Seg Fault from call to destroy malloced struct - c

Question

So when I call the following function I get a seg fault:
void destroyVariableVector(VariableVector* variableVector) {
if (variableVector) {
free(variableVector->variables); // <== Seg Fault here
free(variableVector);
}
}
And this is what my structs look like:
struct _Variable {
char *variableName;
char *arrayOfElements;
int type;
int32_t address;
};
typedef struct _Variable Variable;
struct _VariableVector {
int size; // elements full in array
int capacity; // total available elements
Variable *variables;
};
typedef struct _VariableVector VariableVector;
and here are their init methods:
Variable* initVariable(char *variableName, char *arrayOfElements,
int32_t address, int type) {
Variable* initialVariable = malloc(sizeof(*initialVariable));
if (initialVariable != NULL ) {
initialVariable->variableName = strdup(variableName);
initialVariable->arrayOfElements = strdup(arrayOfElements);
initialVariable->address = address;
initialVariable->type = type;
}
return initialVariable; // may be NULL
}
VariableVector* initVariableVector() {
VariableVector* initialVariableVector = malloc(sizeof(VariableVector));
if (initialVariableVector != NULL ) {
initialVariableVector->size = 0;
initialVariableVector->capacity = VECTOR_INITIAL_CAPACITY;
initialVariableVector->variables = malloc(
sizeof(Variable) * VECTOR_INITIAL_CAPACITY);
}
return initialVariableVector;
}
Can anyone explain how I am getting a seg fault when I call my destroyVariableVector() method???
Here is the code that calls the above functions:
VariableVector* variableVector = initVariableVector();
// add some variables to variableVector
writeOutVariables(variableVector, outputFilePointer);
destroyVariableVector(variableVector);
where the method writeOutVariables looks like:
void writeOutVariables(VariableVector *variableVector, FILE *outputFilePointer) {
// write out all variables within the variableVector to the output file
int variableVectorSize = variableVector->size;
int i = 0;
// start from the first variable in the varaibleVector to the last variable
while (i < variableVectorSize) {
// 0 = label; 1 = variable ascii string; 2 = variable number array;
int currentType = variableVector->variables->type;
if (currentType == 1) {
writeToFileASCIICharArrayInReverseOrder(
variableVector->variables->arrayOfElements,
outputFilePointer);
i++;
} else if (currentType == 2) {
char currentNumberArray[MAXIMUM_LINE_LENGTH + 1]; // + 1 for terminating char
strcpy(currentNumberArray,
variableVector->variables->arrayOfElements);
char* currentNumber = strtok(currentNumberArray, " ,\t\n");
while (currentNumber != NULL ) {
// you have not yet reached the end of the numberArray
int integer = atoi(currentNumber);
writeToFileIntegerAs32bits(integer, outputFilePointer);
currentNumber = strtok(NULL, " ,\t\n");
}
i++;
} else {
i++;
}
variableVector->variables++; // move on to next variable
}
}

Answer 1

In writeOutVariables, you're incrementing variableVector->variables to iterate through your list. However, this because it's a pointer, you're incrementing the actual value and hence moving the pointer beyond the end. Then, when you try to free variables, you're actually freeing memory that wasn't previously malloced.
Instead, initialize a pointer to variableVector->variables inside writeOutVariables so you don't clobber that pointer.
void writeOutVariables(VariableVector *variableVector, FILE *outputFilePointer) {
Variable *tmpVariables = variableVector->varaibles;
// The original code
// ...
tmpVariables++;
}