The strlen()
function computes the length of a string. It returns the number of characters that precede the terminating NULL character. Errors can occur when assumptions are made about the type of data being passed to strlen()
, e.g., in cases where binary data has been read from a file instead of textual data from a users's terminal.
Non-Compliant Code Example
This non-compliant code example is intended to be used 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, takes a size parameter for the destination buffer and copies, at most, size-1
characters from a stream to a string.
char buf[1024]; fgets(buf, sizeof(buf), fp); buf[strlen(buf) - 1] = '\0';
However, if the first character in buf
is a NULL, strlen(buf)
will return 0 and a write-outside-array-bounds error will occur.
Compliant Solution
This compliant solution checks to make sure the first character in the buf
array is not a NULL and that the last character is indeed a newline before replacing it with a NULL.
char buf[1024]; if (fgets(buf, sizeof(buf), fp) != NULL) { if (buf[0] != '\0' && buf[strlen(buf) - 1] == '\n') buf[strlen(buf) - 1] = '\0'; }
Risk Assessment
Rule |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
---|---|---|---|---|---|
FI036-C |
1 (low) |
1 (unlikely) |
3 (low) |
P3 |
L3 |
References
- Lai 06
- Seacord 05 Chapter 2 Strings
- ISO/IEC 9899-1999 Section 7.19.7.2 The fgets function