Page History

When you have a choice of two functions to accomplish the same task, prefer the one with better error checking and reporting.

The following table shows a list of C standard library functions that provide limited or no error checking and reporting along with preferable alternatives:

Noncompliant Code Example (atoi())

This noncompliant code example converts the string token stored in the static array buff to a signed integer value using the atoi() function:

int si;

if (argc > 1) {
  si = atoi(argv[1]);
}

The atoi(), atol(), and atoll() functions convert the initial portion of a string token to int, long int, and long long int representation respectively. Except for the behavior on error, they are equivalent as follows:

Unfortunately, atoi() and related functions lack a mechanism for reporting errors for invalid values. Specifically, the atoi(), atol(), and atoll() functions

• Do not need to set errno on an error.
• Have undefined behavior if the value of the result cannot be represented. (See undefined behavior 119 of Annex J of the C Standard.)
• Return 0 if the string does not represent an integer (which is indistinguishable from a correctly formatted, zero-denoting input string), but the C Standard only specifies the behavior of these functions on success.

See also MSC24-C. Do not use deprecated or obsolescent functions.

Compliant Solution (strtol())

The strtol(), strtoll(), strtoul(), and strtoull() functions convert a null-terminated byte string to long int, long long int, unsigned long int, and unsigned long long int representation respectively.

This compliant solution uses strtol() to convert a string token to an integer and ensures that the value is in the range of int:

long sl;
int si;
char *end_ptr;

if (argc > 1) {
  errno = 0;

  sl = strtol(argv[1], &end_ptr, 10);

  if ((sl == LONG_MIN || sl == LONG_MAX)
   && errno != 0)
  {
    perror("strtol error");
  }
  else if (end_ptr == argv[1]) {
    if (puts("error encountered during conversion") == EOF) {
      /* Handle error */
    }
  }
  else if (sl > INT_MAX) {
    printf("%ld too large!\n", sl);
  }
  else if (sl < INT_MIN) {
    printf("%ld too small!\n", sl);
  }
  else if ('\0' != *end_ptr) {
    if (puts("extra characters on input line\n") == EOF) {
      /* Handle error */
    }
  }
  else {
    si = (int)sl;
  }
}

Both the noncompliant code example and the compliant solution are taken from INT06-C. Use strtol() or a related function to convert a string token to an integer.

Noncompliant Code Example (rewind())

This noncompliant code example sets the file position indicator of an input stream back to the beginning using rewind():

char *file_name;
FILE *fp;

/* Initialize file_name */

fp = fopen(file_name, "r");
if (fp == NULL) {
  /* Handle open error */
}

/* Read data */

rewind(fp);

/* Continue */

It is impossible to determine if rewind() succeeded.

Compliant Solution (fseek())

This compliant solution uses fseek() instead of rewind() and checks to see if the operation succeeded:

char *file_name;
FILE *fp;

/* Initialize file_name */

fp = fopen(file_name, "r");
if (fp == NULL) {
  /* Handle open error */
}

/* Read data */

if (fseek(fp, 0L, SEEK_SET) != 0) {
  /* Handle repositioning error */
}

/* Continue */

Noncompliant Code Example (setbuf())

This noncompliant code example calls setbuf() with a buf argument of NULL:

FILE *file;
/* Setup file */
setbuf(file, NULL);
/* ... */

It is not possible to determine if the call to setbuf() succeeded.

Implementation Details

On 4.2BSD and 4.3BSD systems, setbuf() always uses a suboptimal buffer size and should be avoided.

Compliant Solution (setvbuf())

This compliant solution calls setvbuf(), which returns nonzero if the operation failed:

FILE *file;
char *buf = NULL;
/* Setup file */
if (setvbuf(file, buf, buf ? _IOFBF : _IONBF, BUFSIZ) != 0) {
  /* Handle error */
}
/* ... */

Risk Assessment

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

Automated Detection

This rule in general cannot be detected, although various examples can be detected by simply scanning for functions that have equivalent functions with better error handling.

Related Vulnerabilities

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

Related Guidelines

Bibliography