英文:
Writing doesn't stop for files above ~2gb in C
问题
我有一些C代码，应该生成一个密码并将其保存到文件中。我尝试输入一个大约为10 GB的长度（荒谬地大，但我想看看它能有多快），但它没有在10 GB处停止。
似乎当它超过1 GiB时（我假设是2 GiB，因为这是在使用#define _FILE_OFFSET_BITS 64之前它通常到达的大小），它不会停止写入，而是似乎会无限继续。
我正在使用Raspbian。
非常感谢任何帮助。
英文:
I have some C code that is supposed to generate a password and save it to a file. I tried to enter a length that would be about 10 gb (absurdly big but I wanted to see how fast it could do it), and it didn't stop at 10 gb.
#define _FILE_OFFSET_BITS 64
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
int main(){
int i = 0;
char chars[] = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890_-+={[]}\\|;:'\",<.>/?`~!@#$^&*()";
long long size;
char keypath[512];
char password;
printf("Where should the key be saved? ");
fgets(keypath, sizeof(keypath), stdin);
keypath[strcspn(keypath, "\n")] = 0;
printf("How long should the key be? ");
scanf("%lli",&size);
getchar();
srand((unsigned int)(time(NULL)));
FILE* fptr = fopen(keypath,"a");
if (fptr == NULL) {
printf("Failed to open file for writing\n");
return 1;
}
for (i = 0; i < size; i++){
int choice = rand() % strlen(chars);
password = chars[choice];
fprintf(fptr, "%c", chars[choice]);
}
fclose(fptr);
return 0;
}
It seems like when it goes somewhere over 1 gib (I assume 2 gib since that's the regular amount it goes to before using #define _FILE_OFFSET_BITS 64) it doesn't stop writing and instead keeps going seemingly indefinitely.
I am using Raspbian.
Any help is much appreciated.
答案1
得分: 4
你很好地将变量 size 声明为 long long 类型，这样就确保了它能够表示 10 GB 的文件大小。
然而，你忘记了对变量 i 采取相同的操作。相反，你给了 i 数据类型 int。
由于这个原因，循环
for (i = 0; i < size; i++){
int choice = rand() % strlen(chars);
password = chars[choice];
fprintf(fptr, "%c", chars[choice]);
}
会因为有符号整数溢出而导致未定义的行为，除非你碰巧在一个能够表示 10 GB 文件大小的平台上。在大多数常见的个人电脑平台上，INT_MAX（即 int 能够表示的最大值）的值是 2147483647。在这些平台上，int 无法表示值 10000000000。
英文:
It is good that you made the variable size to be of the type long long, so that it is guaranteed to be able to represent a file size of 10 GB.
However, you forgot to do the same with the variable i. Instead, you gave i the data type int.
Due to this, the loop
for (i = 0; i < size; i++){
int choice = rand() % strlen(chars);
password = chars[choice];
fprintf(fptr, "%c", chars[choice]);
}
will invoke undefined behavior due to signed integer overflow, unless you happen to be on a platform on which int is able to represent a file size of 10 GB. On most common PC platforms, INT_MAX (which is the maximum value an int can represent) has a value of 2147483647. On those platforms, an int is unable to represent the value 10000000000.
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