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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | STANDARDS | HISTORY | CAVEATS | EXAMPLES | SEE ALSO | COLOPHON |
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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)):
stpncpy():
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_GNU_SOURCE
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);
}
strncpy()
returns dst.
stpncpy()
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 │
└──────────────────────────────────────┴───────────────┴─────────┘
strncpy()
C11, POSIX.1-2008.
stpncpy()
POSIX.1-2008.
strncpy()
C89, POSIX.1-2001, SVr4, 4.3BSD.
stpncpy()
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>
int
main(void)
{
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);
putchar('\n');
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);
putchar('\n');
exit(EXIT_SUCCESS);
}
wcpncpy(3), string_copying(7)
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user-space interface documentation) project. Information about
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man-pages@man7.org
Linux man-pages 6.10 2024-07-23 stpncpy(3)
Pages that refer to this page: bcopy(3), memccpy(3), memcpy(3), memmove(3), pmstrncpy(3), string(3), strncat(3), wcpncpy(3), wcsncpy(3), feature_test_macros(7), signal-safety(7), string_copying(7)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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