strcat, strcat_s
Defined in header <string.h> |
||
---|---|---|
(1) | ||
char *strcat( char *dest, const char *src ); |
(until C99) | |
char *strcat( char *restrict dest, const char *restrict src ); |
(since C99) | |
errno_t strcat_s(char *restrict dest, rsize_t destsz, const char *restrict src); |
(2) | (since C11) |
1) Appends a copy of the null-terminated byte string pointed to by
src
to the end of the null-terminated byte string pointed to by
dest
. The character
src[0]
replaces the null terminator at the end of
dest
. The resulting byte string is null-terminated.
The behavior is undefined if the destination array is not large enough for the contents of both
src
and
dest
and the terminating null character. The behavior is undefined if the strings overlap. The behavior is undefined if either
dest
or
src
is not a pointer to a null-terminated byte string.
2) Same as
(1), except that it may clobber the rest of the destination array (from the last character written to
destsz
) with unspecified values and that the following errors are detected at runtime and call the currently installed
constraint handler function:
-
src
ordest
is a null pointer -
destsz
is zero or greater thanRSIZE_MAX
- there is no null terminator in the first
destsz
bytes ofdest
- truncation would occur (the available space at the end of
dest
would not fit every character, including the null terminator, ofsrc
) - overlap would occur between the source and the destination strings
The behavior is undefined if the size of the character array pointed to by
dest
<
strlen(dest)+strlen(src)+1
<=
destsz
; in other words, an erroneous value of
destsz
does not expose the impending buffer overflow. As with all bounds-checked functions,
strcat_s
is only guaranteed to be available if
__STDC_LIB_EXT1__
is defined by the implementation and if the user defines
__STDC_WANT_LIB_EXT1__
to the integer constant 1 before including
string.h
.
Parameters
dest | - | pointer to the null-terminated byte string to append to |
src | - | pointer to the null-terminated byte string to copy from |
destsz | - | maximum number of characters to write, typically the size of the destination buffer |
Return value
1) returns a copy of
dest
2) returns zero on success, returns non-zero on error. Also, on error, writes zero to
dest[0]
(unless
dest
is a null pointer or
destsz
is zero or greater than
RSIZE_MAX
).
Notes
Because strcat
needs to seek to the end of dest
on each call, it is inefficient to concatenate many strings into one using strcat
.
strcat_s
is allowed to clobber the destination array from the last character written up to destsz
in order to improve efficiency: it may copy in multibyte blocks and then check for null bytes.
The function strcat_s
is similar to the BSD function strlcat
, except that.
-
strlcat
truncates the source string to fit in the destination -
strlcat
does not perform all the runtime checks thatstrcat_s
does -
strlcat
does not make failures obvious by setting the destination to a null string or calling a handler if the call fails.
Although strcat_s
prohibits truncation due to potential security risks, it's possible to truncate a string using bounds-checked strncat_s
instead.
Example
#define __STDC_WANT_LIB_EXT1__ 1 #include <string.h> #include <stdio.h> #include <stdlib.h> int main(void) { char str[50] = "Hello "; char str2[50] = "World!"; strcat(str, str2); strcat(str, " ..."); strcat(str, " Goodbye World!"); puts(str); #ifdef __STDC_LIB_EXT1__ set_constraint_handler_s(ignore_handler_s); int r = strcat_s(str, sizeof str, " ... "); printf("str = \"%s\", r = %d\n", str, r); r = strcat_s(str, sizeof str, " and this is too much"); printf("str = \"%s\", r = %d\n", str, r); #endif }
Possible output:
Hello World! ... Goodbye World! str = "Hello World! ... Goodbye World! ... ", r = 0 str = "", r = 22
References
- C11 standard (ISO/IEC 9899:2011):
- 7.24.3.1 The strcat function (p: 364)
- K.3.7.2.1 The strcat_s function (p: 617-618)
- C99 standard (ISO/IEC 9899:1999):
- 7.21.3.1 The strcat function (p: 327)
- C89/C90 standard (ISO/IEC 9899:1990):
- 4.11.3.1 The strcat function
See also
(C11)
|
concatenates a certain amount of characters of two strings (function) |
(C11)
|
copies one string to another (function) |
(C23)
|
copies one buffer to another, stopping after the specified delimiter (function) |
C++ documentation for strcat |
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