What is the best way to return an error from a function when I'm already returning a value?

Question

I wrote a function in C that converts a string to an integer and returns the integer. When I call the function I also want it to let me know if the string is not a valid num

Answer 1

The os-style global errno variable is also popular. Use errno.h.

If errno is non-zero, something went wrong.

Here's a man page reference for errno.

Answer 2

What is the best way to return an error from a function when I'm already returning a value?

Some additional thoughts to the various answers.

---

Return a structure

Code can return a value and an error code. A concern is the proliferation of types.

typedef struct {
  int value;
  int error;
} int_error;

int_error intval(const char *string);

...

int_error = intval(some_string);
if (int_error.error) {
  Process_Error();
}

int only_care_about_value = intval(some_string).value;
int only_care_about_error = intval(some_string).error;

Not-a-number and NULL

Use a special value when the function return type provides it.
Not-a-number's are not required by C, but ubiquitous.

#include <math.h>
#include <stddef.h>

double y = foo(x);
if (isnan(y)) {
  Process_Error();
}

void *ptr = bar(x);
if (ptr == NULL) {
  Process_Error();
}

_Generic/Function Overloading

Considering the pros & cons of error_t foo(&dest, x) vs. dest_t foo(x, &error),

With a cascaded use of _Generic or function overloading as a compiler extension, selecting on 2 or more types, it makes sense to differentiate the underlying function called to be based on the parameters of the call, not the return value. Return the common type, the error status.

Example: a function error_t narrow(destination_t *, source_t) that converted the value of one type to a narrower type, like long long to short and tested if the source value was in range of the target type.

long long ll = ...; 
int i;
char ch; 
error = narrow(&i, ll);
...
error = narrow(&ch, i);