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The byte I/O functions fgetc(), getc(), and getchar() all read a character from a stream and return it as an int. (see See STR00-C. Represent characters using an appropriate type.) . If the stream is at the end of the file, the end-of-file indicator for the stream is set and the function returns EOF. If a read error occurs, the error indicator for the stream is set and the function returns EOF. If these functions succeed, they cast the character returned into an unsigned char.

Because EOF is negative, it should not match any unsigned character value. However, this is only true for implementations where the int type is wider than char. On an implementation where int and char have the same width, a character-reading function can read and return a valid character that has the same bit-pattern as EOF. This could occur, for example, if an attacker inserted a value that looked like EOF into the file or data stream to alter the behavior of the program.

The C Standard requires only that the int type be able to represent a maximum value of +32767 and that a char type be no larger than an int. Although uncommon, this situation can result in the integer constant expression EOF being indistinguishable from a valid character; that is, (int)(unsigned char)65535 == -1. Consequently, failing to use feof() and ferror() to detect end-of-file and file errors can result in incorrectly identifying the EOF character on rare implementations where sizeof(int) == sizeof(char).

This problem is much more common when reading wide characters. The fgetwc(), getwc(), and getwchar() functions return a value of type wint_t. This value can represent the next wide character read, or it can represent WEOF, which indicates end-of-file for wide character streams. On most implementations, the wchar_t type has the same width as wint_t, and so these functions can return a character indistinguishable from WEOF.

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This compliant solution uses feof() and ferror() to test for whether the EOF was an actual character or a real EOF because of end-of-file and ferror() to test for or errors:

#include <stdio.h>

void func(void) {
  int c;

  do {
    c = getchar();
  } while (c != EOF);
 || if (!feof(stdin)) {
    /* Handle end of file */
  } else if (&& !ferror(stdin)) {
    /* Handle file error */
  } else {
    /* Received a character that resembles EOF; handle error */
  }
}
);
}

Noncompliant Code Example (Nonportable)

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Assuming that a char is a signed 8-bit type and an int is a 32-bit type, if getchar() returns the character character value '\xff (decimal 255), it will be interpreted as EOF because this value is sign-extended to 0xFFFFFFFF (the value of EOF) to perform the comparison. (see See STR34-C. Cast characters to unsigned char before converting to larger integer sizes.).

Compliant Solution (Nonportable)

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This compliant solution declares c wc to be a wint_t to match the integer type returned by getwc(). Furthermore, it does not rely on WEOF to determine end-of-file definitively.

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Incorrectly assuming characters from a file cannot match EOF or WEOF has resulted in significant vulnerabilities, including command injection attacks. (see See the *CA-1996-22 advisory.).