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Errors can occur when incorrect assumptions are made about the type of data being read. These assumptions may be violated, for example, when binary data has been read from a file instead of text from a user's terminal . (See or the output of a process is piped to stdin (see FIO14-C. Understand the difference between text mode and binary mode with file streams). ) On some systems, it may also be possible to input a null byte (as well as other binary codes) from the keyboard.

Subclause 7.21.7.2 of the of the C Standard [ISO/IEC 9899:2011]  sayssays,

The fgets function returns s if successful. If end-of-file is encountered and no characters have been read into the array, the contents of the array remain unchanged and a null pointer is returned.

The wide-character function fgetws() has the same behavior. Therefore, if fgets() or fgetws() returns a non-null pointer, it is safe to assume that the array contains data. However, it is erroneous to asume assume that the array contains a nonempty string because the data may contain null characters.

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#include <stdio.h>
#include <string.h>
 
#defineenum { BUFFER_SIZE = 1024 };

void func(void) {
  char buf[BUFFER_SIZE];

  if (fgets(buf, sizeof(buf), stdin) == NULL) {
    /* Handle error */
  }
  buf[strlen(buf) - 1] = '\0';
}

The strlen() function computes the length of a string by determining the number of characters that precede the terminating null character. A problem occurs if the first character read from the input by fgets() happens to be a null character. This may occur, for example, if a binary data file is read by the fgets() call [Lai 2006]. If the first character in buf is a null character, strlen(buf) returns 0, the expression strlen(buf) - 1 wraps around to a large positive value, and a write-outside-array-bounds error occurs.

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This compliant solution uses strchr() to replace the newline character in the string if it exists. (See FIO20-C. Avoid unintentional truncation when using fgets() or fgetws().):

#include <stdio.h>
#include <string.h>
 
#defineenum { BUFFER_SIZE = 1024 };

void func(void) {
  char buf[BUFFER_SIZE];
  char *p;

  if (fgets(buf, sizeof(buf), stdin)) {
    p = strchr(buf, '\n');
    if (p) {
      *p = '\0';
    }
  } else {
    /* Handle error */
  }
}

Risk Assessment

Assuming Incorrectly assuming that character data has been read can result in an out-of-bounds memory write or other flawed logic.

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Bibliography

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