ENV30-C. Do not modify the object referenced by the return value of certain functions

Some functions return a pointer to an object that cannot be modified without causing undefined behavior. These functions include getenv(), setlocale(), localeconv(), asctime(), and strerror(). In such cases, the function call results must be treated as being const-qualified.

The C Standard, 7.24.4.6, paragraph 4 [ISO/IEC 9899:2024], defines getenv() as follows:

The getenv function returns a pointer to a string associated with the matched list member. The string pointed to shall not be modified by the program, but may be overwritten by a subsequent call to the getenv function. If the specified name cannot be found, a null pointer is returned.

If the string returned by getenv() must be altered, a local copy should be created. Altering the string returned by getenv() is undefined behavior. (See undefined behavior 184.)

Similarly, subclause 7.11.1.1, paragraph 8 [ISO/IEC 9899:2024], defines setlocale() as follows:

The pointer to string returned by the setlocale function is such that a subsequent call with that string value and its associated category will restore that part of the program'€™s locale. The string pointed to shall not be modified by the program, but may be overwritten by a subsequent call to the setlocale function.

 And subclause 7.11.2.1, paragraph 8 [ISO/IEC 9899:2024], defines localeconv() as follows:

The localeconv function returns a pointer to the filled-in object. The structure pointed to by the return value shall not be modified by the program, but may be overwritten by a subsequent call to the localeconv function. In addition, calls to the setlocale function with categories LC_ALL, LC_MONETARY, or LC_NUMERIC may overwrite the contents of the structure.

Altering the string returned by setlocale() or the structure returned by localeconv() are undefined behaviors. (See undefined behaviors 120 and 121.) Furthermore, the C Standard imposes no requirements on the contents of the string by setlocale(). Consequently, no assumptions can be made as to the string's internal contents or structure.

Finally, subclause 7.26.6.3, paragraph 4 [ISO/IEC 9899:2024], states

The strerror function returns a pointer to the string, the contents of which are locale-specific. The array pointed to shall not be modified by the program. The behavior is undefined if the returned value is used after a subsequent call to the strerror function, or after the thread which called the function to obtain the returned value has exited.

Altering the string returned by strerror() is undefined behavior. (See undefined behavior 184.)

Noncompliant Code Example (getenv())

This noncompliant code example modifies the string returned by getenv() by replacing all double quotation marks (") with underscores (_):

#include <stdlib.h>
 
void trstr(char *c_str, char orig, char rep) {
  while (*c_str != '\0') {
    if (*c_str == orig) {
      *c_str = rep;
    }
    ++c_str;
  }
}

void func(void) {
  char *env = getenv("TEST_ENV");
  if (env == NULL) {
    /* Handle error */
  }
  trstr(env,'"', '_');
}

Compliant Solution (getenv()) (Environment Not Modified)

If the programmer does not intend to modify the environment, this compliant solution demonstrates how to modify a copy of the return value:

#include <stdlib.h>
#include <string.h>
 
void trstr(char *c_str, char orig, char rep) {
  while (*c_str != '\0') {
    if (*c_str == orig) {
      *c_str = rep;
    }
    ++c_str;
  }
}
 
void func(void) {
  const char *env;
  char *copy_of_env;

  env = getenv("TEST_ENV");
  if (env == NULL) {
    /* Handle error */
  }

  copy_of_env = (char *)malloc(strlen(env) + 1);
  if (copy_of_env == NULL) {
    /* Handle error */
  }

  strcpy(copy_of_env, env);
  trstr(copy_of_env,'"', '_');
  /* ... */
  free(copy_of_env);
}

Compliant Solution (getenv()) (Modifying the Environment in POSIX)

If the programmer's intent is to modify the environment, this compliant solution, which saves the altered string back into the environment by using the POSIX setenv() and strdup() functions, can be used:

#include <stdlib.h>
#include <string.h>
 
void trstr(char *c_str, char orig, char rep) {
  while (*c_str != '\0') {
    if (*c_str == orig) {
      *c_str = rep;
    }
    ++c_str;
  }
}
 
void func(void) {
  const char *env;
  char *copy_of_env;

  env = getenv("TEST_ENV");
  if (env == NULL) {
    /* Handle error */
  }

  copy_of_env = strdup(env);
  if (copy_of_env == NULL) {
    /* Handle error */
  }

  trstr(copy_of_env,'"', '_');

  if (setenv("TEST_ENV", copy_of_env, 1) != 0) {
    /* Handle error */
  }
  /* ... */
  free(copy_of_env);
}

Noncompliant Code Example (localeconv())

In this noncompliant example, the object returned by localeconv() is directly modified:

#include <locale.h>
 
void f2(void) {
  struct lconv *conv = localeconv();
 
  if ('\0' == conv->decimal_point[0]) {
    conv->decimal_point = ".";
  }
}

Compliant Solution (localeconv()) (Copy)

This compliant solution modifies a copy of the object returned by localeconv():

#include <locale.h>
#include <stdlib.h>
#include <string.h>
 
void f2(void) {
  const struct lconv *conv = localeconv();
  if (conv == NULL) {
     /* Handle error */
  }
  
  struct lconv *copy_of_conv = (struct lconv *)malloc(
    sizeof(struct lconv));
  if (copy_of_conv == NULL) {
    /* Handle error */
  }
 
  memcpy(copy_of_conv, conv, sizeof(struct lconv));
 
  if ('\0' == copy_of_conv->decimal_point[0]) {
    copy_of_conv->decimal_point = ".";  
  }
  /* ... */
  free(copy_of_conv);
}

Risk Assessment

Modifying the object pointed to by the return value of getenv(), setlocale(), localeconv(), asctime(), or strerror() is undefined behavior. Even if the modification succeeds, the modified object can be overwritten by a subsequent call to the same function.

Automated Detection

Related Vulnerabilities

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Related Guidelines

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Bibliography