string_copying(7) Miscellaneous Information Manual string_copying(7)
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 re-
quired. 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 buf-
fers, which contain padding null bytes after the charac-
ter sequence, to fill the rest of the buffer without af-
fecting 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 se-
quence is contained.
end is the name of a pointer to one past the last element of a buf-
fer. It is equivalent to &str[sz]. It is used as a sentinel
value, to be able to truncate strings or character sequences in-
stead 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 func-
tion 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 algo-
rithmically 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 re-
turn 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 buf-
fers, so they always have the correct size, and truncation is not nec-
essary.
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 fea-
tures be more precise.
When validating user input, however, it makes sense to truncate. Re-
member 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 buf-
fer, 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 re-
quires always knowing the length of your character sequences, for which
structures can be used. It makes the code much faster, since you al-
ways know the length of your character sequences, and can do the mini-
mal copies and length measurements. mempcpy(3) copies character se-
quences, 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 se-
quences, and need the best performance, using overlapping character se-
quences 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 se-
quences.
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 se-
quence 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 desti-
nation 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 re-
turns 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 desti-
nation 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 des-
tination 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 ter-
minating 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 se-
quence. 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 re-
lated 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 program-
mer is responsible for allocating a buffer large enough. It re-
turns 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 re-
sponsible 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 charac-
ter in the destination character sequence; if truncation occurs, that
pointer is equivalent to a pointer to the end of the destination buf-
fer.
• stpncpy(3)
The following functions return a pointer to one after the last charac-
ter 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 se-
mantics: 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)
Linux man-pages 6.03 2023-02-05 string_copying(7)
Generated by dwww version 1.15 on Sun Jun 23 21:31:17 CEST 2024.