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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | STANDARDS | HISTORY | CAVEATS | EXAMPLES | SEE ALSO | COLOPHON |
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strncpy(3) Library Functions Manual strncpy(3)
strncpy - nonstring copy
Standard C library (libc, -lc)
#include <string.h> // see memory.h(3head)
char *strncpy(size_t dsize;
char dst[restrict dsize], const char *restrict src,
size_t dsize);
This function copies non-null characters 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, it pads 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.
It is equivalent to
stpncpy(dst, src, dsize), dst
dst.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ strncpy() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
C11, POSIX.1-2008.
C89, POSIX.1-2001, SVr4, 4.3BSD.
The name of this function is confusing. This function produces 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.
This function is intended to be used only for writing to fixed-
width null-padded character arrays, such as those in utmp(5)
members.
strncpy(utmp->ut_user, "foo", countof(utmp->ut_user));
memory.h(3head), string_copying(7)
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Linux man-pages 6.19 2026-08-10 strncpy(3)
Pages that refer to this page: bcopy(3), memory.h(3head), pmstrncat(3), pmstrncpy(3), stpncpy(3), strncat(3), wcsncpy(3), feature_test_macros(7), signal-safety(7), string_copying(7)
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HTML rendering created 2026-09-09 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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