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Difference between revisions of "c/string/byte/strcat"

From cppreference.com
< c‎ | string‎ | byte
m (Shorten template names. Use {{lc}} where appropriate.)
m (→‎See also: add `memccpy` as an alternative to `strcat` for chaining C-string concatenation)
 
(15 intermediate revisions by 8 users not shown)
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−
{{c/title| strcat}}
+
{{c/title| strcat|strcat_s}}
 
{{c/string/byte/navbar}}
 
{{c/string/byte/navbar}}
 
{{dcl begin}}
 
{{dcl begin}}
 
{{dcl header | string.h}}
 
{{dcl header | string.h}}
−
{{dcl | notes={{mark until c99}} |
+
{{dcl rev begin|num=1}}
−
char *strcat( char         *dest, const char         *src );
+
{{dcl | until=c99 |
 +
char *strcat( char *dest, const char *src );
 
}}
 
}}
−
{{dcl | notes={{mark since c99}} |
+
{{dcl | since=c99 |
 
char *strcat( char *restrict dest, const char *restrict src );
 
char *strcat( char *restrict dest, const char *restrict src );
 +
}}
 +
{{dcl rev end}}
 +
{{dcl | since=c11 | num=2 |
 +
errno_t strcat_s(char *restrict dest, rsize_t destsz, const char *restrict src);
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}
  
−
Appends a byte string pointed to by {{tt|src}} to a byte string pointed to by {{tt|dest}}. The resulting byte string is null-terminated.  
+
@1@ Appends a copy of the null-terminated byte string pointed to by {{tt|src}} to the end of the null-terminated byte string pointed to by {{tt|dest}}. The character {{tt|src[0]}} replaces the null terminator at the end of {{tt|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 {{tt|src}} and {{tt|dest}} and the terminating null character. The behavior is undefined if the strings overlap. The behavior is undefined if either {{tt|dest}} or {{tt|src}} is not a pointer to a null-terminated byte string.
−
The destination byte string must be large enough for the contents of both {{tt|str}} and {{tt|dest}} and the terminating null character.
+
@2@ Same as {{v|1}}, except that it may clobber the rest of the destination array (from the last character written to {{tt|destsz}}) with unspecified values and that the following errors are detected at runtime and call the currently installed [[c/error/set_constraint_handler_s|constraint handler]] function:
−
 
+
:* {{tt|src}} or {{tt|dest}} is a null pointer
−
The behavior is undefined if the strings overlap.  
+
:* {{tt|destsz}} is zero or greater than {{lc|RSIZE_MAX}}
 +
:* there is no null terminator in the first {{tt|destsz}} bytes of {{tt|dest}}
 +
:* truncation would occur (the available space at the end of {{tt|dest}} would not fit every character, including the null terminator, of {{tt|src}})
 +
:* overlap would occur between the source and the destination strings
 +
@@The behavior is undefined if the size of the character array pointed to by {{tt|dest}} < {{c|strlen(dest)+strlen(src)+1}} <= {{tt|destsz}}; in other words, an erroneous value of {{tt|destsz}} does not expose the impending buffer overflow.
 +
:{{c/ext1 availability|strcat_s}}
  
 
===Parameters===
 
===Parameters===
Line 21: Line 31:
 
{{par | dest | pointer to the null-terminated byte string to append to}}
 
{{par | dest | pointer to the null-terminated byte string to append to}}
 
{{par | src | pointer to the null-terminated byte string to copy from}}
 
{{par | src | pointer to the null-terminated byte string to copy from}}
 +
{{par | destsz | maximum number of characters to write, typically the size of the destination buffer}}
 
{{par end}}
 
{{par end}}
  
 
===Return value===
 
===Return value===
−
{{tt|dest}}
+
@1@ returns a copy of {{tt|dest}}
 +
@2@ returns zero on success, returns non-zero on error. Also, on error, writes zero to {{c|dest[0]}} (unless {{tt|dest}} is a null pointer or {{tt|destsz}} is zero or greater than {{lc|RSIZE_MAX}}).
 +
 
 +
===Notes===
 +
Because {{tt|strcat}} needs to seek to the end of {{tt|dest}} on each call, it is inefficient to concatenate many strings into one using {{tt|strcat}}.
 +
 
 +
{{tt|strcat_s}} is allowed to clobber the destination array from the last character written up to {{tt|destsz}} in order to improve efficiency: it may copy in multibyte blocks and then check for null bytes.
 +
 
