination with trailing null bytes. If the destination buffer, limited by its size, isn't large enough to hold the copy, the resulting character sequence is truncated. For the difference between the two functions, see RETURN VALUE.

An implementation of these functions might be:

char *
strncpy(char *restrict dst, const char *restrict src, size_t dsize)
    stpncpy(dst, src, dsize);
    return dst;
char *
stpncpy(char *restrict dst, const char *restrict src, size_t dsize)
    size_t  dlen;
    dlen = strnlen(src, dsize);
    return memset(mempcpy(dst, src, dlen), 0, dsize - dlen);

returns dst.
returns a pointer to one after the last character in the destination character sequence.

For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
stpncpy (), strncpy () Thread safety MT-Safe

C11, POSIX.1-2008.

C89, POSIX.1-2001, SVr4, 4.3BSD.
glibc 1.07. POSIX.1-2008.

The name of these functions is confusing. These functions produce a null-padded character sequence, not a string (see string_copying(7)). For example:

strncpy(buf, "1", 5);       // { '1',   0,   0,   0,   0 }
strncpy(buf, "1234", 5);    // { '1', '2', '3', '4',   0 }
strncpy(buf, "12345", 5);   // { '1', '2', '3', '4', '5' }
strncpy(buf, "123456", 5);  // { '1', '2', '3', '4', '5' }

It's impossible to distinguish truncation by the result of the call, from a character sequence that just fits the destination buffer; truncation should be detected by comparing the length of the input string with the size of the destination buffer.

If you're going to use this function in chained calls, it would be useful to develop a similar function that accepts a pointer to the end (one after the last element) of the destination buffer instead of its size.

#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
    char    *p;
    char    buf1[20];
    char    buf2[20];
    size_t  len;
    if (sizeof(buf2) < strlen("Hello world!"))
        errx("strncpy: truncating character sequence");
    strncpy(buf2, "Hello world!", sizeof(buf2));
    len = strnlen(buf2, sizeof(buf2));
    printf("[len = %zu]: ", len);
    fwrite(buf2, 1, len, stdout);
    if (sizeof(buf1) < strlen("Hello world!"))
        errx("stpncpy: truncating character sequence");
    p = stpncpy(buf1, "Hello world!", sizeof(buf1));
    len = p - buf1;
    printf("[len = %zu]: ", len);
    fwrite(buf1, 1, len, stdout);

wcpncpy(3), string_copying(7)

2024-06-15 Linux man-pages 6.9.1