如何确保 strtol() 已成功返回？

Question

according documentation:

On success, the function returns the converted integral number as a long int value. If no valid conversion could be performed, a zero value is returned. If the correct value is out of the range of representable values, LONG_MAX or LONG_MIN is returned, and the global variable errno is set to ERANGE.

Consider strtol(str, (char**)NULL, 10); if str is "0\\0" how to know if the function failed or only has converted the string with "0" number?

Answer 1

You need to pass a real pointer address if you want error checking, so you can distinguish 0 values arising from "0" and similar from 0 values arising from "pqr" :

char *endptr;
errno = 0;
long result = strtol(str, &endptr, 10);
if (endptr == str)
{
    // nothing parsed from the string, handle errors or exit
}
if ((result == LONG_MAX || result == LONG_MIN) && errno == ERANGE)
{
    // out of range, handle or exit
}
// all went fine, go on

Answer 2

Since the accepted answer is not actually a correct way to check for failure.

You should not check for errors by examining the return value of strtol, because the string might be a valid representation of 0l, LONG_MAX, or LONG_MIN. Instead, check whether tailptr points to what you expect after the number (eg '\\0' if the string should end after the number). You also need to clear errno before the call and check it afterward, in case there was overflow.

Answer 3

IMHO, I prefer sscanf() to atoi() or strtol() . The main reason is that you cannot reliably check for error status on some platforms (ie Windows) unless you use sscanf() (which returns 1 if you succeed, and 0 if you fail).

Answer 4

I think I checked all the edge cases. If anyone thinks of an edge case that I missed please let me know in the comments and I will update this post. I tried to keep the error messages simple. If you disagree with this decision please feel free to change them as you see fit.

// Copyright (C) 2021 by cmwt
// Permission to use, copy, modify, and/or distribute this software for any purpose with or without fee is hereby granted.
// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

#include <errno.h>    // errno, ERANGE
#include <limits.h>   // LONG_MAX, LONG_MIN
#include <stdbool.h>  // true, false
#include <stddef.h>   // ptrdiff_t
#include <stdio.h>    // printf()
#include <stdlib.h>   // strtol()

struct ResultToLong {
  const char *err_msg;
  long        answer;
  ptrdiff_t   num_read;
  bool        is_func_success;
};

struct ResultToLong stringToLong(const char* start, int base) {
  struct ResultToLong result = {NULL, 0, 0, false};
  int save_errno = 0;
  char *end = NULL;

  if (base < 0 || base > 36) {
    result.err_msg = "Bad base: expect (0 <= base <= 36)";
    return result;
  }
  if (start == NULL) {
    result.err_msg = "Bad start: expect (start != NULL)";
    return result;
  }
  if (*start == '\0') {
    result.err_msg = "Bad start: start empty (const char* start == \"\";)";
    return result;
  }

  errno = 0;
  result.answer = strtol(start, &end, base);
  save_errno = errno;

  if (result.answer == 0 && *(start - 1) != '0') {
    result.err_msg = "Bad start: not a number";
    result.num_read = end - start;
    return result;
  }
  if (result.answer == LONG_MIN && save_errno == ERANGE) {
    result.err_msg = "Bad start: result < LONG_MIN";
    result.num_read = end - start;
    return result;
  }
  if (result.answer == LONG_MAX && save_errno == ERANGE) {
    result.err_msg = "Bad start: result > LONG_MAX";
    result.num_read = end - start;
    return result;
  }
  if (*end != '\0') {
  result.err_msg = "Warning: number in start is not '\\0' terminated";
  result.num_read = end - start;
  result.is_func_success = true;
  return result;
  }

  result.err_msg = "Success";
  result.num_read = end - start;
  result.is_func_success = true;
  return result;
}

int main() {
  struct ResultToLong result;
  const char* str;
  
  printf("Starting...\n\n");

  str= NULL;
  result = stringToLong(str, 0);
  printf("Response message: %s\n", result.err_msg);
  printf("Input string: %s\n", "<NULL>");
  printf("Number of chars processed: %ld\n", result.num_read);
  printf("Result: %ld\n\n", result.answer);
  
  str= "";
  result = stringToLong(str, 0);
  printf("Response message: %s\n", result.err_msg);
  printf("Input string: '%s'\n", str);
  printf("Number of chars processed: %ld\n", result.num_read);
  printf("Result: %ld\n\n", result.answer);

  str= "42";
  result = stringToLong(str, -1);
  printf("Response message: %s\n", result.err_msg);
  printf("Input string: '%s'\n", str);
  printf("Number of chars processed: %ld\n", result.num_read);
  printf("Result: %ld\n\n", result.answer);

  str= "42";
  result = stringToLong(str, 0);
  printf("Response message: %s\n", result.err_msg);
  printf("Input string: '%s'\n", str);
  printf("Number of chars processed: %ld\n", result.num_read);
  printf("Result: %ld\n\n", result.answer);

  str= "42 ";
  result = stringToLong(str, 0);
  printf("Response message: %s\n", result.err_msg);
  printf("Input string: '%s'\n", str);
  printf("Number of chars processed: %ld\n", result.num_read);
  printf("Result: %ld\n\n", result.answer);

  str= "                42";
  result = stringToLong(str, 0);
  printf("Response message: %s\n", result.err_msg);
  printf("Input string: '%s'\n", str);
  printf("Number of chars processed: %ld\n", result.num_read);
  printf("Result: %ld\n\n", result.answer);

  str= "0x42";
  result = stringToLong(str, 0);
  printf("Response message: %s\n", result.err_msg);
  printf("Input string: '%s'\n", str);
  printf("Number of chars processed: %ld\n", result.num_read);
  printf("Result: %ld\n\n", result.answer);

  str= "042";
  result = stringToLong(str, 0);
  printf("Response message: %s\n", result.err_msg);
  printf("Input string: '%s'\n", str);
  printf("Number of chars processed: %ld\n", result.num_read);
  printf("Result: %ld\n\n", result.answer);

  str= "+9999999999999999999";
  result = stringToLong(str, 0);
  printf("Response message: %s\n", result.err_msg);
  printf("Input string: '%s'\n", str);
  printf("Number of chars processed: %ld\n", result.num_read);
  printf("Result: %ld\n\n", result.answer);

  str= "-9999999999999999999";
  result = stringToLong(str, 0);
  printf("Response message: %s\n", result.err_msg);
  printf("Input string: '%s'\n", str);
  printf("Number of chars processed: %ld\n", result.num_read);
  printf("Result: %ld\n\n", result.answer);

  str= "?";
  result = stringToLong(str, 0);
  printf("Response message: %s\n", result.err_msg);
  printf("Input string: '%s'\n", str);
  printf("Number of chars processed: %ld\n", result.num_read);
  printf("Result: %ld\n\n", result.answer);

  printf("Done.\n");
}

If you object to returning a struct of this size by value then pass a pointer to the struct as an additional argument, just don't forget to handle the case where the struct pointer is NULL . My goal was to make this code easy to understand. You probbaly want combine checks and centralize setting the the return values.

Other Thoughts

I kinda wish strtol() would also return where the number starts. You could figure this out by iterating back from the end pointer but it might be slow.

Answer 5

You can either check the errno or pass a non-NULL value for the second argument and compare its resulting value to str , like:

char * endptr;
long result = strtol(str, &endptr, 10);
if (endptr > str)
{
    // Use result...
}