errno in C

There are three common error methods:

//1) global error code, C language method
errno() and strerror()
//1) status in object , C++ method
cin.fail()

//2) return value by parameter or return value
//3) Exception.

errno() and strerror() is a typical C language style.

Besides above, you can use some custom function pointer to do some custom error handling behavior, such as set_new_handler function for new operator

In general, you should detect errors by checking return values, and use errno or perror() only to distinguish among the various causes of an error, such as "File not found" or "Permission denied."
FILE * pFile = fopen ("unexist.ent","rb");
if (pFile==NULL)
perror ("The␣following␣error␣occurred");
It’s only necessary to detect errors with errno when a function does not have a unique, unambiguous, out-of-band error return (that is, because all of its possible return values are valid; one example is atoi()). In these cases (and in these cases only; check the documentation to be sure whether a function allows this),
#include <cerrno>
errno = 0
// set it zero before call any math library function.
double not_a_number = std::log(-1.0);
  if (errno == EDOM) {
        std::cout << "log(-1)␣failed:␣" <<
                      std::strerror(errno) << ’\n’;
you can detect errors by setting errno to 0, calling the function, and then testing errno. (Setting errno to 0 first is important, as no library function ever does that for you.)

C11 provide fenv.h file to expand errno

#include <math.h>       /* math_errhandling */
#include <errno.h>      /* errno, EDOM */
#include <fenv.h>
/* feclearexcept, fetestexcept, FE_ALL_EXCEPT, FE_INVALID */

#pragma STDC FENV_ACCESS on
  errno = 0;
  if (math_errhandling & MATH_ERREXCEPT)
            feclearexcept(FE_ALL_EXCEPT);

  sqrt (-1);
  if (math_errhandling & MATH_ERRNO) {
    if (errno==EDOM) printf("errno␣set␣to␣EDOM\n");
  }

  if (math_errhandling  &MATH_ERREXCEPT) {
    if (fetestexcept(FE_INVALID)) printf("FE_INVALID␣raised\n");
  }

1.4.3.1 errno in C