Segmentation fault after realloc(). Can't assign allocated memory to pointer - c

Question

I'm trying to allocate some memory with realloc(). This works so far. But if I want to assign the allocated memory to a pointer in a struct variable, I get a segmentation fault:
// in header
typedef struct {
int a;
char test[20];
} MyContent;
typedef struct {
MyContent* values;
// simmilar to: MyContent values[]
// ... some other stuff
} MyData;
// in source
void myFunction(MyData* dataPtr) {
dataPtr->values = NULL;
MyData* tempPtr = NULL;
for (int i = 1; i < 10; i++) {
tempPtr = (MyContent*) realloc(dataPtr->values, i * sizeof(MyContent));
if (tempPtr == NULL) {
free(dataPtr->values);
break;
}
dataPtr->values = tempPtr; // Here I get the segmentation fault
dataPtr->values[(i-1)].a = 42;
// ...
}
}
I can't figure out what's going wrong here. Any suggestions? Thanks for your help.

Answer 1

Seems like you edited your code. The edited code works just fine.
#include<stdio.h>
#include<malloc.h>
#include<string.h>
// in header
typedef struct {
int a;
char test[20];
} MyContent;
typedef struct {
MyContent* values;
// simmilar to: MyContent values[]
// ... some other stuff
} MyData;
// in source
void myFunction(MyData* dataPtr) {
dataPtr->values = NULL;
MyData* tempPtr;
for (int i = 1; i < 10; i++) {
tempPtr = (MyData*) realloc(dataPtr->values, i * sizeof(MyContent));
if (tempPtr == NULL) {
if(dataPtr->values)
free(dataPtr->values);
printf("realloc() failed\n");
return ;
}
dataPtr->values = (MyContent*)tempPtr; // Here I get the segmentation fault
dataPtr->values[(i-1)].a = 42+i;
strcpy(dataPtr->values[(i-1)].test,"name");
}
}
void PrintData(MyData* dataPtr) {
for (int i = 1; i < 10; i++)
printf("We have %s at %d\n",dataPtr->values[(i-1)].test,dataPtr->values[(i-1)].a);
}
main() {
MyData Sample;
myFunction(&Sample);
PrintData(&Sample);
}

Answer 2

At first glance, I don't see a problem that could cause a crash there - that ones-based addressing is a bit odd, but not incorrect. There could be a problem in the code that you're not showing that results in heap or stack corruption that the realloc call makes worse. Or if you are compiling with optimizations, your debugger might be confused about where the crash is actually occurring. You're also confusing MyData and MyContent, but I'll assume that's just because you made an error while redacting the code.
Note also that if realloc fails, you will crash on the line after the one you indicated, as you'll be writing to a null pointer. You need to abort if tempPtr is NULL, not just free the old pointer. Again, though, this causes a fault on a different line than you indicated.
I'd recommend running your program under valgrind to see where it reports errors - the first such error to occur is likely to be the culprit.

Answer 3

Your condition is wrong for checking the newly allocated memory. It should be:
if (tempPtr == NULL) {
// handle error condition or continue with original 'dataPtr->values'
}
else {
dataPtr->values = tempPtr;
}
Remember that realloc() doesn't necessarily transfer one block to the another block. Sometimes it may allocate the memory in the same pointer region.