If either of the C Standard fgets() or fgetws() functions fail, the contents of the array being written is indeterminate. (See undefined behavior 170.)  It is necessary to reset the string to a known value to avoid errors on subsequent string manipulation functions.

Noncompliant Code Example

In this noncompliant code example, an error flag is set if fgets() fails. However, buf is not reset and has indeterminate contents:

#include <stdio.h>
 
enum { BUFFER_SIZE = 1024 };
void func(FILE *file) {
  char buf[BUFFER_SIZE];

  if (fgets(buf, sizeof(buf), file) == NULL) {
    /* Set error flag and continue */
  }
}

Compliant Solution

In this compliant solution, buf is set to an empty string if fgets() fails. The equivalent solution for fgetws() would set buf to an empty wide string.

#include <stdio.h>
 
enum { BUFFER_SIZE = 1024 };

void func(FILE *file) {
  char buf[BUFFER_SIZE];

  if (fgets(buf, sizeof(buf), file) == NULL) {
    /* Set error flag and continue */
    *buf = '\0';
  }
}

Exceptions

FIO40-C-EX1: If the string goes out of scope immediately following the call to fgets() or fgetws() or is not referenced in the case of a failure, it need not be reset.

Risk Assessment

Making invalid assumptions about the contents of an array modified by fgets() or fgetws() can result in undefined behavior and abnormal program termination.

Automated Detection

Tool	Version	Checker	Description
CodeSonar		LANG.MEM.UVAR	Uninitialized Variable
Helix QAC		DF4861, DF4862, DF4863
LDRA tool suite		44 S	Enhanced enforcement
Parasoft C/C++test		CERT_C-FIO40-a	Reset strings on fgets() or fgetws() failure
Polyspace Bug Finder		CERT C: Rule FIO40-C	Checks for use of indeterminate string (rule partially covered)
PVS-Studio		V1024

Related Vulnerabilities

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