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ccastingfgetc

What will happen if fgetc reads 0xFF?

fgetc reads a character from a file at a time, and returns the character as type of int. If the file ends, then fgetc returns EOF ("probably" (int)(-1)). However, I have seen this so frequently:

char ch;

while ((ch = fgetc(fp)) != EOF)  {  /* Do something. */  }
       |<--  PARN  -->|

This is what I concern:

  1. fgetc reads OxFF and returns 0x000000FF.
  2. 0xFF is assigned to ch. (type casting from int to char)
  3. If PARN is type of char, then it will be promoted to 0xFFFFFFFF.
  4. Break out of the while loop because PARN == EOF. (stop reading from the file)

How can we tell reading OxFF and returning EOF apart?

Solution

  • The whole reason the return type of fgetc is int is because it can return either a value of an unsigned char, or EOF. This is clearly documented in the specification and in any good resource on learning C. If you use an object of type char to store the result, this is programmer error.