FI036-C. Do not assume a newline character is read when using fgets()

The fgets() function is typically used to read a newline-terminated line of input from a stream. The fgets() function takes a size parameter for the destination buffer and copies, at most, size-1 characters from a stream to a string. Truncation errors can occur if the programmer blindly assumes that the last character in the destination string will be a newline.

Non-Compliant Code Example

This non-compliant code example is intended to be used to remove the trailing newline (\n) from an input line.

char buf[BUFSIZ + 1];

fgets(buf, sizeof(buf), fp);
buf[strlen(buf) - 1] = '\0';

However, if the last character in buf is not a newline, this code overwrites an otherwise-valid character.

Compliant Solution

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

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

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

Risk Assessment

Assuming a newline character is read can result in data truncation.

References

[[Lai 06]]
[[Seacord 05]] Chapter 2, "Strings"
[[ISO/IEC 9899-1999]] Section 7.19.7.2, "The fgets function"