Manpage of STRCPY

STRCPY

Section: Linux Programmer's Manual (3)
Updated: 2015-08-08
Index
 

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 (aq\0aq), to the buffer pointed to by dest. The strings may not overlap, and the destination string destmust be large enough to receive the copy. Beware of buffer overruns!(See BUGS.)

The strncpy() function is similar, except that at most nbytes of srcare copied. Warning: If there is no null byte among the first nbytes of src, the string placed in destwill not be null-terminated.

If the length of srcis less than n, strncpy() writes additional null bytes to destto ensure that a total of nbytes are written.

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

    return dest;
}
 

RETURN VALUE

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

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).
InterfaceAttributeValue
strcpy(), strncpy() Thread safetyMT-Safe
 

CONFORMING TO

POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.  

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 destis greater than the length of src, then strcpy() can be used.

One valid (and intended) use of strncpy() is to copy a C string to a fixed-length buffer while ensuring both that the buffer is not overflowed and that unused bytes in the target buffer are zeroed out (perhaps to prevent information leaks if the buffer is to be written to media or transmitted to another process via an interprocess communication technique).

If there is no terminating null byte in the first nbytes of src, strncpy() produces an unterminated string in dest. If bufhas length buflen, you can force termination using something like the following:

strncpy(buf, str, buflen - 1);
if (buflen > 0)
    buf[buflen - 1]= aq\0aq;

(Of course, the above technique ignores the fact that, if srccontains more than buflen - 1bytes, information is lost in the copying to dest.)  

strlcpy()

Some systems (the BSDs, Solaris, and others) provide the following function:


    size_t strlcpy(char *dest, const char *src, size_t size);

This function is similar to strncpy(), but it copies at most size-1bytes to dest, always adds a terminating null byte, and does not pad the target with (further) null bytes. This function fixes some of the problems of strcpy() and strncpy(), but the caller must still handle the possibility of data loss if sizeis too small. The return value of the function is the length of src, which allows truncation to be easily detected: if the return value is greater than or equal to size, truncation occurred. If loss of data matters, the caller musteither check the arguments before the call, or test the function return value. strlcpy() is not present in glibc and is not standardized by POSIX, but is available on Linux via the libbsdlibrary.  

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)


 

Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ATTRIBUTES
CONFORMING TO
NOTES
strlcpy()
BUGS
SEE ALSO

This document was created by man2html, using the manual pages.
Time: 22:27:49 GMT, June 20, 2016 Click Here!