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strcpy, strcpy_s

Defined in header `<string.h>`
	(1)
char strcpy( char dest, const char *src );		(until C99)
char strcpy( char restrict dest, const char *restrict src );		(since C99)
errno_t strcpy_s( char restrict dest, rsize_t destsz, const char restrict src );	(2)	(since C11)

1) Copies the null-terminated byte string pointed to by src, including the null terminator, to the character array whose first element is pointed to by dest.

The behavior is undefined if the dest array is not large enough. The behavior is undefined if the strings overlap. The behavior is undefined if either dest is not a pointer to a character array 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 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
destsz is less or equal strnlen_s(src, destsz); 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. As with all bounds-checked functions, strcpy_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 character array to write 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

strcpy_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 strcpy_s is similar to the BSD function strlcpy, except that.

strlcpy truncates the source string to fit in the destination (which is a security risk)
strlcpy does not perform all the runtime checks that strcpy_s does
strlcpy does not make failures obvious by setting the destination to a null string or calling a handler if the call fails.

Although strcpy_s prohibits truncation due to potential security risks, it's possible to truncate a string using bounds-checked strncpy_s instead.

Example

#define __STDC_WANT_LIB_EXT1__ 1
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
 
int main(void)
{
    char *src = "Take the test.";
//  src[0] = 'M' ; // this would be undefined behavior
    char dst[strlen(src) + 1]; // +1 to accomodate for the null terminator
    strcpy(dst, src);
    dst[0] = 'M'; // OK
    printf("src = %s\ndst = %s\n", src, dst);
 
#ifdef __STDC_LIB_EXT1__
    set_constraint_handler_s(ignore_handler_s);
    int r = strcpy_s(dst, sizeof dst, src);
    printf("dst = \"%s\", r = %d\n", dst, r);
    r = strcpy_s(dst, sizeof dst, "Take even more tests.");
    printf("dst = \"%s\", r = %d\n", dst, r);
#endif
}

Possible output:

src = Take the test.
dst = Make the test.
dst = "Take the test.", r = 0
dst = "", r = 22

References

C17 standard (ISO/IEC 9899:2018):

7.24.2.3 The strcpy function (p: 264-265)
K.3.7.1.3 The strcpy_s function (p: 447)

C11 standard (ISO/IEC 9899:2011):

7.24.2.3 The strcpy function (p: 363)
K.3.7.1.3 The strcpy_s function (p: 615-616)

C99 standard (ISO/IEC 9899:1999):

7.21.2.3 The strcpy function (p: 326)

C89/C90 standard (ISO/IEC 9899:1990):

4.11.2.3 The strcpy function