Why is my string not showing after transfer via SHM? - c

Question

I've created the following types to transfer a list of nodes from one process to the other one:
typedef struct {
char *from;
char *to;
} Edge;
typedef struct {
size_t numEdges;
Edge *fst;
} EdgeList;
typedef struct {
EdgeList buffer[BUFFER_SIZE];
...
} CircularBuffer; /**< used as shared memory. */
This is what the process that sends the EdgeList looks like:
// open shared memory -> shm_open()
int shm_fd = shm_open(SHM_PATH, O_RDWR, S_IRUSR | S_IWUSR);
...
// map shared memory into memory -> mmap()
CircularBuffer *shm = mmap(NULL, sizeof(CircularBuffer),
PROT_READ | PROT_WRITE, MAP_SHARED, shm_fd, 0);
...
while (true) {
shm->buffer[...] = getSolution(...);
...
}
...
This is what the process that receives the EdgeList looks like:
// create shared memory -> shm_open()
int shm_fd = shm_open(SHM_PATH, O_RDWR | O_CREAT | O_EXCL, S_IRUSR | S_IWUSR);
...
// set shared memory size -> ftruncate()
ftruncate(shm_fd, sizeof(CircularBuffer);
...
// map shared memory into memory -> mmap()
CircularBuffer *shm = mmap(NULL, sizeof(CircularBuffer),
PROT_READ | PROT_WRITE, MAP_SHARED, shm_fd, 0);
...
EdgeList bestSolution = {.numEdges = SIZE_MAX, .fst = NULL};
while (true) {
EdgeList solution = shm->buffer[...];
if (solution.numEdges < bestSolution.numEdges) {
bestSolution = solution;
// I can read 'bestSolution.numEdges' here just fine.
// But i can't read 'bestSolution.fst[i].from' and 'bestSolution.fst[i].to' for some reason.
}
...
}
...
Now here is where things behave unexpectedly: The receiving process is not showing me any strings from the shared memory but everything else works just as expected.
I have access to bestSolution.numEdges after it being read from the shared memory, but not to bestSolution.fst[i].from and bestSolution.fst[i].to (with i being a legal index from that array).
When I try to read these strings, I only see (null) instead of the strings. Can I fix this issue?