Strtol在溢出转换时未设置errno

Question

My strtol function fails to set errno during overflown conversion.

#include <stdio.h>
#include <string.h>
#include <malloc.h>
#include <getopt.h>
#include <errno.h>
#include <stdlib.h>    

int main(int argc, char **argv) {

    errno = 0;
    int e = strtol("1000000000000000", NULL, 10);
    printf("%d %d\n", errno, e);

    return 0;
}

returns

0 -1530494976

What do I do wrong?

Compiler

gcc (Ubuntu 4.9.2-10ubuntu13) 4.9.2

Options

gcc -Wall -std=gnu99 -O2

Answer 1

There is nothing wrong with the implementation of strtol() but there is with your code.

The return type of this function is long (see the trailing l ) and apparently the value 1000000000000000 can be represented by the long integer type. However the return value is assigned to e whose type is int which is unable to represent this value. What then happens is implementation-defined.

So change int e to long e and "%d %d\\n" to "%d %ld\\n" . If you want to keep it as int , then you have to check if the value is outside of its range of representable values by yourself:

#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <limits.h>  // for INT_{MIN,MAX}
int
main(void)
{
    errno = 0;
    long f = strtol("1000000000000000", NULL, 10);
    if (errno == ERANGE) {
        puts("value not representable by long (or int)");
    } else if (f < INT_MIN || f > INT_MAX) {
        puts("value not representable by int");
    } else {
        int e = f;
        printf("%d\n", e);
    }
}

Answer 2

You are doing well but you are not checking the errno value for strtol but for printf() ....Not understand let me explain... After long f = strtol("1000000000000000", NULL, 10); statement you have to first save the value of errno. Because after this statement you are going to call printf() statement that also set errno accordingly.

printf("errno is: %d\n", errno); 

So in this statement "errno" gives the error indication for printf() not for strtol() ... To do so save "errno" before calling any library function because most of the library function interact with "errno". The correct use is :

long f = strtol("1000000000000000", NULL, 10);
int saved_error = errno;       // Saving the error...
printf("errno is: %d\n", saved_error);

Now check it. It will give correct output surely...And one more thing to convert this errno to some meaningful string to represent error. Use strerror() function as :

printf("Error is: %s\n", strerror(saved_error)); 

Cremno's answer is correct but this is more generic.

Answer 3

It seems like both Microsoft [1] and Apple [2] implementations have the setting of errno commented out.

[1] http://research.microsoft.com/en-us/um/redmond/projects/invisible/src/crt/strtol.c.htm

[2] http://www.opensource.apple.com/source/xnu/xnu-1456.1.26/bsd/libkern/strtol.c