错误：常量 255 与“char”类型表达式的比较结果始终为 false

Question

我正在研究 cs50 pset4 恢复，进展顺利。但是，当我使用 if 语句检查该文件是否为 JPEG 时，clang 开始编写一些奇怪的错误消息。这是我的代码：

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

int main(int argc, char *argv[])
{
  FILE *pFile;
  char *buffer = NULL;
  char *filename = NULL;
  // If user didn't print 2 items
  if (argc != 2) {
    printf("Usage: ./recover image\n");
    return 1;
  }
  // Open the file
  pFile = fopen(argv[1], "r");
  int j = 0;
  // checking the card by 512b chunks

  // loop (i=0,  i++);
  while (5 < 6) {
    int i = 0;
    i++;

    // k=fread (buffer, 512, i, *file);
    int k = fread(buffer, 512, i, pFile);
    // if buffer [0]== 0xff // checking if it's the header. If yes - creating a new jpeg; if not -
    // i++
    if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff &&
        (buffer[3] & 0xf0) == 0xe0) {
      // if it's not the first file, we should close the last one

      // sprintf
      sprintf(filename, "%03i.jpg", 2);
      // FILE = fopen (W)
      pFile = fopen(filename, "w");
      // fwrite (buffer, 512, j, *file1)
      fwrite(buffer, 512, j, pFile);
      // j=j+1
      j = j + 1;
    }
    // if k<512 - end of the loop
    if (k < 512) {
      return 0;
    }
  }
}

我还没有完成剩下的所有工作，但是当我看到这些错误消息时我停下了：

clang -ggdb3 -O0 -std=c11 -Wall -Werror -Wextra -Wno-sign-compare -Wno-unused-parameter -Wno-unused-variable -Wshadow    recover.c  -lcrypt -lcs50 -lm -o recover
recover.c:29:15: error: result of comparison of constant 255 with expression of type 'char' is always false [-Werror,-Wtautological-constant-out-of-range-compare]
if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xf0) == 0xe0)
    ~~~~~~~~~ ^  ~~~~
recover.c:29:36: error: result of comparison of constant 216 with expression of type 'char' is always false [-Werror,-Wtautological-constant-out-of-range-compare]
if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xf0) == 0xe0)
                         ~~~~~~~~~ ^  ~~~~
recover.c:29:57: error: result of comparison of constant 255 with expression of type 'char' is always false [-Werror,-Wtautological-constant-out-of-range-compare]
if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xf0) == 0xe0)
                                              ~~~~~~~~~ ^  ~~~~
3 errors generated.
<builtin>: recipe for target 'recover' failed
make: *** [recover] Error 1

我尝试使用 help50，但发生了这种情况：

clang -ggdb3 -O0 -std=c11 -Wall -Werror -Wextra -Wno-sign-compare -Wno-unused-parameter -Wno-unused-variable -Wshadow    recover.c  -lcrypt -lcs50 -lm -o recover
recover.c:29:15: error: result of comparison of constant 255 with expression of type 'char' is always false [-Werror,-Wtautological-constant-out-of-range-compare]
if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xf0) == 0xe0)
    ~~~~~~~~~ ^  ~~~~
recover.c:29:36: error: result of comparison of constant 216 with expression of type 'char' is always false [-Werror,-Wtautological-constant-out-of-range-compare]
if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xf0) == 0xe0)
                         ~~~~~~~~~ ^  ~~~~
recover.c:29:57: error: result of comparison of constant 255 with expression of type 'char' is always false [-Werror,-Wtautological-constant-out-of-range-compare]
if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xf0) == 0xe0)
                                              ~~~~~~~~~ ^  ~~~~
3 errors generated.
<builtin>: recipe for target 'recover' failed
make: *** [recover] Error 1

Asking for help...

recover.c:29:15: error: result of comparison of constant 255 with expression of type 'char' is always false [-Werror,-Wtautological-constant-out-of-range-compare]

Not quite sure how to help, but focus your attention on line 29 of recover.c!

当我尝试谷歌搜索时，我只是得到了整个问题的答案。我不知道该怎么办，请帮帮我。

谢谢， 迷失在代码中:)

Answer 1

char 在此实现上的范围从 -127 到 127（最小值）。因此，将其与 216 或 255 进行比较是徒劳的。所以将其设为unsigned char *buffer，至少存储 0 到 255。

还有，

隐式转换丢失整数精度：“size_t”（又名“unsigned long”）到“int”

是解释性的：size_t 内部是unsigned long，它可以存储比int 更多的正数。所以理论上你可能有 int 溢出。

• https://en.wikipedia.org/wiki/Integer_overflow

• https://en.wikipedia.org/wiki/C_data_types#Main_types

---

一个类似的问题，但我的答案是 C++：

• How do I save recoded data?

Answer 2

将缓冲区声明为 uint8_t 缓冲区[512]，因为没有分配空间来保存 512 个字节。 完成后请记住关闭文件，例如fclose(pFile)

Answer 3

在 MSCV 编译器上我收到了一些投诉

 while (5 < 6)

第 23 行，

警告 C4127：条件表达式是常量

它确实是一个常数。你可以改变它

        int k = fread(buffer, 512, i, pFile);

第 29 行，

从“size_t”转换为“int”，可能会丢失数据

fread() 返回size_t 无符号且k 是int，因此如果fread() 读取的字节数超过INT_MAX 字节，它可以变为负数。

我们也有

'buffer' 可能是 '0'：这不符合规范 函数'fread'

确保它出现在其他部分。您已将 buffer 声明为

char* buffer = NULL;

并且从未为它分配内存。

我变了

char _buffer[512];
char* buffer = _buffer;

和while 到

while (pFile) // pFile is not 0 

因为pFile指的是打开的文件，所以是真的

并向fread()添加了演员表

int k = (int) fread(buffer, 512, (size_t)i, pFile);

所以编译好了