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Do not use deprecated or obsolescent functions when more secure equivalent functions are available. Deprecated functions are defined by the C Standard. Obsolescent functions are defined by this recommendation.

Deprecated Functions

The gets() function was deprecated by Technical Corrigendum 3 to C99 and eliminated from C11.

Obsolescent Functions

Functions in the first column of the following table are hereby defined to be obsolescent functions. To remediate invocations of obsolescent functions, an application might use inline coding that, in all respects, conforms to this guideline, or an alternative library that, in all respects, conforms to this guideline, or alternative non-obsolescent functions.

The atof, atoi, atol, and atoll functions are obsolescent because the strod, strtof, strtol, strtold, strtoll, strotul, and strtoull functions can emulate their usage and have more robust error handling capabilities. See guideline INT05-C. Do not use input functions to convert character data if they cannot handle all possible inputs.

The fopen and freopen functions are obsolescent because the fopen_s and freopen_s functions can emulate their usage and improve security by protecting the file from unauthorized access by setting its file protection and opening the file with exclusive access [ISO/IEC WG14 N1173].

The setbuf function is obsolescent because setbuf does not return a value and can be emulated using setvbuf. See guideline FIO12-C. Prefer setvbuf() to setbuf().

The rewind function is obsolescent because rewind does not return a value and can be emulated using fseek. See guideline FIO07-C. Prefer fseek() to rewind().

The asctime and ctime functions are obsolescent because they use non-reentrant static buffers and can be emulated using asctime_s and ctime_s.

Unchecked Obsolescent Functions

The following are hereby defined to be unchecked obsolescent functions:

To remediate invocations of unchecked obsolescent functions, an application might use inline coding that, in all respects, conforms to this guideline, or an alternative library that, in all respects, conforms to this guideline, or alternative nonobsolescent functions from C11 Annex K:

or alternative nonobsolescent functions from ISO/IEC TR 24731-2, Extensions to the C Library—Part II: Dynamic Allocation Functions [ISO/IEC TR 24731-2]:

Noncompliant Code Example

In this noncompliant code example, the obsolescent functions strcat() and strcpy() are used:

#include <string.h>
#include <stdio.h>
 
enum { BUFSIZE = 32 };
void complain(const char *msg) {

  static const char prefix[] = "Error: ";
  static const char suffix[] = "\n";
  char buf[BUFSIZE];

  strcpy(buf, prefix);
  strcat(buf, msg);
  strcat(buf, suffix);
  fputs(buf, stderr);
}

Compliant Solution

In this compliant solution, strcat() and strcpy() are replaced by strcat_s() and strcpy_s():

#define __STDC_WANT_LIB_EXT1__
#include <string.h>
#include <stdio.h>
 
enum { BUFFERSIZE = 256 };

void complain(const char *msg) {
  static const char prefix[] = "Error: ";
  static const char suffix[] = "\n";
  char buf[BUFFERSIZE];

  strcpy_s(buf, BUFFERSIZE, prefix);
  strcat_s(buf, BUFFERSIZE, msg);
  strcat_s(buf, BUFFERSIZE, suffix);
  fputs(buf, stderr);
}

Risk Assessment

The deprecated and obsolescent functions enumerated in this guideline are commonly associated with software vulnerabilities.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography