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where rbytes is a pointer to a size_t. If no error occurs, and rbytes is not NULL, its value is set to the total number of bytes read, and read() returns 0. If an error occurs, read() returns a nonzero value indicating the error.

Noncompliant Code Example (

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C11, Annex K)

In this noncompliant code example, the error handler returns normally, but the strcpy_s() function's return value is not checked:

constraint_handler_t handle_errors(void) {
  constraint_handler_t data;
  /* Define what to do when error occurs */
  return data;
}

/*...*/

set_constraint_handler(handle_errors);

/*...*/

/* Returns zero on success */
errno_t function(char *dst1){
  char src1[100] = "hello";

  strcpy_s(dst1, sizeof(dst1), src1);
  /* At this point strcpy_s may have yielded an
     error and handle_errors() might have returned */

  /* ... */
  return 0;
}

Compliant Solution (

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C11, Annex K)

In this compliant solution, the error handler terminates the program, ensuring that strcpy_s() never returns unless it fully succeeds:

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ERR02-EX2: You may use a function returning in-band error indicators if you can securely guarantee the program will not try to continue processing should an error occur in the function. For example, the functions defined in TR24731-1 C11 Annex K provide hooks for internal constraint violations. If a constraint violation handler is guaranteed not to return upon an error, then you may safely ignore errors returned by these functions. You might accomplish this by having the constraint-violation handler call abort() or longjmp(), for instance.

See ERR03-C. Use runtime-constraint handlers when calling the bounds-checking interfaces for more on the functions defined in TR24731-1C11 Annex K.

Risk Assessment

The risk in using in-band error indicators is difficult to quantify and is consequently given as low. However, if the use of in-band error indicators results in programmers' failing to check status codes or incorrectly checking them, the consequences can be more severe.

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Bibliography

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