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NAME
stpcpy, strcpy, strcat, stpecpy, strlcpy, strlcat, stpncpy, strncpy, zustr2ustp,
zustr2stp, strncat, ustpcpy, ustr2stp - copying strings and character sequences
SYNOPSIS
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.
size_t strlcpy(char dst[restrict .sz], const char *restrict src,
size_t sz);
size_t strlcat(char dst[restrict .sz], const char *restrict src,
size_t sz);
Null-padded character sequences
// Zero a fixed-width buffer, and
// copy a string into a character sequence with truncation.
char *stpncpy(char dst[restrict .sz], const char *restrict src,
size_t sz);
// Zero a fixed-width buffer, and
// copy a string into a character sequence with truncation.
char *strncpy(char dest[restrict .sz], const char *restrict src,
size_t sz);
// Chain-copy a null-padded character sequence into a character sequence.
char *zustr2ustp(char *restrict dst, const char src[restrict .sz],
size_t sz);
// Chain-copy a null-padded character sequence into a string.
char *zustr2stp(char *restrict dst, const char src[restrict .sz],
size_t sz);
// Catenate a null-padded character sequence into a string.
char *strncat(char *restrict dst, const char src[restrict .sz],
size_t sz);
Measured character sequences
// Chain-copy a measured character sequence.
char *ustpcpy(char *restrict dst, const char src[restrict .len],
size_t len);
// Chain-copy a measured character sequence into a string.
char *ustr2stp(char *restrict dst, const char src[restrict .len],
size_t len);
DESCRIPTION
Terms (and abbreviations)
string (str)
is a sequence of zero or more non-null characters followed by a null byte.
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 (zustr)
Character sequences can be contained in fixed-width 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.
measured character sequence (ustr)
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(ustr, sz).
size (sz)
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[sz]. 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 byte 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 byte 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 byte (or one
after the last character in a character sequence) after the call, so that the
programmer can use it to chain such calls.
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 byte 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, however, it makes sense to truncate. Remember to check the
return value of such function calls.
Functions that truncate:
• stpecpy(3) is the most efficient string copy function that performs truncation. It
only requires to check for truncation once after all chained calls.
• strlcpy(3bsd) and strlcat(3bsd) are designed to crash if the input string is invalid
(doesn't contain a terminating null byte).
• 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, such as utmpx(5), use null-padded character
sequences in fixed-width buffers. To interface with them, specialized functions need to
be used.
To copy strings into them, use stpncpy(3).
To copy from an unterminated string within a fixed-width buffer into a string, ignoring
any trailing null bytes in the source fixed-width buffer, you should use zustr2stp(3) or
strncat(3).
To copy from an unterminated string within a fixed-width buffer into a character sequence,
ignoring any trailing null bytes in the source fixed-width buffer, you should use
zustr2ustp(3).
Measured 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 byte if you need a string.
However, for keeping type safety, it's good to add a wrapper that uses char * instead of
void *: ustpcpy(3).
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 measured character sequence, use ustpcpy(3).
To copy a measured character sequence into a string, use ustr2stp(3).
Because these functions ask for the length, and a string is by nature composed of a
character sequence of the same length plus a terminating null byte, a string is also
accepted as input.
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(3)
• 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:
• zustr2stp(3)
• strncat(3)
• ustr2stp(3)
Other functions operate on an input character sequence to create an output character
sequence. List of functions:
• ustpcpy(3)
• zustr2stp(3)
Functions
stpcpy(3)
This function copies 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)
These functions 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(3)
This function copies 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.
strlcpy(3bsd)
strlcat(3bsd)
These functions 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. These functions
force a SIGSEGV if the src pointer is not a string.
stpecpy(3) is a simpler alternative to these functions.
stpncpy(3)
This function copies the input string into a destination null-padded character
sequence in a fixed-width 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
byte. 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.
zustr2ustp(3)
This function copies the input character sequence contained in a null-padded wixed-
width buffer, into a destination character sequence. The programmer is responsible
for allocating a buffer large enough. It returns a pointer suitable for chaining.
A truncating version of this function doesn't exist, since the size of the original
character sequence is always known, so it wouldn't be very useful.
This function is not provided by any library; See EXAMPLES for a reference
implementation.
zustr2stp(3)
This function copies the input character sequence contained in a null-padded wixed-
width buffer, into a destination string. The programmer is responsible for
allocating a buffer large enough. It returns a pointer suitable for chaining.
A truncating version of this function doesn't exist, since the size of the original
character sequence is always known, so it wouldn't be very useful.
This function is not provided by any library; See EXAMPLES for a reference
implementation.
strncat(3)
Do not confuse this function with strncpy(3); they are not related at all.
This function catenates the input character sequence contained in a null-padded
wixed-width buffer, into a destination string. The programmer is responsible for
allocating a buffer large enough. The return value is useless.
zustr2stp(3) is a faster alternative to this function.
ustpcpy(3)
This function copies 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.
ustr2stp(3)
This function copies the input character sequence, limited by its length, into a
destination string. The programmer is responsible for allocating a buffer large
enough. It returns a pointer suitable for chaining.
