strtol（）返回不正确的值

Question

I've got a weird one here. strtol, atol and atoi all return an incorrect value when I pass in the following string:

long test = strtol("3087663490", &p, 10);

According to my debugger it returns 2147483647. I'm completely stumped on this. Any advice?

Answer 1

Your value is larger than what a signed long type can represent. Try:

unsigned long test = strtoul("3087663490", &p, 10);

(You're getting 2147483647 since it is LONG_MAX or 0x7FFFFFFF)

Answer 2

Correctly testing for errors from strtol() is tricky. You need code similar to the below:

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <limits.h>

int main()
{
    char *n = "3087663490";
    char *p;
    long test;

    errno = 0;
    test = strtol(n, &p, 10);

    if (p == n)
        printf("No digits found.\n");
    else if ((test == LONG_MIN || test == LONG_MAX) && errno == ERANGE)
        printf("Value out of range.\n");
    else
        printf("Value is %ld\n", test);

    return 0;
}

Running this shows what your problem is - Value out of range. . Use a wider type, like long long (and strtoll() ).

Answer 3

It appears long is a 32-bit type on your system. If that's the case, 3087663490 is larger than LONG_MAX . From the strtol(3) man page :

If an overflow occurs, strtol() returns LONG_MAX .

You need to use an unsigned type to represent that number on your machine:

unsigned long test = strtoul("3087663490", &p, 10);

Should work fine.

Answer 4

After returning from strtol check the value of p . I believe that if you went over the size limit the return value will be LONG_MAX and the pointer will still be pointer at a digit rather than whitespace or text.

And I also see after checking that errno will be set to ERANGE .