strtoul 给出意外的输出

Question

我正在尝试使用strtoul 函数，但如下所示，它返回了一个意外的值（在开头添加了ff）：

#include <stdio.h>
#include <string.h>
#include <limits.h>

main() {
    unsigned long temp ;
    char *err;
    temp = strtoul("3334444444",&err,10);
    if (temp > UINT_MAX) {
        printf("%lx %x %x\n",temp,3334444444,UINT_MAX);
    }else 
        printf("%lx %x\n",temp,3334444444);
}

$./a.out
ffffffffc6bf959c c6bf959c ffffffff 

上面的输出对应于if 部分为真，尽管我希望else 部分在这里执行。谁能解释为什么strtoul 会这样？为什么它返回ffffffffc6bf959c 而不仅仅是c6bf959c？如果我在上面的代码中使用"333444444"（即少一个4）而不是"3334444444"，那么我得到与else部分对应的正确输出（即13dff55c 13dff55c）。

Note : As pointed by melpomene in his reply below, stdlib.h header file should have been included and that will resolve the issue. Can anyone please let me know what is being done by the program by assuming the incorrect return type (int in this case) during compile time which can't be undone (or atleast it is not getting undone in this case) even after knowing the correct return type (unsigned long in this case) during link time ? In short, i want to know how c6bf959c is getting converted to ffffffffc6bf959c because of prototype not provided.

Answer 1

使用 gcc 编译代码并启用警告会给出：

try.c:5:1: warning: return type defaults to ‘int’ [-Wimplicit-int]
 main() {
 ^~~~
try.c:5:1: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
try.c: In function ‘main’:
try.c:8:12: warning: implicit declaration of function ‘strtoul’ [-Wimplicit-function-declaration]
     temp = strtoul("3334444444",&err,10);
            ^~~~~~~
try.c:8:5: warning: nested extern declaration of ‘strtoul’ [-Wnested-externs]
     temp = strtoul("3334444444",&err,10);
     ^~~~
try.c:10:22: warning: format ‘%x’ expects argument of type ‘unsigned int’, but argument 3 has type ‘long long int’ [-Wformat=]
         printf("%lx %x %x\n",temp,3334444444,UINT_MAX);
                      ^
try.c:12:22: warning: format ‘%x’ expects argument of type ‘unsigned int’, but argument 3 has type ‘long long int’ [-Wformat=]
         printf("%lx %x\n",temp,3334444444);
                      ^

主要问题是implicit declaration of function ‘strtoul’，表示函数没有声明（因此假设返回int），因为你忘了#include <stdlib.h>。添加缺少的#include 修复了temp 的值。

但您还应该查看针对 printf 报告的警告并修复这些问题。