strncpy (3) - Linux Manuals

strncpy: copy a string

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. Beware of buffer overruns! (See BUGS.)

The strncpy() function is similar, except that at most n bytes of src are copied. 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() writes additional null bytes to dest to ensure that a total of n bytes 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] != '\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.

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).

Interface	Attribute	Value
strcpy(), strncpy()	Thread safety	MT-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 dest is 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 destination 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 n bytes of src, strncpy() produces an unterminated string in dest. If buf has length buflen, you can force termination using something like the following:

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

(Of course, the above technique ignores the fact that, if src contains more than buflen - 1 bytes, 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-1 bytes to dest, always adds a terminating null byte, and does not pad the destination 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 size is 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 must either 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 libbsd library.

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.

COLOPHON

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