RETURN VALUE
The following functions return a pointer to the terminating null byte in the destination
string.
• stpcpy(3)
• ustr2stp(3)
• zustr2stp(3)
The following function returns a pointer to the terminating null byte in the destination
string, except when truncation occurs; if truncation occurs, it returns a pointer to the
end of the destination buffer.
• stpecpy(3)
The following function returns a pointer to one after the last character in the
destination character sequence; if truncation occurs, that pointer is equivalent to a
pointer to the end of the destination buffer.
• stpncpy(3)
The following functions return a pointer to one after the last character in the
destination character sequence.
• zustr2ustp(3)
• ustpcpy(3)
The following functions return the length of the total string that they tried to create
(as if truncation didn't occur).
• strlcpy(3bsd), strlcat(3bsd)
The following functions return the dst pointer, which is useless.
• strcpy(3), strcat(3)
• strncpy(3)
• strncat(3)
NOTES
The Linux kernel has an internal function for copying strings, which is similar to
stpecpy(3), except that it can't be chained:
strscpy(9)
This function copies 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). It
returns the length of the destination string, or -E2BIG on truncation.
stpecpy(3) is a simpler and faster alternative to this function.
CAVEATS
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.
BUGS
All catenation functions share the same performance problem: Shlemiel the painter
⟨https://www.joelonsoftware.com/2001/12/11/back-to-basics/⟩.
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(3)
end = buf + sizeof(buf);
p = buf;
p = stpecpy(p, end, "Hello ");
p = stpecpy(p, end, "world");
p = stpecpy(p, end, "!");
if (p == end) {
p--;
goto toolong;
}
len = p - buf;
puts(buf);
strlcpy(3bsd)
strlcat(3bsd)
if (strlcpy(buf, "Hello ", sizeof(buf)) >= sizeof(buf))
goto toolong;
if (strlcat(buf, "world", sizeof(buf)) >= sizeof(buf))
goto toolong;
len = strlcat(buf, "!", sizeof(buf));
if (len >= sizeof(buf))
goto toolong;
puts(buf);
strscpy(9)
len = strscpy(buf, "Hello world!", sizeof(buf));
if (len == -E2BIG)
goto toolong;
puts(buf);
stpncpy(3)
p = stpncpy(buf, "Hello world!", sizeof(buf));
if (sizeof(buf) < strlen("Hello world!"))
goto toolong;
len = p - buf;
for (size_t i = 0; i < sizeof(buf); i++)
putchar(buf[i]);
strncpy(3)
strncpy(buf, "Hello world!", sizeof(buf));
if (sizeof(buf) < strlen("Hello world!"))
goto toolong;
len = strnlen(buf, sizeof(buf));
for (size_t i = 0; i < sizeof(buf); i++)
putchar(buf[i]);
zustr2ustp(3)
p = buf;
p = zustr2ustp(p, "Hello ", 6);
p = zustr2ustp(p, "world", 42); // Padding null bytes ignored.
p = zustr2ustp(p, "!", 1);
len = p - buf;
printf("%.*s\n", (int) len, buf);
zustr2stp(3)
p = buf;
p = zustr2stp(p, "Hello ", 6);
p = zustr2stp(p, "world", 42); // Padding null bytes ignored.
p = zustr2stp(p, "!", 1);
len = p - buf;
puts(buf);
strncat(3)
buf[0] = '\0'; // There's no 'cpy' function to this 'cat'.
strncat(buf, "Hello ", 6);
strncat(buf, "world", 42); // Padding null bytes ignored.
strncat(buf, "!", 1);
len = strlen(buf);
puts(buf);
ustpcpy(3)
p = buf;
p = ustpcpy(p, "Hello ", 6);
p = ustpcpy(p, "world", 5);
p = ustpcpy(p, "!", 1);
len = p - buf;
printf("%.*s\n", (int) len, buf);
ustr2stp(3)
p = buf;
p = ustr2stp(p, "Hello ", 6);
p = ustr2stp(p, "world", 5);
p = ustr2stp(p, "!", 1);
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)
{
char *p;
if (src[strlen(src)] != '\0')
raise(SIGSEGV);
if (dst == end)
return end;
p = memccpy(dst, src, '\0', end - dst);
if (p != NULL)
return p - 1;
/* truncation detected */
end[-1] = '\0';
return end;
}
char *
zustr2ustp(char *restrict dst, const char *restrict src, size_t sz)
{
return ustpcpy(dst, src, strnlen(src, sz));
}
char *
zustr2stp(char *restrict dst, const char *restrict src, size_t sz)
{
char *p;
p = zustr2ustp(dst, src, sz);
*p = '\0';
return p;
}
char *
ustpcpy(char *restrict dst, const char *restrict src, size_t len)
{
return mempcpy(dst, src, len);
}
char *
ustr2stp(char *restrict dst, const char *restrict src, size_t len)
{
char *p;
p = ustpcpy(dst, src, len);
*p = '\0';
return p;
}
SEE ALSO
bzero(3), memcpy(3), memccpy(3), mempcpy(3), stpcpy(3), strlcpy(3bsd), strncat(3),
stpncpy(3), string(3)