---
name: "strncpy"
section: "3"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/strncpy"
---

# strncpy(3)

nonstring copy

## NAME

strncpy - nonstring copy

## LIBRARY

Standard C library (*libc*, *-lc*)

## SYNOPSIS

```
#include <string.h>  // see memory.h(3head)
```

**char \*strncpy(**size\_t dsize;               **char** *dst***\[restrict** *dsize***\], const char \*restrict** *src***,**               **size\_t** *dsize***);**

## DESCRIPTION

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
```

## RETURN VALUE

*dst*.

## ATTRIBUTES

For an explanation of the terms used in this section, see [**attributes**(7)](/attributes/7.md).

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **strncpy**() | Thread safety | MT-Safe |

## STANDARDS

C11, POSIX.1-2008.

## HISTORY

C89, POSIX.1-2001, SVr4, 4.3BSD.

## CAVEATS

The name of this function is confusing. This function produces a null-padded character sequence, not a string (see [**string\_copying**(7)](/string_copying/7.md)). 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.

## EXAMPLES

This function is intended to be used only for writing to fixed-width null-padded character arrays, such as those in [**utmp**(5)](/utmp/5.md) members.

```
strncpy(utmp->ut_user, "foo", countof(utmp->ut_user));
```

## SEE ALSO

[**memory.h**(3head)](/memory.h/3head.md), [**string\_copying**(7)](/string_copying/7.md)