如何通过函数将指针传递给c中未声明的可修改1 / 2D数组并保持其值？

Question

I am writing a function in C that needs to modify multiple variables as it progresses, specifically the last 3 args. The function declaration is shown below:

void Sample_v2(double cyl_radius, double thetaOx, double binsize, double ** newRx, double * binCenterThetas, double ** binCenters) 

And my variables are as follows:

int main() {
    // initialize function outputs
    double ** GPU_binCenters;
    double ** GPU_Rx;
    double * GPU_binCenterThetas;
    // set testing values
    double cyl_rad = 0.3;
    double theta = 1.5708;
    double binsiz = 0.0025;
    int n;

    Sample_v2(cyl_rad, theta, binsiz, GPU_Rx, GPU_binCenterThetas, GPU_binCenters);

    fprintf(stdout,"GPU_binCenters\n");
    for (n = 0; n < 188; n++) {
        fprintf(stdout, "%G\n", GPU_binCenterThetas[n]);
    }
}

I give my arrays values with the following code:

newRx = (double**) calloc((int)ceil(howmany) * 2, sizeof(double*));
for (ii = 0; ii < (int)ceil(howmany) * 2; ii++) {
    newRx[ii] = (double*) calloc(2, sizeof(double));
}

or

binCenterThetas = (double*) calloc((int)ceil(howmany)*2+1, sizeof(double));

And get a segfault when I try to run this on the fprintf statement in the for loop, the first time it runs. Using gdb I am not able to manually read values as well, but I can see the pointer address. I think the pointer is being copied when I pass it, but I tried implementing another solution passing my variables that need to be modified by reference and had issues with that as well. The code for that is below:

Sample_v2(cyl_rad, theta, binsiz, &GPU_Rx, &GPU_binCenterThetas, &GPU_binCenters);

    ...........

void Sample_v2(double cyl_radius, double thetaOx, double binsize, double *** newRx, double ** binCenterThetas, double *** binCenters) {

    ...........

*newRx = (double**) malloc((int)ceil(howmany) * 2 * sizeof(double*));
for (ii = 0; ii < (int)ceil(howmany) * 2; ii++) {
    *newRx[ii] = (double*) malloc(2 * sizeof(double));
}

That code segfaults when allocating the memory within the for loop. Any help on how I can actually get my array to either be passed out or initialize without segfaulting. Note that I am not using a return statement as I have 3 arrays that are modified within the function.

Answer 1

When you call

Sample_v2(cyl_rad, theta, binsiz, GPU_Rx, GPU_binCenterThetas, GPU_binCenters);

the arguments received by Sample_v2 are copies of the pointers in main , and any changes done in Sample_v2 to those pointers don't influence the pointers in main , so when you enter the fprintf loop, GPU_binCenterThetas is still an uninitialised pointer, and accessing GPU_binCenterThetas[n] is undefined behaviour and likely to cause a segfault.

The correct way to handle that is indeed to add a layer of indirection and pass the addresses of these pointers (or to define a struct containing three pointers of appropriate types and return that; if there was only one pointer to modify, returning the pointer would be my preferred way - if the pointers are returned, they need not be passed as arguments).

The problem with your attempt

Sample_v2(cyl_rad, theta, binsiz, &GPU_Rx, &GPU_binCenterThetas, &GPU_binCenters);

    ...........

void Sample_v2(double cyl_radius, double thetaOx, double binsize, double *** newRx, double ** binCenterThetas, double *** binCenters) {

    ...........

*newRx = (double**) malloc((int)ceil(howmany) * 2 * sizeof(double*));
for (ii = 0; ii < (int)ceil(howmany) * 2; ii++) {
    *newRx[ii] = (double*) malloc(2 * sizeof(double));
}

is that you have gotten the precedence wrong.

*newRx[ii]

is

*(newRx[ii])

but newRx[ii] is only a valid pointer for ii == 0 . What you meant was

(*newRx)[ii]

(and similar for the others).