FI036-C. Don't assume a newline character is read

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[1024];

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

However, if the last character in buf is not a newline, it will truncate an otherwise-valid character.

Compliant Code Example

The following example uses strchr() to replace the newline character in the string only if it exists.

char buf[1024];

if (fgets(buf, sizeof(buf), fp))
{
  char *p;
  if (p=strchr(buf, '\n'))
    *p = 0;
}

Risk Assessment

References

• Lai 06
• Seacord 05 Chapter 2 Strings
• ISO/IEC 9899-1999 Section 7.19.7.2 The fgets function