 +
The function {{tt|strcat_s}} is similar to the [https://www.freebsd.org/cgi/man.cgi?query=strlcat&sektion=3 BSD function {{tt|strlcat}}], except that
 +
* {{tt|strlcat}} truncates the source string to fit in the destination
 +
* {{tt|strlcat}} does not perform all the runtime checks that {{tt|strcat_s}} does
 +
* {{tt|strlcat}} does not make failures obvious by setting the destination to a null string or calling a handler if the call fails.
 +
 
 +
Although {{tt|strcat_s}} prohibits truncation due to potential security risks, it's possible to truncate a string using bounds-checked {{lc|strncat|strncat_s}} instead.
  
 
===Example===
 
===Example===
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  |
 
  |
 
  | code=
 
  | code=
 +
#define __STDC_WANT_LIB_EXT1__ 1
 
#include <string.h>  
 
#include <string.h>  
 
#include <stdio.h>
 
#include <stdio.h>
 +
#include <stdlib.h>
  
−
int main()  
+
int main(void)  
 
{
 
{
 
     char str[50] = "Hello ";
 
     char str[50] = "Hello ";
 
     char str2[50] = "World!";
 
     char str2[50] = "World!";
 
     strcat(str, str2);
 
     strcat(str, str2);
 +
    strcat(str, " ...");
 
     strcat(str, " Goodbye World!");
 
     strcat(str, " Goodbye World!");
 
     puts(str);
 
     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
 
}
 
}
−
| output=
+
|p=true
−
Hello World! Goodbye World!
+
| output=
 +
Hello World! ... Goodbye World!
 +
str = "Hello World! ... Goodbye World! ... ", r = 0
 +
str = "", r = 22
 
}}
 
}}
 +
 +
===References===
 +
{{ref std c11}}
 +
{{ref std | section=7.24.3.1  | title=The strcat function | p=364}}
 +
{{ref std | section=K.3.7.2.1 | title=The strcat_s function | p=617-618}}
 +
{{ref std c99}}
 +
{{ref std | section=7.21.3.1  | title=The strcat function | p=327}}
 +
{{ref std c89}}
 +
{{ref std | section=4.11.3.1 | title=The strcat function }}
 +
{{ref std end}}
  
 
===See also===
 
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
−
{{dsc inc | c/string/byte/dcl list strncat}}
+
{{dsc inc | c/string/byte/dsc strncat}}
−
{{dsc inc | c/string/byte/dcl list strcpy}}
+
{{dsc inc | c/string/byte/dsc strcpy}}
 +
{{dsc inc | c/string/byte/dsc memccpy}}
 
{{dsc see cpp | cpp/string/byte/strcat}}
 
{{dsc see cpp | cpp/string/byte/strcat}}
 
{{dsc end}}
 
{{dsc end}}
  
−
[[de:c/string/byte/strcat]]
+
{{langlinks|ar|cs|de|es|fr|it|ja|ko|pl|pt|ru|tr|zh}}
−
[[es:c/string/byte/strcat]]
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−
[[fr:c/string/byte/strcat]]
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−
[[it:c/string/byte/strcat]]
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−
[[ja:c/string/byte/strcat]]
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−
[[pt:c/string/byte/strcat]]
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−
[[ru:c/string/byte/strcat]]
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−
[[zh:c/string/byte/strcat]]
+

Latest revision as of 09:08, 14 November 2020

 
C
 
Strings library
 
Null-terminated byte strings
Functions
Character manipulation
Conversions to and from numeric formats
String manipulation
(C11)
(C11)
strcatstrcat_s
(C11)
String examination
Memory manipulation
Miscellaneous
(C11)(C11)
 
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 or dest is a null pointer
  • destsz is zero or greater than RSIZE_MAX
  • there is no null terminator in the first destsz bytes of dest
  • truncation would occur (the available space at the end of dest would not fit every character, including the null terminator, of src)
  • 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>.

Contents

[edit] 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

[edit] 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).

[edit] 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 that strcat_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.

[edit] Example

Run this code
#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

[edit] 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

[edit] See also

(C11)
 concatenates a certain amount of characters of two strings
(function) [edit]
(C11)
 copies one string to another
(function) [edit]
(C23)
 copies one buffer to another, stopping after the specified delimiter
(function) [edit]
C++ documentation for strcat
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