How to allocate memory for array of structs? - c++

Question

I've a C++ library (really a wrapper for another C++ library) and I need pass some structs to my C application.
I don't know how allocate the memory dynamically.
I get a segmentation fault.
library.h
struct my_substruct {
unsigned char id ;
time_t date ;
char *info ;
};
typedef struct my_substruct My_substruct ;
struct my_struct {
char *description ;
unsigned char value ;
My_substruct *substruct ;
};
typedef my_struct My_struct ;
library.cpp
unsigned char getStructs(My_struct *structs)
{
vector <structCPPLibrary> structsCPPLibrary = getFromCPPLibrary();
unsigned char numStructs = structsCPPLibrary.size();
structs = (My_struct *)malloc(numStructs*sizeof(My_struct));
unsigned char indexStruct = 0;
for (auto s : structsCPPLibrary)
{
structs[indexStruct].description = (char *)malloc(s.description.size() + 1);
strcpy(structs[indexStruct].description, s.description.c_str()); // In 's' is a std::string
structs[indexStruct].value = s.value; // In 's' is an unsigned char too
unsigned char numSubstructs = s.substructs.size(); // In 's' is a vector of Substructs
structs[indexStruct].substruct = (My_substruct *)malloc(numSubstructs*sizeof(My_substruct));
unsigned char indexSubstruct = 0;
for (auto sub : s.substruct) {
structs[indexStruct].substruct[indexSubstruct].id = sub.id; // In 'sub' is an unsigned char too
structs[indexStruct].substruct[indexSubstruct].date = sub.date; // In 'sub' is a time_t too
structs[indexStruct].substruct[indexSubstruct].info = (char *)malloc(sub.info.size() + 1);
strcpy(structs[indexStruct].substruct[indexSubstruct].info, sub.info.c_str()); // In 'sub' is a std::string
indexSubstruct++;
}
indexStruct++;
}
return indexStruct;
}
main.c
void getStructFromWrapper(void)
{
My_struct *structs;
unsigned char numStruct = getStructs(structs);
show_content(structs);
}

Answer 1

Change
unsigned char getStructs(My_struct *structs) {
...
}
getStructs(structs);
To
unsigned char getStructs(My_struct **p_structs) {
// C function can't be pass by reference, so convert to a reference here
auto& struct = *p_structs;
...
}
...
getStructs(&structs);
Your problem is that your struct = ... line is not modifying the value of structs in getStructFromWrapper.