strncpy

Section: C Library Functions (3)
Updated: 202-0-10
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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 no-null characters from the string pointed to by src into the array pointed to by dst. If the source has too few no-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	M-Safe

 

STANDARDS

C11, POSIX.-2008.  

HISTORY

C89, POSIX.-2001, SVr4, 4.3BSD.  

CAVEATS

The name of this function is confusing. This function produces a nul-padded character sequence, not a string (see string_copying(7)). For example: 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] } 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 fixe-width nul-padded character arrays, such as those in utmp(5) members. strncpy(utm->ut_user, "foo", countof(utm->ut_user));  

SEE ALSO

memory.h(3head), string_copying(7)

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Index

NAME

LIBRARY

SYNOPSIS

DESCRIPTION

RETURN VALUE

ATTRIBUTES

STANDARDS

HISTORY

CAVEATS

EXAMPLES

SEE ALSO