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Do not use the same variable name in two scopes where one scope is contained in another. Examples includeFor example,

• No other variable should share the name of a global variable if the other value variable is in a subscope of the global variable.
• A block should not declare a variable with the same name as a variable declared in any block that contains it.

Reusing variable names leads to programmer confusion about which variable is being modified. Additionally, if variable names are reused, generally one or both of the variable names are too generic.

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Noncompliant Code Example

This non-compliant noncompliant code example reuses declares the msg identifier twice: at the start of the compilation unit (with file scope ) and also and reuses the same identifier to declare a character array local to the report_error() function. Consequently, the The programmer unintentionally copies a string may unintentionally copy the function argument to the locally declared msg array within the report_error() function, failing . Depending on the programmer's intention, it either fails to initialize the assign global variable and resulting in a potential buffer overflow msg or allows the local msg buffer to overflow by using the global value msgsize as a bounds for the local buffer.

#include <stdio.h>
 
static char msg[100];
static const size_t msgsize = sizeof( msg);

void report_error(const char const *error_msgstr) {
  char msg[80];
  snprintf(msg, msgsize, "Error: %s\n", str);
  /* ... */
}

int main(void) {
  /* ... */
  strncpy(msg, error_msg, sizeof(msg));
report_error("some error");
 
  return 0;
}

int 

Compliant Solution

This compliant solution uses different, more descriptive variable names:

#include <stdio.h>
 
static char message[100];
static const size_t message_size = sizeof( message);

void report_error(const char *strmain(void) {
  char error_msg[80];
  snprintf(msg, sizeof( msg), "Error: %s\n", str);
  /* Ensure error_msg isn't too long... */
}

int main(void) {
  /* ... */
  report_error("some error");
 
  return 0;
}

When the block is small, the danger of reusing variable names is mitigated by the visibility of the immediate declaration. Even in this case, however, variable name reuse is not desirable. In general, the larger the declarative region of an identifier, the more descriptive and verbose should be the name of the identifier.

By using different variable names globally and locally, the compiler forces the developer to be more precise and descriptive with variable names.

Noncompliant Code Example

This noncompliant code example declares two variables with the same identifier, but in slightly different scopes. The scope of the identifier i declared in the for loop's initial clause terminates after the closing curly brace of the for loop. The scope of the identifier i declared in the for loop's compound statement terminates before the closing curly brace. Thus, the inner declaration of i hides the outer declaration of i, which can lead to unintentionally referencing the wrong object.

void f(void)if (strlen(error_msg) >= sizeof(msg)) {
  for  error_msg[sizeof(error_msg) - 1] = '\0';
  }
  report_error(error_msg)(int i = 0; i < 10; i++) {
    long i;
    /* ... */
  }
}

Compliant Solution

This compliant solution uses different, more descriptive variable namesa unique identifier for the variable declared within the for loop.

char system_msg[100];

void report_error(char const *error_msgvoid f(void) {
  for (int i = 0; i < 10; i++) {
    char default_msg[80]long j;
    /* ... */
  }
}

Exceptions

DCL01-C-EX1: A function argument in a function declaration may clash with a variable in a containing scope provided that when the function is defined, the argument has a name that clashes with no variables in any containing scopes.

extern int ifname;
void f(char *name);  /* Declaration: no problem here */
/* ... */
void f(char *arg) {  /* Definition: no problem; arg doesn't hide name */
  /* Use arg */
}

DCL01-C-EX2: A temporary variable within a new scope inside of a macro can override an identifier in a containing scope. However,this exception does not apply to to the arguments of the macro itself.

#define SWAP(type, a, b) do { type tmp = a; a = b; b = tmp; } while(0)
 
void func(error_msg)
    strncpy(system_msg, error_msg, sizeof(system_msg));
  else
    strncpy(system_msg, default_msg, sizeof(system_msg));
  return;
}

int main(void) {
  char error_msg[80]int tmp = 100;
  /*int Ensurea error_msg isn't too long */= 10, b = 20;
  if (strlen(error_msg) >= sizeof(system_msg)) {
    error_msg[sizeof(error_msg) - 1] = '\0';
  }
  report_error(error_msg);
  /* ... */
}

When the block is small, the danger of reusing variable names is mitigated by the visibility of the immediate declaration. Even in this case, however, variable name reuse is not desirable.

By using different variable names globally and locally, the compiler forces the developer to be more precise and descriptive with variable names.

SWAP(int, a, b); /* Hidden redeclaration of tmp is acceptable */
  SWAP(int, tmp, b); /* NONCOMPLIANT: Hidden redeclaration of tmp clashes with argument */
}

Risk Assessment

Reusing a variable name in a subscope can lead to unintentionally referencing an incorrect variable.

Automated Detection

The LDRA tool suite Version 7.6.0 can detect violations of this recommendation.

Splint Version 3.1.1 can detect violations of this recommendation.

Compass / ROSE currently does not detect violations of this recommendation. However, detecting violations should be easy. One merely has to note every variable declaration. If the variable is static, it should go into some global list. If the variable is not static, but its name exists in the global list, then the rule has been violated.

Related Vulnerabilities

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

References

\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 5.2.4.1, "Translation limits"
\[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\] "BRS Leveraging human experience" and "YOW Identifier name reuse"
\[[MISRA 04|AA. C References#MISRA 04]\] Rule 5.2

Related Guidelines

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Image Added Image Added Image AddedDCL00-A. Const-qualify immutable objects      02. Declarations and Initialization (DCL)       DCL02-A. Use visually distinct identifiers