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).

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)). 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) members.

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