Linux manual pages Section 3  

Name

strcpy, strncpy — copy a string

Synopsis

#include <string.h>
char *strcpy( char *dest,
  const char *src);
 
char *strncpy( char *dest,
  const char *src,
  size_t n);
 

DESCRIPTION

The strcpy() function copies the string pointed to by src, including the terminating null byte ('\0'), to the buffer pointed to by dest. The strings may not overlap, and the destination string dest must be large enough to receive the copy.

The strncpy() function is similar, except that at most n bytes of src are copied.

[Warning] Warning

If there is no null byte among the first n bytes of src, the string placed in dest will not be null-terminated.

If the length of src is less than n, strncpy() pads the remainder of dest with null bytes.

A simple implementation of strncpy() might be:

char *
strncpy(char *dest, const char *src, size_t n)
{
    size_t i;

    for (i = 0; i < n && src[i] != '\0'; i++)
        dest[i] = src[i];
    for ( ; i < n; i++)
        dest[i] = '\0';

    return dest;
}

RETURN VALUE

The strcpy() and strncpy() functions return a pointer to the destination string dest.

CONFORMING TO

SVr4, 4.3BSD, C89, C99.

NOTES

Some programmers consider strncpy() to be inefficient and error prone. If the programmer knows (i.e., includes code to test!) that the size of dest is greater than the length of src, then strcpy() can be used.

If there is no terminating null byte in the first n characters of src, strncpy() produces an unterminated string in dest. Programmers often prevent this mistake by forcing termination as follows:

strncpy(buf, str, n);
if (n > 0)
    buf[n − 1]= '\0';

BUGS

If the destination string of a strcpy() is not large enough, then anything might happen. Overflowing fixed-length string buffers is a favorite cracker technique for taking complete control of the machine. Any time a program reads or copies data into a buffer, the program first needs to check that there's enough space. This may be unnecessary if you can show that overflow is impossible, but be careful: programs can get changed over time, in ways that may make the impossible possible.

SEE ALSO

bcopy(3), memccpy(3), memcpy(3), memmove(3), stpcpy(3), stpncpy(3), strdup(3), string(3), wcscpy(3), wcsncpy(3)

COLOPHON

This page is part of release 3.35 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://man7.org/linux/man-pages/.


  Copyright (C) 1993 David Metcalfe (davidprism.demon.co.uk)

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References consulted:
    Linux libc source code
    Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
    386BSD man pages
Modified Sat Jul 24 18:06:49 1993 by Rik Faith (faithcs.unc.edu)
Modified Fri Aug 25 23:17:51 1995 by Andries Brouwer (aebcwi.nl)
Modified Wed Dec 18 00:47:18 1996 by Andries Brouwer (aebcwi.nl)
2007-06-15, Marc Boyer <marc.boyerenseeiht.fr> + mtk
    Improve discussion of strncpy().