Do not use the same variable name in two scopes where one scope is contained in another. For example,
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.
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, 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 *str) { char msg[80]; snprintf(msg, msgsize, "Error: %s\n", str); /* ... */ } int main(void) { /* ... */ report_error("some error"); return 0; } |
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.
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; /* ... */ } } |
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; /* ... */ } } |
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 */ /* ... */ 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(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 */ } |
Reusing a variable name in a subscope can lead to unintentionally referencing an incorrect variable.
Recommendation | Severity | Likelihood | Remediation Cost | Priority | Level |
---|---|---|---|---|---|
DCL01-C | Low | Unlikely | Medium | P2 | L3 |
Tool | Version | Checker | Description |
---|---|---|---|
Astrée | Supported indirectly via MISRA C:2012 Rule 5.3. | ||
Axivion Bauhaus Suite | CertC-DCL01 | ||
CodeSonar | LANG.ID.ND.NEST | Non-distinct identifiers: nested scope | |
Compass/ROSE | |||
CC2.DCL01 | Fully implemented | ||
Helix QAC | C0795, C0796, C2547, C3334 | ||
Klocwork | MISRA.VAR.HIDDEN | ||
LDRA tool suite | 131 S | Fully implemented | |
Parasoft C/C++test | CERT_C-DCL01-a | Identifier declared in a local or function prototype scope shall not hide an identifier declared in a global or namespace scope | |
PC-lint Plus | 578 | Fully supported | |
Polyspace Bug Finder | Checks for variable shadowing (rule fully covered) | ||
PVS-Studio | V561, V688, V703, V711, V2015 | ||
RuleChecker | Supported indirectly via MISRA C:2012 Rule 5.3. | ||
Splint |
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
SEI CERT C++ Coding Standard | VOID DCL01-CPP. Do not reuse variable names in subscopes |
MISRA C:2012 | Rule 5.3 (required) |