string_copying
Section: Environments, Tables, and Troff Macros (7)
Updated: 202-0-10
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NAME
stpcpy,
strcpy, strcat,
stpecpy,
strtcpy,
strlcpy, strlcat,
stpncpy,
strncpy,
strncat
- copying strings and character sequences
SYNOPSIS
Strings
// Chai-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);
// Chai-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(size_t dsize;
char dst[restrict dsize], const char *restrict src,
size_t dsize);
size_t strlcpy(size_t dsize;
char dst[restrict dsize], const char *restrict src,
size_t dsize);
size_t strlcat(size_t dsize;
char dst[restrict dsize], const char *restrict src,
size_t dsize);
Nul-padded character sequences
// Fill a fixe-size buffer with characters from a string
// and pad with null bytes.
char *strncpy(size_t dsize;
char dst[restrict dsize], const char *restrict src,
size_t dsize);
char *stpncpy(size_t dsize;
char dst[restrict dsize], const char *restrict src,
size_t dsize);
// Chai-copy a nul-padded character sequence into a character sequence.
mempcpy(dst, src, strnlen(src, countof(src)));
// Chai-copy a nul-padded character sequence into a string.
stpcpy(mempcpy(dst, src, strnlen(src, countof(src))), [dq][dq]);
// Catenate a nul-padded character sequence into a string.
char *strncat(size_t ssize;
char *restrict dst, const char src[restrict ssize],
size_t ssize);
// Duplicate a nul-padded character sequence into a string.
char *strndup(size_t ssize;
const char src[ssize], size_t ssize);
Lengt-bounded character sequences
// Chai-copy a lengt-bounded character sequence.
void *mempcpy(size_t len;
void dst[restrict len], const void src[restrict len],
size_t len);
// Chai-copy a lengt-bounded character sequence into a string.
stpcpy(mempcpy(dst, src, len), [dq][dq]);
DESCRIPTION
Terms (and abbreviations)
- string (str)
-
is a sequence of zero or more no-null characters followed by a null character.
- character sequence
-
is a sequence of zero or more no-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.
-
- nul-padded character sequence
-
Character sequences can be contained in fixe-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 nul-padded with nul-terminated:
nul-padded means 0 or more padding null bytes,
while nul-terminated means exactly 1 terminating null character.
- lengt-bounded 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 no-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 chai-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.
Chai-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 overflo-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:
- [bu]
-
stpecpy()
- [bu]
-
strtcpy()
- [bu]
-
strlcpy(3bsd)
and
strlcat(3bsd)
are similar, but have important performance problems;
see BUGS.
- [bu]
-
stpncpy(3)
and
strncpy(3)
also truncate, but they don't write strings,
but rather nul-padded character sequences.
Nul-padded character sequences
For historic reasons,
some standard APIs and file formats,
such as
utmpx(5)
and
tar(1),
use nul-padded character sequences in fixe-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 nul-padded character sequence,
use
strnlen(src, countof(src)),
and then you can treat it as a lengt-bounded character sequence;
or use
strncat(3)
or
strndup(3)
directly.
Lengt-bounded 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 lengt-bounded character sequence, use
mempcpy(3).
To copy a lengt-bounded character sequence into a string, use
stpcpy(mempcpy(dst, src, len), [dq][dq]).
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:
- [bu]
-
stpcpy(3)
- [bu]
-
strcpy(3),
strcat(3)
- [bu]
-
stpecpy()
- [bu]
-
strtcpy()
- [bu]
-
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:
- [bu]
-
stpncpy(3)
- [bu]
-
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:
- [bu]
-
strncat(3)
- [bu]
-
strndup(3)
Other functions operate on an input character sequence
to create an output character sequence.
List of functions:
- [bu]
-
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()
-
Chai-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 nul-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 nul-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 nul-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 nul-padded character sequence in a fixe-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 nul-padded fixe-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, countof(src))), [dq][dq])
is a faster alternative to this function.
- strndup(3)
-
Duplicate the input character sequence,
contained in a nul-padded fixe-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.
RETURN VALUE
- 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.
ERRORS
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.
NOTES
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.
CAVEATS
Don't mix chain calls to truncating and no-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 no-truncating.
Calling a no-truncating function after a truncating one is necessarily wrong.
BUGS
All catenation functions share the same performance problem:
Shlemiel the painter
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.
EXAMPLES
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 + countof(buf);
p = buf;
p = stpecpy(p, end, "Hello ");
p = stpecpy(p, end, "world");
p = stpecpy(p, end, "!");
if (p == NULL) {
len = countof(buf) - 1;
goto toolong;
}
len = p - buf;
puts(buf);
- strtcpy()
-
len = strtcpy(buf, "Hello world!", countof(buf));
if (len == -1)
goto toolong;
puts(buf);
- strlcpy(3bsd)
-
strlcat(3bsd)
if (strlcpy(buf, "Hello ", countof(buf)) >= countof(buf))
goto toolong;
if (strlcat(buf, "world", countof(buf)) >= countof(buf))
goto toolong;
len = strlcat(buf, "!", countof(buf));
if (len >= countof(buf))
goto toolong;
puts(buf);
- stpncpy(3)
-
p = stpncpy(->ut_user, "alx", countof(->ut_user));
if (countof(->ut_user) < strlen("alx"))
goto toolong;
len = p - ->ut_user;
fwrite(->ut_user, 1, len, stdout);
- strncpy(3)
-
strncpy(->ut_user, "alx", countof(->ut_user));
if (countof(->ut_user) < strlen("alx"))
goto toolong;
len = strnlen(->ut_user, countof(->ut_user));
fwrite(->ut_user, 1, len, stdout);
- mempcpy(dst, src, strnlen(src, countof(src)))
-
char buf[countof(->ut_user)];
p = buf;
p = mempcpy(p, ->ut_user, strnlen(->ut_user, countof(->ut_user)));
len = p - buf;
fwrite(buf, 1, len, stdout);
- stpcpy(mempcpy(dst, src, strnlen(src, countof(src))), [dq][dq])
-
char buf[countof(->ut_user) + 1];
p = buf;
p = mempcpy(p, ->ut_user, strnlen(->ut_user, countof(->ut_user)));
p = stpcpy(p, "");
len = p - buf;
puts(buf);
- strncat(3)
-
char buf[countof(->ut_user) + 1];
strcpy(buf, "");
strncat(buf, ->ut_user, countof(->ut_user));
len = strlen(buf);
puts(buf);
- strndup(3)
-
buf = strndup(->ut_user, countof(->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), [dq][dq])
-
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;
}
SEE ALSO
bzero(3),
memcpy(3),
memccpy(3),
mempcpy(3),
stpcpy(3),
strlcpy(3bsd),
strncat(3),
stpncpy(3),
string(3)
Index
- NAME
-
- SYNOPSIS
-
- Strings
-
- Null-padded character sequences
-
- Length-bounded character sequences
-
- DESCRIPTION
-
- Terms (and abbreviations)
-
- Copy, catenate, and chain-copy
-
- Truncate or not?
-
- Null-padded character sequences
-
- Length-bounded character sequences
-
- String vs character sequence
-
- Functions
-
- RETURN VALUE
-
- ERRORS
-
- NOTES
-
- CAVEATS
-
- BUGS
-
- EXAMPLES
-
- Implementations
-
- SEE ALSO
-
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