'\" t
.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH strncpy 3 2026-08-10 "Linux man-pages 6.19"
.SH NAME
strncpy \- nonstring copy
.SH LIBRARY
Standard C library
.RI ( libc ,\~ \-lc )
.SH SYNOPSIS
.nf
.BR #include\~<string.h> "  // see memory.h(3head)"
.P
.BR "char *strncpy(" "size_t dsize;"
.BI "              char " dst "[restrict " dsize "], const char *restrict " src ,
.BI "              size_t " dsize );
.fi
.SH DESCRIPTION
This function
copies non-null characters
from the string pointed to by
.I src
into the array pointed to by
.IR 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.
.P
It is equivalent to
.P
.in +4n
.EX
stpncpy(dst, src, dsize), dst
.EE
.in
.SH RETURN VALUE
.IR dst .
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.na
.nh
.BR strncpy ()
T}	Thread safety	MT-Safe
.TE
.SH STANDARDS
C11, POSIX.1-2008.
.SH HISTORY
C89, POSIX.1-2001, SVr4, 4.3BSD.
.SH CAVEATS
The name of this function is confusing.
This function produces a null-padded character sequence,
not a string (see
.BR string_copying (7)).
For example:
.P
.in +4n
.EX
strncpy(buf, "1", 5);       // { \[aq]1\[aq],   0,   0,   0,   0 }
strncpy(buf, "1234", 5);    // { \[aq]1\[aq], \[aq]2\[aq], \[aq]3\[aq], \[aq]4\[aq],   0 }
strncpy(buf, "12345", 5);   // { \[aq]1\[aq], \[aq]2\[aq], \[aq]3\[aq], \[aq]4\[aq], \[aq]5\[aq] }
strncpy(buf, "123456", 5);  // { \[aq]1\[aq], \[aq]2\[aq], \[aq]3\[aq], \[aq]4\[aq], \[aq]5\[aq] }
.EE
.in
.P
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.
.SH EXAMPLES
This function is intended to be used
only for writing to fixed-width null-padded character arrays,
such as those in
.BR utmp (5)
members.
.P
.in +4n
.EX
strncpy(utmp->ut_user, "foo", countof(utmp->ut_user));
.EE
.in
.SH SEE ALSO
.BR memory.h (3head),
.BR string_copying (7)