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

From cppreference.com
< c‎ | string‎ | byte
(Undo revision 83415 by DevSolar (talk) and reword to maybe make it clearer what this UB note meant)
m (+C17 ref, langlinks, fmt)
 
(8 intermediate revisions by 6 users not shown)
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{{dcl rev end}}
 
{{dcl rev end}}
 
{{dcl | since=c11 |num=2|
 
{{dcl | since=c11 |num=2|
−
errno_t strncpy_s(char *restrict dest, rsize_t destsz,
+
errno_t strncpy_s( char *restrict dest, rsize_t destsz,
−
                  const char *restrict src, rsize_t count);
+
                  const char *restrict src, rsize_t count );
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}
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@2@ Same as {{v|1}}, except that the function does not continue writing zeroes into the destination array to pad up to {{tt|count}}, it stops after writing the terminating null character (if there was no null in the source, it writes one at {{c|dest[count]}} and then stops). Also, the following errors are detected at runtime and call the currently installed [[c/error/set_constraint_handler_s|constraint handler]] function:
 
@2@ Same as {{v|1}}, except that the function does not continue writing zeroes into the destination array to pad up to {{tt|count}}, it stops after writing the terminating null character (if there was no null in the source, it writes one at {{c|dest[count]}} and then stops). Also, 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
 
:* {{tt|src}} or {{tt|dest}} is a null pointer
−
:* {{tt|destsz}} or {{tt|count}} is zero or greater than {{lc|RSIZE_MAX}}
+
:* {{tt|destsz}} is zero or greater than {{lc|RSIZE_MAX}}
 +
:* {{tt|count}} is greater than {{lc|RSIZE_MAX}}
 
:* {{tt|count}} is greater or equal {{tt|destsz}}, but {{tt|destsz}} is less or equal {{c|strnlen_s(src, count)}}, in other words, truncation would occur
 
:* {{tt|count}} is greater or equal {{tt|destsz}}, but {{tt|destsz}} is less or equal {{c|strnlen_s(src, count)}}, in other words, truncation would occur
 
:* overlap would occur between the source and the destination strings
 
:* 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|strnlen_s(src, destsz)}} <= {{tt|destsz}}; in other words, an erroneous value of {{tt|destsz}} does not expose the impending buffer overflow. The behavior is undefined if the size of the character array pointed to by {{tt|src}} < {{c|strnlen_s(src, count)}} < {{tt|destsz}}; in other words, an erroneous value of {{tt|count}} does not expose the impending buffer overflow.
 
@@The behavior is undefined if the size of the character array pointed to by {{tt|dest}} < {{c|strnlen_s(src, destsz)}} <= {{tt|destsz}}; in other words, an erroneous value of {{tt|destsz}} does not expose the impending buffer overflow. The behavior is undefined if the size of the character array pointed to by {{tt|src}} < {{c|strnlen_s(src, count)}} < {{tt|destsz}}; in other words, an erroneous value of {{tt|count}} does not expose the impending buffer overflow.
−
 
+
:{{c/ext1 availability|strncpy_s}}
−
:As all bounds-checked functions, {{tt|strncpy_s}} is only guaranteed to be available if {{c|__STDC_LIB_EXT1__}} is defined by the implementation and if the user defines {{c|__STDC_WANT_LIB_EXT1__}} to the integer constant {{c|1}} before including {{tt|string.h}}.
+
  
 
===Parameters===
 
===Parameters===
Line 57: Line 57:
 
