C: Passing one dimension of a 2D array results in segfault - c

Question

it has been a while since I wrote my last lines of C and I'm having issues getting into the hang of C at this moment. I guess this is probably a stupid mistake I'm making, but after having researched the issue for some time I still couldn't figure it out what I'm doing wrong. So this is how my code looks like:
int read_signal_from_file(const char* path, float* signal, SF_INFO * info) {
SNDFILE *sndFile;
sndFile = sf_open(path, SFM_READ, info);
signal = malloc(info->frames * sizeof(float));
long numFrames = sf_readf_float(sndFile, signal, info->frames);
...
}
and
int main(int argc, char *argv[]) {
float** signals = malloc(NUM_FILES * sizeof(float*));
float avg = 0.0;
SF_INFO tmp_info;
for(int i = 0; i < NUM_FILES; i++) {
read_signal_from_file(INPUT_FILES[i], signals[i], &tmp_info);
for(long j = 0; j < tmp_info.frames; j++) {
printf("Sample no #%ld: %f\n", j, signals[i][j]);
}
}
}
inside of read_signal_from_file(), I have no troubles accessing the allocated memory portion. But as soon as I'm trying to access it from the scope of the main function (e.g. as in the printf() example above, I get corrupted results that look suspiciously like uninitialized memory or overflows. If I try to operate on that data, (e.g. by calling vDSP_meanv(), the application crashes with a segfault.

Answer 1

Firstly, you don't have a 2D array, you have a pointer-to-pointer.
Then, the pointers themselves don't point to anything, they are uninitialized because you don't allocate memory for them - thus your program invokes undefined behavior.
So, either allocate memory for them using malloc() in a for loop, or even better, use a real array:
float (*arr)[COLUMN_SIZE] = malloc(sizeof(*arr) * ROW_SIZE);

Answer 2

You are allocating the array of pointers, but you aren't setting them to anything:
float** signals = malloc(NUM_FILES * sizeof(float*));
Which means that you are passing an uninitialized pointer (signals[i]) here:
read_signal_from_file(INPUT_FILES[i], signals[i], &tmp_info)
Because signals[i] is passed by value, it isn't going to be changed in the function call, so it is still going to be uninitialized after read_signal_from_file returns.
A simple change would be to pass by reference:
int read_signal_from_file(const char* path, float* &signal, SF_INFO * info) { ... }

Answer 3

(As mentioned) You are allocating the signals array of pointers, but you don't assign the pointers in this array. When you allocate signal in read_signal_from_file you do reserve the memory but you're assigning the pointer to the local signal variable. If you want to assign the allocated signal to the signals array in your main function you should pass a pointer to the pointer:
read_signal_from_file(INPUT_FILES[i], &signals[i], &tmp_info);
and change your read_signal_from_file function to accept it:
int read_signal_from_file(const char* path, float ** signal, SF_INFO * info) {
// ...
*signal = malloc(info->frames * sizeof(float));
// ...

Answer 4

You pass float *signal to read_signal_from_file() and reassign signal there but that just changes the local var signal, not signals[i] in main().
Change the signature of read_signal_from_file() to
int read_signal_from_file(const char* path, float** signal, SF_INFO * info)
change your malloc() to
*signal = malloc(info->frames * sizeof(float));
and pass signals+i instead of signals[i]