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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 recommendation 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.

C99, Section 7.1921.7.2 , "The fgets function", paragraph 3 saysof 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 non-empty nonempty null-terminated byte string (NTBS) is erroneous because the data it placed in the array may contain null characters.

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This noncompliant code example attempts to remove the trailing new-line newline (\n) from an input line. The fgets() function is typically used to read a newnewline-line 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';

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This compliant solution uses strchr() to replace the new-line newline character in the string , if it exists. (See rue FIO36-C. Do not assume a new-line 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 */
}

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Related Vulnerabilities

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

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CERT C++ Secure Coding Standard: FIO37-CPP. Do not assume character data has been read

ISO/IEC 9899:1999 Section 2011 Section 7.1921.7.2, "The fgets function"

MITRE CWE: CWE-119, "Failure to Constrain Operations constrain operations within the Bounds bounds of an Allocated Memory Bufferallocated memory buffer"

MITRE CWE: CWE-241, "Failure to Handle Wrong Data Typehandle wrong data type"

Bibliography

[Lai 2006]
[Seacord 2005a] Chapter 2, "Strings"

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