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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | NOTES | CAVEATS | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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string_copying(7) Miscellaneous Information Manual string_copying(7)
stpcpy, strcpy, strcat, stpecpy, strtcpy, strlcpy, strlcat,
stpncpy, strncpy, strncat - copying strings and character
sequences
Strings
// Chain-copy a string.
char *stpcpy(char *restrict dst, const char *restrict src);
// Copy/catenate a string.
char *strcpy(char *restrict dst, const char *restrict src);
char *strcat(char *restrict dst, const char *restrict src);
// Chain-copy a string with truncation.
char *stpecpy(char *dst, char end[0], const char *restrict src);
// Copy/catenate a string with truncation.
ssize_t strtcpy(char dst[restrict .dsize], const char *restrict src,
size_t dsize);
size_t strlcpy(char dst[restrict .dsize], const char *restrict src,
size_t dsize);
size_t strlcat(char dst[restrict .dsize], const char *restrict src,
size_t dsize);
Null-padded character sequences
// Fill a fixed-size buffer with characters from a string
// and pad with null bytes.
char *strncpy(char dst[restrict .dsize], const char *restrict src,
size_t dsize);
char *stpncpy(char dst[restrict .dsize], const char *restrict src,
size_t dsize);
// Chain-copy a null-padded character sequence into a character sequence.
mempcpy(dst, src, strnlen(src, NITEMS(src)));
// Chain-copy a null-padded character sequence into a string.
stpcpy(mempcpy(dst, src, strnlen(src, NITEMS(src))), "");
// Catenate a null-padded character sequence into a string.
char *strncat(char *restrict dst, const char src[restrict .ssize],
size_t ssize);
// Duplicate a null-padded character sequence into a string.
char *strndup(const char src[.ssize], size_t ssize);
Known-length character sequences
// Chain-copy a known-length character sequence.
void *mempcpy(void dst[restrict .len], const void src[restrict .len],
size_t len);
// Chain-copy a known-length character sequence into a string.
stpcpy(mempcpy(dst, src, len), "");
Terms (and abbreviations)
string (str)
is a sequence of zero or more non-null characters followed
by a null character.
character sequence
is a sequence of zero or more non-null characters. A
program should never use a character sequence where a
string is required. However, with appropriate care, a
string can be used in the place of a character sequence.
null-padded character sequence
Character sequences can be contained in fixed-size
buffers, which contain padding null bytes after the
character sequence, to fill the rest of the buffer
without affecting the character sequence; however,
those padding null bytes are not part of the
character sequence. Don't confuse null-padded with
null-terminated: null-padded means 0 or more
padding null bytes, while null-terminated means
exactly 1 terminating null character.
known-length character sequence
Character sequence delimited by its length. It may
be a slice of a larger character sequence, or even
of a string.
length (len)
is the number of non-null characters in a string or
character sequence. It is the return value of strlen(str)
and of strnlen(buf, size).
size refers to the entire buffer where the string or character
sequence is contained.
end is the name of a pointer to one past the last element of a
buffer. It is equivalent to &str[size]. It is used as a
sentinel value, to be able to truncate strings or
character sequences instead of overrunning the containing
buffer.
copy This term is used when the writing starts at the first
element pointed to by dst.
catenate
This term is used when a function first finds the
terminating null character in dst, and then starts writing
at that position.
chain This term is used when it's the programmer who provides a
pointer to the terminating null character in the string
dst (or one after the last character in a character
sequence), and the function starts writing at that
location. The function returns a pointer to the new
location of the terminating null character (or one after
the last character in a character sequence) after the
call, so that the programmer can use it to chain such
calls.
duplicate
Allocate a new buffer where a copy is placed.
Copy, catenate, and chain-copy
Originally, there was a distinction between functions that copy
and those that catenate. However, newer functions that copy
while allowing chaining cover both use cases with a single API.
They are also algorithmically faster, since they don't need to
search for the terminating null character of the existing string.
However, functions that catenate have a much simpler use, so if
performance is not important, it can make sense to use them for
improving readability.
The pointer returned by functions that allow chaining is a
byproduct of the copy operation, so it has no performance costs.
Functions that return such a pointer, and thus can be chained,
have names of the form *stp*(), since it's common to name the
pointer just p.
Chain-copying functions that truncate should accept a pointer to
the end of the destination buffer, and have names of the form
*stpe*(). This allows not having to recalculate the remaining
size after each call.
Truncate or not?
The first thing to note is that programmers should be careful
with buffers, so they always have the correct size, and
truncation is not necessary.
In most cases, truncation is not desired, and it is simpler to
just do the copy. Simpler code is safer code. Programming
against programming mistakes by adding more code just adds more
points where mistakes can be made.
Nowadays, compilers can detect most programmer errors with
features like compiler warnings, static analyzers, and
_FORTIFY_SOURCE (see ftm(7)). Keeping the code simple helps
these overflow-detection features be more precise.
When validating user input, code should normally not truncate,
but instead fail and prevent the copy at all.
In some cases, however, it makes sense to truncate.
Functions that truncate:
• stpecpy()
• strtcpy()
• strlcpy(3bsd) and strlcat(3bsd) are similar, but have
important performance problems; see BUGS.
• stpncpy(3) and strncpy(3) also truncate, but they don't write
strings, but rather null-padded character sequences.
Null-padded character sequences
For historic reasons, some standard APIs and file formats, such
as utmpx(5) and tar(1), use null-padded character sequences in
fixed-size buffers. To interface with them, specialized
functions need to be used.
To copy bytes from strings into these buffers, use strncpy(3) or
stpncpy(3).
To read a null-padded character sequence, use
strnlen(src, NITEMS(src)), and then you can treat it as a known-
length character sequence; or use strncat(3) or strndup(3)
directly.
Known-length character sequences
The simplest character sequence copying function is mempcpy(3).
It requires always knowing the length of your character
sequences, for which structures can be used. It makes the code
much faster, since you always know the length of your character
sequences, and can do the minimal copies and length measurements.
mempcpy(3) copies character sequences, so you need to explicitly
set the terminating null character if you need a string.
In programs that make considerable use of strings or character
sequences, and need the best performance, using overlapping
character sequences can make a big difference. It allows holding
subsequences of a larger character sequence, while not
duplicating memory nor using time to do a copy.
However, this is delicate, since it requires using character
sequences. C library APIs use strings, so programs that use
character sequences will have to take care of differentiating
strings from character sequences.
To copy a known-length character sequence, use mempcpy(3).
To copy a known-length character sequence into a string, use
stpcpy(mempcpy(dst, src, len), "").
A string is also accepted as input, because mempcpy(3) asks for
the length, and a string is composed of a character sequence of
the same length plus a terminating null character.
String vs character sequence
Some functions only operate on strings. Those require that the
input src is a string, and guarantee an output string (even when
truncation occurs). Functions that catenate also require that
dst holds a string before the call. List of functions:
• stpcpy(3)
• strcpy(3), strcat(3)
• stpecpy()
• strtcpy()
• strlcpy(3bsd), strlcat(3bsd)
Other functions require an input string, but create a character
sequence as output. These functions have confusing names, and
have a long history of misuse. List of functions:
• stpncpy(3)
• strncpy(3)
Other functions operate on an input character sequence, and
create an output string. Functions that catenate also require
that dst holds a string before the call. strncat(3) has an even
more misleading name than the functions above. List of
functions:
• strncat(3)
• strndup(3)
Other functions operate on an input character sequence to create
an output character sequence. List of functions:
• mempcpy(3)
Functions
stpcpy(3)
Copy the input string into a destination string. The
programmer is responsible for allocating a buffer large
enough. It returns a pointer suitable for chaining.
strcpy(3)
strcat(3)
Copy and catenate the input string into a destination
string. The programmer is responsible for allocating a
buffer large enough. The return value is useless.
stpcpy(3) is a faster alternative to these functions.
stpecpy()
Chain-copy the input string into a destination string. If
the destination buffer, limited by a pointer to its end,
isn't large enough to hold the copy, the resulting string
is truncated (but it is guaranteed to be null-terminated).
It returns a pointer suitable for chaining. Truncation
needs to be detected only once after the last chained
call.
This function is not provided by any library; see EXAMPLES
for a reference implementation.
strtcpy()
Copy the input string into a destination string. If the
destination buffer isn't large enough to hold the copy,
the resulting string is truncated (but it is guaranteed to
be null-terminated). It returns the length of the string,
or -1 if it truncated.
This function is not provided by any library; see EXAMPLES
for a reference implementation.
strlcpy(3bsd)
strlcat(3bsd)
Copy and catenate the input string into a destination
string. If the destination buffer, limited by its size,
isn't large enough to hold the copy, the resulting string
is truncated (but it is guaranteed to be null-terminated).
They return the length of the total string they tried to
create.
Check BUGS before using these functions.
strtcpy() and stpecpy() are better alternatives to these
functions.
stpncpy(3)
Copy the input string into a destination null-padded
character sequence in a fixed-size buffer. If the
destination buffer, limited by its size, isn't large
enough to hold the copy, the resulting character sequence
is truncated. Since it creates a character sequence, it
doesn't need to write a terminating null character. 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.
strncpy(3)
This function is identical to stpncpy(3) except for the
useless return value.
stpncpy(3) is a more useful alternative to this function.
strncat(3)
Catenate the input character sequence, contained in a
null-padded fixed-size buffer, into a destination string.
The programmer is responsible for allocating a buffer
large enough. The return value is useless.
Do not confuse this function with strncpy(3); they are not
related at all.
stpcpy(mempcpy(dst, src, strnlen(src, NITEMS(src))), "")
is a faster alternative to this function.
strndup(3)
Duplicate the input character sequence, contained in a
null-padded fixed-size buffer, into a newly allocated
destination string.
The string must be freed with free(3).
mempcpy(3)
Copy the input character sequence, limited by its length,
into a destination character sequence. The programmer is
responsible for allocating a buffer large enough. It
returns a pointer suitable for chaining.
stpcpy(3)
A pointer to the terminating null character in the
destination string.
stpecpy()
A pointer to the terminating null character in the
destination string, on success. On error, NULL is
returned, and errno is set to indicate the error.
mempcpy(3)
stpncpy(3)
A pointer to one after the last character in the
destination character sequence.
strtcpy()
The length of the string, on success. On error, -1 is
returned, and errno is set to indicate the error.
strlcpy(3bsd)
strlcat(3bsd)
The length of the total string that they tried to create
(as if truncation didn't occur).
strcpy(3)
strcat(3)
strncpy(3)
strncat(3)
The dst pointer, which is useless.
strndup(3)
The newly allocated string.
Most of these functions don't set errno.
stpecpy()
strtcpy()
ENOBUFS
dsize was 0.
E2BIG The string has been truncated.
strndup(3)
ENOMEM Insufficient memory available to allocate duplicate
string.
The Linux kernel has an internal function for copying strings,
strscpy(9), which is identical to strtcpy(), except that it
returns -E2BIG instead of -1 and it doesn't set errno.
Don't mix chain calls to truncating and non-truncating functions.
It is conceptually wrong unless you know that the first part of a
copy will always fit. Anyway, the performance difference will
probably be negligible, so it will probably be more clear if you
use consistent semantics: either truncating or non-truncating.
Calling a non-truncating function after a truncating one is
necessarily wrong.
All catenation functions share the same performance problem:
Shlemiel the painter
⟨https://www.joelonsoftware.com/2001/12/11/back-to-basics/⟩. As a
mitigation, compilers are able to transform some calls to
catenation functions into normal copy functions, since
strlen(dst) is usually a byproduct of the previous copy.
strlcpy(3) and strlcat(3) need to read the entire src string,
even if the destination buffer is small. This makes them
vulnerable to Denial of Service (DoS) attacks if an attacker can
control the length of the src string. And if not, they're still
unnecessarily slow.
