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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.

...

Noncompliant Code Example

In this non-compliant code example, the programmer sets the value of the msg variable, expecting to reuse it outside the block. Due to the reuse of the variable name, however, the outside msg variable value is not changedThis 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, it either fails to initialize the global variable 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 *error_msgstr) {
  char msg[80];
  snprintf(msg, msgsize, "Error: %s\n", str);
  /* ... */
}

int main(void) {
  /* Assume error_msg isn't too long ... */
  strcpy(msg,report_error("some error_msg");
 
  return 0;
}
/* ... */

/* Ensure error_msg isn't too long */
if (strlen( error_msg) >= sizeof( msg)) {
  error_msg[sizeof(msg) - 1] = '\0';
}
report_error( error_msg); /* oops! */

Furthermore, if the length of the null-terminated byte string referenced by error_msg is greater than 79 characters in length, a buffer overflow will occur on the stack, which may be exploitable. This occurs in spite of the outer function's attempt to prevent buffer overflow!

Compliant Solution

This compliant solution uses different, more descriptive variable names.

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;
    

char system_msg[100];
/* ... */
void report_error(const char *error_msg  }
}

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++) {
  char default_msg[80]  long j;
    /* ... */
  /* Assume error_msg isn't too long */
  strcpy(system_msg, error}
}

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);  /* Declaration: no problem here */
_msg);
  return;
}
/* ... */

void f(char *arg) {  /* Ensure error_msg isn't too long */
if (strlen( error_msg) >= sizeof( system_msg)) {
  error_msg[sizeof(msg) - 1] = '\0';
}
report_error( error_msg); /* good */

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.

...

 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(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 unintended values for the unintentionally referencing an incorrect variable.

Automated Detection

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The LDRA tool suite V 7.6.0 is able to detect violations of this recommendation.

Tool	Version	Checker	Description
Astrée	Include Page Astrée_V Astrée_V		Supported indirectly via MISRA C:2012 Rule 5.3.
Axivion Bauhaus Suite	Include Page Axivion Bauhaus Suite_V Axivion Bauhaus Suite_V	CertC-DCL01
CodeSonar	Include Page CodeSonar_V CodeSonar_V	LANG.ID.ND.NEST	Non-distinct identifiers: nested scope
Compass/ROSE
ECLAIR	Include Page ECLAIR_V ECLAIR_V	CC2.DCL01	Fully implemented
Helix QAC	Include Page Helix QAC_V Helix QAC_V	C0795, C0796, C2547, C3334
Klocwork	Include Page Klocwork_V Klocwork_V	MISRA.VAR.HIDDEN
LDRA tool suite	Include Page LDRA_V LDRA_V	131 S	Fully implemented
Parasoft C/C++test	Include Page Parasoft_V Parasoft_V	CERT_C-DCL01-a CERT_C-DCL01-b	Identifier declared in a local or function prototype scope shall not hide an identifier declared in a global or namespace scope Identifiers declared in an inner local scope should not hide identifiers declared in an outer local scope
PC-lint Plus	Include Page PC-lint Plus_V PC-lint Plus_V	578	Fully supported
Polyspace Bug Finder	Include Page Polyspace Bug Finder_V Polyspace Bug Finder_V	CERT C: Rec. DCL01-C	Checks for variable shadowing (rule fully covered)
PVS-Studio	Include Page PVS-Studio_V PVS-Studio_V	V561, V688, V703, V711, V2015
RuleChecker	Include Page RuleChecker_V RuleChecker_V		Supported indirectly via MISRA C:2012 Rule 5.3.
Splint	Include Page Splint_V Splint_V

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"
\[[MISRA 04|AA. C References#MISRA 04]\] Rule 5.2

Related Guidelines

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Image Added Image Added Image AddedDCL00-A. Declare immutable values using enum or const      02. Declarations and Initialization (DCL)       DCL02-A. Use visually distinct identifiers