Question

I’m a real C++ noob, so bear with me.

I have a simple program. When I compile it with g++ -Wall prog.cpp -o prog and then run it with ./prog it just segfaults.

After some tinkering I wrote a makefile (see below). If I run make test, the program compiles and runs fine. If I run it with ./prog, it segfaults. (The exact error message is Segmentation fault (core dumped))

Can anyone explain why?

Here is the program:

#include <iostream>
#include <stdlib.h>
#include <stdio.h>
#include <sys/time.h>

int main() {

    srand(time(NULL));

    //int i, j, k, i2, j2, k2;
    int N = 1000;

    double mul1[N][N];
    double mul2[N][N];
    double res[N][N];

    printf("N: %d\n", N);

    for(int x=0; x<N; x++) {
        for(int y=0; y<N; y++) {
            mul1[x][y] = rand() % 100;
            mul2[x][y] = rand() % 100;
            res[x][y] = 0;
        }
    }

    return 0;
}

Here is the makefile:

all: prog

prog: prog.cpp
    $(CXX) -Wall -g -o $@ prog.cpp

test: prog
    ./prog

clean: 
    rm -f prog

Answer 1

First:

int N = 1000;

double mul1[N][N];
double mul2[N][N];
double res[N][N];

You have a stack overflow.

Assuming double is 8 bytes on your platform, your 3 arrays are more than 20 MB (3 * 8 * 1000 * 1000) in size. For example on my Linux machine the stack size allocated for every process is 8192 kB.

Second:

Regarding why it does not work on the shell but it works in the Makefile context you can try this:

Change N to 10 (so it does no longer segfault) and add this function call at the beginning of your program:

 system("ulimit -s");  // system is in stdlib.h

You’ll see that the stack size is different in the shell (for example the printed value of system is 8192) than in the Makefile context (for example the printed value of system is unlimited).