SonarCFamily for C/C++ Rule List

Stack allocated memory, like memory allocated with the functions alloca, _alloca, _malloca, __builtin_alloca, is automatically released at the end of the function, and should not be released with free. Explicitly free-ing such memory results in undefined behavior.

Noncompliant Code Example

void fun() {
 char *name = (char *) alloca(size);
 // ...
 free(name); // Noncompliant, memory allocated on the stack
 char *name2 = "name";
 // ...
 free(name2); // Noncompliant, memory allocated on the stack
}

Compliant Solution

void fun() {
 char *name = (char *) alloca(size);
 // ...
 char *name2 = "name";
 // ...
}

Using the value of a pointer to a FILE object after the associated file is closed is undefined behavior.

Noncompliant Code Example

void fun() {
 FILE * pFile;
 pFile = fopen(fileName, "w");
if (condition) {
 fclose(pFile);
 // ...
 }
fclose(pFile); // Noncompliant, the file has already been closed
}

Compliant Solution

void fun() {
 FILE * pFile;
 pFile = fopen(fileName, "w");
if (condition) {
 // ...
 }
fclose(pFile);
}

See
CERT, FIO46-C. - Do not access a closed file

Memory allocated dynamically with calloc(...), malloc(...), realloc(...) or new should be released when it's not needed anymore. Failure to do so will result in a memory leak that could bring the box to its knees.

This rule raises an issue when memory is allocated and not freed in the same function. Allocated memory is ignored if a pointer to it is returned to the caller or stored in a structure that's external to the function.

Noncompliant Code Example

int fun() {
 char* name = (char *) malloc (size);
 if (!name) {
 return 1;
 }
 // ...
 return 0; // Noncompliant, memory pointed by "name" has not been released
}

Compliant Solution

int fun() {
 char* name = (char *) malloc (size);
 if (!name) {
 return 1;
 }
 // ...
 free(name);
 return 0;
}

See
MITRE, CWE-401 - Improper Release of Memory Before Removing Last Reference ('Memory Leak')
MEM00-C. - Allocate and free memory in the same module, at the same level of abstraction
CERT, MEM31-C. - Free dynamically allocated memory when no longer needed

Once a block of memory has been freed, it becomes available for other memory requests. Whether it's re-used immediately, some time later, or not at all is random, and may vary based on load. Because of that randomness, tests may pass when running locally, but the odds are that such code will fail spectacularly in production by returning strange values, executing unexpected code, or causing a program crash.

Noncompliant Code Example

char *cp = malloc(sizeof(char)*10);
// ...
free(cp);
cp[9] = 0; // Noncompliant

See

MITRE, CWE-416 - Use After Free
CERT, MEM30-C. - Do not access freed memory
CERT, MEM50-CPP. - Do not access freed memory
CERT, EXP54-CPP. - Do not access an object outside of its lifetime