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

• No other variable should share the name of a global variable if the other 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.

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This noncompliant code example declares the msg identifier at file scope and reuses the same identifier to declare a character array local to the report_error() function. The programmer may unintentionally copy the function argument to the locally declared msg array within the report_error() function. Depending on the programmer's intention, this it either fails to initialize the global variable msg, or this 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 *str) {
  char msg[80];
  snprintf(msg, msgsize, "Error: %s\n", str);
  /* ... */
}

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

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 *str) {
  char msg[80];
  snprintf(msg, sizeof( msg), "Error: %s\n", str);
  /* ... */
}

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) {
  for (int i = 0; i < 10; i++) {
    long i;
    /* ... */
  }
}

Compliant Solution

This compliant solution uses a unique identifier for the variable declared within the for loop.

void f(void) {
  for (int i = 0; i < 10; i++) {
    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 name;
void f(char *name);   //* declarationDeclaration: no problem here */
//* ... */
void f(char *arg) {   //* definitionDefinition: no problem,; arg doesn't hide name */
  //* useUse 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(void) {
  int tmp = 100;
  int a = 10, b = 20;
  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

Related Vulnerabilities

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

Related Guidelines

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ISO/IEC 9899:1999 Section 5.2.4.1, "Translation limits"

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Bibliography

Image Removed      02. Declarations and Initialization (DCL)      Image Modified