#include <stdio.h>
 
#include <stdio.h>
 
#include <stdlib.h>
 
#include <stdlib.h>
 +
#include <errno.h>
  
 
int main(void)
 
int main(void)
Line 64: Line 65:
 
     strncpy(dest, src, 5); // writes five characters 'h', 'i', '\0', '\0', '\0' to dest
 
     strncpy(dest, src, 5); // writes five characters 'h', 'i', '\0', '\0', '\0' to dest
 
     printf("strncpy(dest, src, 5) to a 6-byte dest gives : ");
 
     printf("strncpy(dest, src, 5) to a 6-byte dest gives : ");
−
     for(size_t n = 0; n < sizeof dest; ++n) {
+
     for (size_t n = 0; n < sizeof dest; ++n) {
 
         char c = dest[n];
 
         char c = dest[n];
 
         c ? printf("'%c' ", c) : printf("'\\0' ");
 
         c ? printf("'%c' ", c) : printf("'\\0' ");
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     char dest2[2];
 
     char dest2[2];
 
     strncpy(dest2, src, 2); // truncation: writes two characters 'h', 'i', to dest2
 
     strncpy(dest2, src, 2); // truncation: writes two characters 'h', 'i', to dest2
−
     for(size_t n = 0; n < sizeof dest2; ++n) {
+
     for (size_t n = 0; n < sizeof dest2; ++n) {
 
         char c = dest2[n];
 
         char c = dest2[n];
 
         c ? printf("'%c' ", c) : printf("'\\0' ");
 
         c ? printf("'%c' ", c) : printf("'\\0' ");
Line 81: Line 82:
 
     set_constraint_handler_s(ignore_handler_s);
 
     set_constraint_handler_s(ignore_handler_s);
 
     char dst1[6], src1[100] = "hello";
 
     char dst1[6], src1[100] = "hello";
−
     int r1 = strncpy_s(dst1, 6, src1, 100);     // writes 0 to r1, 6 characters to dst1
+
     errno_t r1 = strncpy_s(dst1, 6, src1, 100); // writes 0 to r1, 6 characters to dst1
 
     printf("dst1 = \"%s\", r1 = %d\n", dst1,r1); // 'h','e','l','l','o','\0' to dst1
 
     printf("dst1 = \"%s\", r1 = %d\n", dst1,r1); // 'h','e','l','l','o','\0' to dst1
  
 
     char dst2[5], src2[7] = {'g','o','o','d','b','y','e'};
 
     char dst2[5], src2[7] = {'g','o','o','d','b','y','e'};
−
     int r2 = strncpy_s(dst2, 5, src2, 7);       // copy overflows the destination array
+
     errno_t r2 = strncpy_s(dst2, 5, src2, 7);   // copy overflows the destination array
 
     printf("dst2 = \"%s\", r2 = %d\n", dst2,r2); // writes nonzero to r2,'\0' to dst2[0]
 
     printf("dst2 = \"%s\", r2 = %d\n", dst2,r2); // writes nonzero to r2,'\0' to dst2[0]
  
 
     char dst3[5];
 
     char dst3[5];
−
     int r3 = strncpy_s(dst3, 5, src2, 4);       // writes 0 to r3, 5 characters to dst3
+
     errno_t r3 = strncpy_s(dst3, 5, src2, 4);   // writes 0 to r3, 5 characters to dst3
−
     printf("dst3 = \"%s\", r3 = %d\n", dst3,r3); // 'g', 'o', 'o', 'd', '\0' to dst3  
+
     printf("dst3 = \"%s\", r3 = %d\n", dst3,r3); // 'g', 'o', 'o', 'd', '\0' to dst3
 
#endif
 
#endif
 
}
 
}
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===References===
 
===References===
 +
{{ref std c17}}
 +
{{ref std | section=7.24.2.4  | title=The strncpy function | p=265}}
 +
{{ref std | section=K.3.7.1.4 | title=The strncpy_s function | p=447-448}}
 +
{{ref std end}}
 
{{ref std c11}}
 
{{ref std c11}}
 
{{ref std | section=7.24.2.4  | title=The strncpy function | p=363-364}}
 
{{ref std | section=7.24.2.4  | title=The strncpy function | p=363-364}}
 
{{ref std | section=K.3.7.1.4 | title=The strncpy_s function | p=616-617}}
 
{{ref std | section=K.3.7.1.4 | title=The strncpy_s function | p=616-617}}
 +
{{ref std end}}
 
