stpncpy, strncpy - zero a fixed-width buffer and copy a string into a character
sequence with truncation and zero the rest of it
Standard C library (
libc,
-lc)
#include <string.h>
char *stpncpy(char dst[restrict .sz], const char *restrict src,
size_t sz);
char *strncpy(char dst[restrict .sz], const char *restrict src,
size_t sz);
stpncpy():
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_GNU_SOURCE
These functions copy the string pointed to by
src into a null-padded
character sequence at the fixed-width buffer pointed to by
dst. 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 *
stpncpy(char *restrict dst, const char *restrict src, size_t sz)
{
bzero(dst, sz);
return mempcpy(dst, src, strnlen(src, sz));
}
char *
strncpy(char *restrict dst, const char *restrict src, size_t sz)
{
stpncpy(dst, src, sz);
return dst;
}
-
stpncpy()
- returns a pointer to one after the last character in the
destination character sequence.
-
strncpy()
- returns dst.
For an explanation of the terms used in this section, see
attributes(7).
Interface |
Attribute |
Value |
stpncpy (), strncpy () |
Thread safety |
MT-Safe |
-
stpncpy()
- POSIX.1-2008.
-
strncpy()
- POSIX.1-2001, POSIX.1-2008, C99, SVr4, 4.3BSD.
The name of these functions is confusing. These functions produce a null-padded
character sequence, not a string (see
string_copying(7)).
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>
int
main(void)
{
char *p;
char buf1[20];
char buf2[20];
size_t len;
if (sizeof(buf1) < strlen("Hello world!"))
warnx("stpncpy: truncating character sequence");
p = stpncpy(buf1, "Hello world!", sizeof(buf1));
len = p - buf1;
printf("[len = %zu]: ", len);
printf("%.*s\n", (int) len, buf1); // "Hello world!"
if (sizeof(buf2) < strlen("Hello world!"))
warnx("strncpy: truncating character sequence");
strncpy(buf2, "Hello world!", sizeof(buf2));
len = strnlen(buf2, sizeof(buf2));
printf("[len = %zu]: ", len);
printf("%.*s\n", (int) len, buf2); // "Hello world!"
exit(EXIT_SUCCESS);
}
wcpncpy(3),
string_copying(7)