The following are examples of correct use of each of these
functions.
stpcpy(3)
p = buf;
p = stpcpy(p, "Hello ");
p = stpcpy(p, "world");
p = stpcpy(p, "!");
len = p - buf;
puts(buf);
strcpy(3)
strcat(3)
strcpy(buf, "Hello ");
strcat(buf, "world");
strcat(buf, "!");
len = strlen(buf);
puts(buf);
stpecpy()
end = buf + NITEMS(buf);
p = buf;
p = stpecpy(p, end, "Hello ");
p = stpecpy(p, end, "world");
p = stpecpy(p, end, "!");
if (p == NULL) {
len = NITEMS(buf) - 1;
goto toolong;
}
len = p - buf;
puts(buf);
strtcpy()
len = strtcpy(buf, "Hello world!", NITEMS(buf));
if (len == -1)
goto toolong;
puts(buf);
strlcpy(3bsd)
strlcat(3bsd)
if (strlcpy(buf, "Hello ", NITEMS(buf)) >= NITEMS(buf))
goto toolong;
if (strlcat(buf, "world", NITEMS(buf)) >= NITEMS(buf))
goto toolong;
len = strlcat(buf, "!", NITEMS(buf));
if (len >= NITEMS(buf))
goto toolong;
puts(buf);
stpncpy(3)
p = stpncpy(u->ut_user, "alx", NITEMS(u->ut_user));
if (NITEMS(u->ut_user) < strlen("alx"))
goto toolong;
len = p - u->ut_user;
fwrite(u->ut_user, 1, len, stdout);
strncpy(3)
strncpy(u->ut_user, "alx", NITEMS(u->ut_user));
if (NITEMS(u->ut_user) < strlen("alx"))
goto toolong;
len = strnlen(u->ut_user, NITEMS(u->ut_user));
fwrite(u->ut_user, 1, len, stdout);
mempcpy(dst, src, strnlen(src, NITEMS(src)))
char buf[NITEMS(u->ut_user)];
p = buf;
p = mempcpy(p, u->ut_user, strnlen(u->ut_user, NITEMS(u->ut_user)));
len = p - buf;
fwrite(buf, 1, len, stdout);
stpcpy(mempcpy(dst, src, strnlen(src, NITEMS(src))), "")
char buf[NITEMS(u->ut_user) + 1];
p = buf;
p = mempcpy(p, u->ut_user, strnlen(u->ut_user, NITEMS(u->ut_user)));
p = stpcpy(p, "");
len = p - buf;
puts(buf);
strncat(3)
char buf[NITEMS(u->ut_user) + 1];
strcpy(buf, "");
strncat(buf, u->ut_user, NITEMS(u->ut_user));
len = strlen(buf);
puts(buf);
strndup(3)
buf = strndup(u->ut_user, NITEMS(u->ut_user));
len = strlen(buf);
puts(buf);
free(buf);
mempcpy(3)
p = buf;
p = mempcpy(p, "Hello ", 6);
p = mempcpy(p, "world", 5);
p = mempcpy(p, "!", 1);
len = p - buf;
fwrite(buf, 1, len, stdout);
stpcpy(mempcpy(dst, src, len), "")
p = buf;
p = mempcpy(p, "Hello ", 6);
p = mempcpy(p, "world", 5);
p = mempcpy(p, "!", 1);
p = stpcpy(p, "");
len = p - buf;
puts(buf);
Implementations
Here are reference implementations for functions not provided by
libc.
/* This code is in the public domain. */
char *
stpecpy(char *dst, char end[0], const char *restrict src)
{
size_t dlen;
if (dst == NULL)
return NULL;
dlen = strtcpy(dst, src, end - dst);
return (dlen == -1) ? NULL : dst + dlen;
}
ssize_t
strtcpy(char *restrict dst, const char *restrict src, size_t dsize)
{
bool trunc;
size_t dlen, slen;
if (dsize == 0) {
errno = ENOBUFS;
return -1;
}
slen = strnlen(src, dsize);
trunc = (slen == dsize);
dlen = slen - trunc;
stpcpy(mempcpy(dst, src, dlen), "");
if (trunc)
errno = E2BIG;
return trunc ? -1 : slen;
}
bzero(3), memcpy(3), memccpy(3), mempcpy(3), stpcpy(3),
strlcpy(3bsd), strncat(3), stpncpy(3), string(3)
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Linux man-pages 6.9.1 2024-05-14 string_copying(7)
Pages that refer to this page: stpncpy(3), strcpy(3), strncat(3)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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