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

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 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 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.

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 assume that the array contains a nonempty string because the data 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.

#include <stdio.h>
#include <string.h>
 
enum { 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.

Compliant Solution

This compliant solution uses strchr() to replace the newline character in the string if it exists:

#include <stdio.h>
#include <string.h>
 
enum { 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

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

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography