strncpy - Man Page

copy fixed length string, returning a pointer to the array end


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.


#include <string.h>

char *stpncpy(char *restrict s1, const char *restrict s2, size_t n);
char *strncpy(char *restrict s1, const char *restrict s2, size_t n);


For strncpy(): The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2017 defers to the ISO C standard.

The stpncpy() and strncpy() functions shall copy not more than n bytes (bytes that follow a NUL character are not copied) from the array pointed to by s2 to the array pointed to by s1.

If the array pointed to by s2 is a string that is shorter than n bytes, NUL characters shall be appended to the copy in the array pointed to by s1, until n bytes in all are written.

If copying takes place between objects that overlap, the behavior is undefined.

Return Value

If a NUL character is written to the destination, the stpncpy() function shall return the address of the first such NUL character. Otherwise, it shall return &s1[n].

The strncpy() function shall return s1.

No return values are reserved to indicate an error.


No errors are defined.

The following sections are informative.



Application Usage

Applications must provide the space in s1 for the n bytes to be transferred, as well as ensure that the s2 and s1 arrays do not overlap.

Character movement is performed differently in different implementations. Thus, overlapping moves may yield surprises.

If there is no NUL character byte in the first n bytes of the array pointed to by s2, the result is not null-terminated.



Future Directions


See Also

strcpy(), wcsncpy()

The Base Definitions volume of POSIX.1-2017, <string.h>

Referenced By

strcpy(3p), string.h(0p), wcsncpy(3p).

2017 IEEE/The Open Group POSIX Programmer's Manual