Problem with string conversion to number ( strtod )

Question

I am using strtod( ) function to extract an environment variable as a string, and then changing it to double using strtod:

enter code here
 char strEnv[32];
 strncpy(strEnv, getenv("LT_LEAK_START"), 31);
 // How to make sure before parsing that env LT_LEAK_START is indeed a number?
 double d = strtod(strEnv, NULL);

Now i want to make sure that this number entered by user is a number and not a string or special character. How can i make sure of that?

A code snippet would be of great help.

Thanks in advance.

Answer 1

The 2nd argument to the strtod function is useful.

char *err;
d = strtod(userinput, &err);
if (*err == 0) { /* very probably ok */ }
if (!isspace((unsigned char)*err)) { /* error */ }

---

Edit: examples added

The strtod function tries to convert the initial portion of the 1st argument to a double and stops either when there are no more chars, or there is a char that can't be used to make a double.

input         result
----------    ----------------------------
"42foo"       will return 42
              and leave err pointing to the "foo" (*err == 'f')

"     4.5"    will return 4.5
              and leave err pointing to the empty string (*err == 0)

"42         " will return 42
              and leave `err` pointing to the spaces (*err == ' ')

Answer 2

man strtod: If no conversion is performed, zero is returned and the value of nptr is stored in the location referenced by endptr.

char * endptr;
double d = strtod(strEnv, &endptr);
if (strEnv == endptr)
   /* invalid number */
else
   ...