FIO37-C. Do not assume that fgets() returns a nonempty string when successful

Errors can occur when 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 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.

Section 7.21.7.2 of the C Standard [ISO/IEC 9899:2011] says,

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.

Therefore, if fgets() returns a non-null pointer, we can assume it actually filled the array with data. However, assuming that it filled the array with a nonempty null-terminated byte string (NTBS) is erroneous because the data it placed in the array may contain null characters.

Noncompliant Code Example

This noncompliant code example attempts to remove the trailing newline (\n) from an input line. The fgets() function is typically used to read a newline-terminated line of input from a stream. It takes a size parameter for the destination buffer and copies, at most, size - 1 characters from a stream to a character array.

char buf[BUFSIZ + 1];

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 and a write-outside-array-bounds error occurs.

Compliant Solution

This compliant solution uses strchr() to replace the newline character in the string if it exists. (See FIO36-C. Do not assume a newline character is read when using fgets().)

char buf[BUFSIZ + 1];
char *p;

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

Risk Assessment

Assuming character data has been read can result in an out-of-bounds memory write.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography