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errno_t strcpy_s(
   char * restrict s1,
   rsize_t s1max,
   char const * restrict s2
);

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When a runtime-constraint violation is detected, the destination string is set to the NULL null string (as long as it is not a null pointer and the maximum length of the destination buffer is greater than zero and not greater than RSIZE_MAX) and the function returns a nonzero value. In the following example, the strcpy_s() function is used to copy src1 to dst1.

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However, the call to copy {{src2}} to {{dst2}} fails because there is insufficient space available to copy the entire string, which consists of eight characters, to the destination buffer. As a result, {{r2}} is assigned a nonzero value and {{dst2\[0\]}} is set to the null character. 

Users of the ISO/IEC TR 24731-1 functions are less likely to introduce a security flaw because the size of the destination buffer and the maximum number of characters to append must be specified.  ISO/IEC TR 24731 Part II (24731-2, in progress) will offer another approach, supplying functions that allocate enough memory for their results.   ISO/IEC TR 24731 functions also ensure NULL null termination of the destination string.

ISO/IEC TR 24731-1 functions are still capable of overflowing a buffer if the maximum length of the destination buffer and number of characters to copy are incorrectly specified.    ISO/IEC TR 24731-2 functions may make it more difficult to keep track of memory that must be freed, leading to memory leaks.   As a result, the ISO/IEC TR 24731 functions are not especially secure but may be useful in preventive maintenance to reduce the likelihood of vulnerabilities in an existing legacy code base.

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void complain(char const *msg) {
  errno_t err;
  static char const prefix[] = "Error: ";
  static char const suffix[] = "\n";
  char buf[BUFSIZ];

  /* Ensure that more than one character 
   * is available for msg. */
  static_assert(sizeof(buf) > sizeof(prefix) + sizeof(suffix),
                "Buffer for complain() is too small");
  strcpy(buf, prefix);

  err = strcat_s(buf, sizeof(buf), msg);
  if (err != 0) {
    /* handle error */
  }

  err = strcat_s(buf, sizeof(buf), suffix);
  if (err != 0) {
    /* handle error */
  }
  fputs(buf, stderr);
}

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