Do not use functions that input characters and convert them to integers if the functions cannot handle all possible inputs. For example, formatted input functions such as scanf(), fscanf(), vscanf(), and vfscanf() can be used to read string data from stdin or (in the cases of fscanf() and vfscanf()) other input streams. These functions work fine for valid integer values but lack robust error handling for invalid values.

Alternatively, input character data as a null-terminated byte string and convert to an integer value using strtol() or a related function. (See ERR34-C. Detect errors when converting a string to a number.)

Noncompliant Code Example

This noncompliant code example uses the scanf() function to read a string from stdin and convert it to a long. The scanf() and fscanf() functions have undefined behavior if the value of the result of this operation cannot be represented as an integer.

long num_long;

if (scanf("%ld", &num_long) != 1) {
  /* Handle error */

In general, do not use scanf() to parse integers or floating-point numbers from input strings because the input could contain numbers not representable by the argument type.

Compliant Solution (Linux)

This compliant example uses the Linux scanf() implementation's built-in error handling to validate input. On Linux platforms, scanf() sets errno to ERANGE if the result of integer conversion cannot be represented within the size specified by the format string [Linux 2008]. Note that this solution is platform dependent, so it should be used only where portability is not a concern.

long num_long;
errno = 0;

if (scanf("%ld", &num_long) != 1) {
  /* Handle error */
else if (ERANGE == errno) {
  if (puts("number out of range\n") == EOF) {
      /* Handle error */

Compliant Solution

This compliant example uses fgets() to input a string and strtol() to convert the string to an integer. Error checking is provided to make sure the value is a valid integer in the range of long.

char buff[25];
char *end_ptr;
long num_long;

if (fgets(buff, sizeof(buff), stdin) == NULL) {
  if (puts("EOF or read error\n") == EOF) {
    /* Handle error */
} else {
  errno = 0;

  num_long = strtol(buff, &end_ptr, 10);

  if (ERANGE == errno) {
    if (puts("number out of range\n") == EOF) {
      /* Handle error */
  else if (end_ptr == buff) {
    if (puts("not valid numeric input\n") == EOF) {
      /* Handle error */
  else if ('\n' != *end_ptr && '\0' != *end_ptr) {
    if (puts("extra characters on input line\n") == EOF) {
      /* Handle error */

Note that this solution treats any trailing characters, including whitespace characters, as an error condition.

Risk Assessment

Although it is relatively rare for a violation of this recommendation to result in a security vulnerability, it can easily result in lost or misinterpreted data.




Remediation Cost









Automated Detection





Axivion Bauhaus Suite


MISC.NEGCHARNegative Character Value

Can detect violations of this recommendation. In particular, it notes uses of the scanf() family of functions where on the type specifier is a floating-point or integer type

Helix QAC


LDRA tool suite
44 SEnhanced Enforcement
Parasoft C/C++test


Avoid using unsafe string functions that do not check bounds
PC-lint Plus



Fully supported

Related Vulnerabilities

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

Related Guidelines



  1. This rule needs to make clearer exactly what is verbotem in the scanf() family. My interpretation is that one must not pass as input parameter a pointer to any integer smaller than intmax_t (signed or unsigned), because a smaller integer might not be able to hold the object read in, resulting in undefined behavior.

    On second thought, both intmax_t and uintmax_t are also insufficient, because uintmax_t can't hold -1 and intmax_t can't hold UINT_MAX. So maybe this vul is unavoidable and we should forbid using scanf() on all ints, like the CCE illustrates.

    So can scanf() be done securely on ints?

    1. rcs sez:

      > i think we are telling people not to use scanf() because you can't check for invalid values, and its > a rule. at least the remediation cost is appropriately given as "high".

  2. Removing my comment as it does not apply.