Convert errno to exit codes - c++

Question

I'm working on lib which uses a lot of file system functions.
What I want is that my function returns various of error codes (not just -1 as error) depending on errno in case file system function fails.
Although I could use errno values directly but I want to create some abstraction layer between my functions error codes and system errno (e.g. my error values begins on -1000 and are negative whereas errno values are positive).
My question what is the best way to implement it.
For now I see two possible solution:
use an enum with error codes and switch case function to translate, e.g.:
typedef enum {
MY_ERROR_EPERM = -1104, /* Operation not permitted */
MY_ERROR_ENOENT = -1105, /* No such file or directory */
// ...
} MyReturnCodes_t;
int ErrnoToErrCode(unsigned int sysErrno) {
int error = ENOSYS;
switch(sysErrno) {
case EPERM: error = MY_ERROR_EPERM; break;
case ENOENT: error = MY_ERROR_ENOENT; break;
// ...
}
return error;
}
use translation directly in enum:
#define ERR_OFFSET -1000
typedef enum {
MY_ERROR_EPERM = ERR_OFFSET - EPERM, /* Operation not permitted */
MY_ERROR_ENOENT = ERR_OFFSET - ENOENT, /* No such file or directory */
MY_ERROR_ESRCH = ERR_OFFSET - ESRCH, /* No such process */
// ...
} MyReturnCodes_t;
Which way is more constant?
One more point: This library should be used both on QNX and Linux OS, what is the proper way to align errno codes (which different in some cases)?

Answer 1

I´d go for a std::map in a dedicated function. You don't have to care about gaps or anything as long as you use the provided error macros:
#include <iostream>
#include <errno.h>
#include <map>
namespace MyError
{
enum MyReturnCode: int
{
MY_INVALID_VAL = 0 , /* Invalid Mapping */
MY_ERROR_EPERM = -1104, /* Operation not permitted */
MY_ERROR_ENOENT = -1105, /* No such file or directory */
};
MyReturnCode fromErrno(int e)
{
static const std::map<int, MyReturnCode> mapping {
{ EPERM, MY_ERROR_EPERM},
{ ENOENT, MY_ERROR_ENOENT}
};
if(mapping.count(e))
return mapping.at(e);
else
return MY_INVALID_VAL;
}
}
int main()
{
std::cout << MyError::fromErrno(ENOENT) << std::endl;
std::cout << MyError::fromErrno(42) << std::endl;
return 0;
}
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