{{ref std c99}}
 
{{ref std c99}}
 
{{ref std | section=7.21.2.4  | title=The strncpy function | p=326-327}}
 
{{ref std | section=7.21.2.4  | title=The strncpy function | p=326-327}}
 +
{{ref std end}}
 
{{ref std c89}}
 
{{ref std c89}}
 
{{ref std | section=4.11.2.4  | title=The strncpy function}}
 
{{ref std | section=4.11.2.4  | title=The strncpy function}}
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{{dsc inc | c/string/byte/dsc strcpy}}
 
{{dsc inc | c/string/byte/dsc strcpy}}
 
{{dsc inc | c/string/byte/dsc memcpy}}
 
{{dsc inc | c/string/byte/dsc memcpy}}
 +
{{dsc inc | c/experimental/dynamic/dsc strndup}}
 
{{dsc see cpp | cpp/string/byte/strncpy}}
 
{{dsc see cpp | cpp/string/byte/strncpy}}
 
{{dsc end}}
 
{{dsc end}}
  
−
[[ar:c/string/byte/strncpy]]
+
{{langlinks|ar|cs|de|es|fr|it|ja|ko|pl|pt|ru|tr|zh}}
−
[[cs:c/string/byte/strncpy]]
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[[de:c/string/byte/strncpy]]
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[[es:c/string/byte/strncpy]]
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[[fr:c/string/byte/strncpy]]
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[[it:c/string/byte/strncpy]]
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[[ja:c/string/byte/strncpy]]
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[[ko:c/string/byte/strncpy]]
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[[pl:c/string/byte/strncpy]]
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[[pt:c/string/byte/strncpy]]
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[[ru:c/string/byte/strncpy]]
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[[tr:c/string/byte/strncpy]]
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[[zh:c/string/byte/strncpy]]
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Latest revision as of 12:17, 27 June 2022

 
C
 
Strings library
 
Null-terminated byte strings
Functions
Character manipulation
Conversions to and from numeric formats
String manipulation
(C11)
strncpystrncpy_s
(C11)
(C11)
String examination
Memory manipulation
Miscellaneous
(C11)(C11)
 
Defined in header <string.h>
(1)
char *strncpy( char *dest, const char *src, size_t count );
 (until C99)
char *strncpy( char *restrict dest, const char *restrict src, size_t count );
 (since C99)
errno_t strncpy_s( char *restrict dest, rsize_t destsz,
                   const char *restrict src, rsize_t count );
 (2) (since C11)
1) Copies at most count characters of the character array pointed to by src (including the terminating null character, but not any of the characters that follow the null character) to character array pointed to by dest.
If count is reached before the entire array src was copied, the resulting character array is not null-terminated.
If, after copying the terminating null character from src, count is not reached, additional null characters are written to dest until the total of count characters have been written.
The behavior is undefined if the character arrays overlap, if either dest or src is not a pointer to a character array (including if dest or src is a null pointer), if the size of the array pointed to by dest is less than count, or if the size of the array pointed to by src is less than count and it does not contain a null character.
2) Same as (1), except that the function does not continue writing zeroes into the destination array to pad up to count, it stops after writing the terminating null character (if there was no null in the source, it writes one at dest[count] and then stops). Also, 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
  • count is greater than RSIZE_MAX
  • count is greater or equal destsz, but destsz is less or equal strnlen_s(src, count), in other words, truncation would occur
  • 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 < strnlen_s(src, destsz) <= destsz; in other words, an erroneous value of destsz does not expose the impending buffer overflow. The behavior is undefined if the size of the character array pointed to by src < strnlen_s(src, count) < destsz; in other words, an erroneous value of count does not expose the impending buffer overflow.
As with all bounds-checked functions, strncpy_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 character array to copy to
src - pointer to the character array to copy from
count - maximum number of characters to copy
destsz - 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) and may clobber the rest of the destination array with unspecified values.

[edit] Notes

As corrected by the post-C11 DR 468, strncpy_s, unlike strcpy_s, is only allowed to clobber the remainder of the destination array if an error occurs.

Unlike strncpy, strncpy_s does not pad the destination array with zeroes, This is a common source of errors when converting existing code to the bounds-checked version.

Although truncation to fit the destination buffer is a security risk and therefore a runtime constraints violation for strncpy_s, it is possible to get the truncating behavior by specifying count equal to the size of the destination array minus one: it will copy the first count bytes and append the null terminator as always: strncpy_s(dst, sizeof dst, src, (sizeof dst)-1);

[edit] Example

Run this code
#define __STDC_WANT_LIB_EXT1__ 1
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
 
int main(void)
{
    char src[] = "hi";
    char dest[6] = "abcdef"; // no null terminator
    strncpy(dest, src, 5); // writes five characters 'h', 'i', '\0', '\0', '\0' to dest
    printf("strncpy(dest, src, 5) to a 6-byte dest gives : ");
    for (size_t n = 0; n < sizeof dest; ++n) {
        char c = dest[n];
        c ? printf("'%c' ", c) : printf("'\\0' ");
    }
 
    printf("\nstrncpy(dest2, src, 2) to a 2-byte dst gives : ");
    char dest2[2];
    strncpy(dest2, src, 2); // truncation: writes two characters 'h', 'i', to dest2
    for (size_t n = 0; n < sizeof dest2; ++n) {
        char c = dest2[n];
        c ? printf("'%c' ", c) : printf("'\\0' ");
    }
    printf("\n");
 
#ifdef __STDC_LIB_EXT1__
    set_constraint_handler_s(ignore_handler_s);
    char dst1[6], src1[100] = "hello";
    errno_t r1 = strncpy_s(dst1, 6, src1, 100);  // writes 0 to r1, 6 characters to dst1
    printf("dst1 = \"%s\", r1 = %d\n", dst1,r1); // 'h','e','l','l','o','\0' to dst1
 
    char dst2[5], src2[7] = {'g','o','o','d','b','y','e'};
    errno_t r2 = strncpy_s(dst2, 5, src2, 7);    // copy overflows the destination array
    printf("dst2 = \"%s\", r2 = %d\n", dst2,r2); // writes nonzero to r2,'\0' to dst2[0]
 
    char dst3[5];
    errno_t r3 = strncpy_s(dst3, 5, src2, 4);    // writes 0 to r3, 5 characters to dst3
    printf("dst3 = \"%s\", r3 = %d\n", dst3,r3); // 'g', 'o', 'o', 'd', '\0' to dst3
#endif
}

Possible output: 

strncpy(dest, src, 5) to a 6-byte dst gives : 'h' 'i' '\0' '\0' '\0' 'f'
strncpy(dest2, src, 2) to a 2-byte dst gives : 'h' 'i'
dst1 = "hello", r1 = 0
dst2 = "", r2 = 22
dst3 = "good", r3 = 0

[edit] References

  • C17 standard (ISO/IEC 9899:2018):
  • 7.24.2.4 The strncpy function (p: 265)
  • K.3.7.1.4 The strncpy_s function (p: 447-448)
  • C11 standard (ISO/IEC 9899:2011):
  • 7.24.2.4 The strncpy function (p: 363-364)
  • K.3.7.1.4 The strncpy_s function (p: 616-617)
  • C99 standard (ISO/IEC 9899:1999):
  • 7.21.2.4 The strncpy function (p: 326-327)
  • C89/C90 standard (ISO/IEC 9899:1990):
  • 4.11.2.4 The strncpy function

[edit] See also

(C11)
 copies one string to another
(function) [edit]
(C11)
 copies one buffer to another
(function) [edit]
(dynamic memory TR)
 allocate a copy of a string up to specified size
(function) [edit]
C++ documentation for strncpy
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