Xcode 错误：“EXC_BAD_ACCESS”

Question

I am attempting to compile and run a C program in Xcode. This program requires a text file for reading input and another text file for writing the data. I have put the program, and these two text files in the Source folder. The program builds successfully, but when I try to run the program I get the error: GDB: Program received signal: "EXC_BAD_ACCESS"

What could be causing this?

 int main() {

     FILE *fp; 
     FILE *fr;

     //Declare and intialize all variables to be used
     float ax = 0, ay = 0, x = 0, y = 0, vx = 0, vy = 0; 
     float time = 0, deltaTime = .001; 
     float timeImpact = 0, vyImpact = 0, vxImpact = 0, xImpact = 0, yImpact = 0; 

     int numBounces = 0;

     //Coefficient of Restitution; epsilon = ex = ey
     float ex = .5;
     float ey = .5;

     fr = fopen("input_data.txt", "rt"); //Open file for reading

     fp = fopen( "output_data.txt", "w" ); // Open file for writing

     if(fr == NULL){ printf("File not found");} //if text file is not in directory...

     if(fp == NULL){ printf("File not found");} //if text file is not in directory...

     fscanf(fr, "ax: %f ay: %f x: %f y: %f vx: %f vy: %f\n", &ax, &ay, &x, &y, &vx, &vy); 

     while (time < 5) {

          //time = time + deltaTime
          time = time + deltaTime;
          //velocity[new] = velocity[old] + acc * deltaTime
          vx = vx + ax*deltaTime;
          vy = vy + ay*deltaTime;
          //position[new] = position[old] + velocity*deltaTime + .5*acc*(deltaTime)^2
          x = x + vx*deltaTime + (.5*ax*deltaTime*deltaTime);
          y = y + vy*deltaTime + (.5*ay*deltaTime*deltaTime);  

          //fprintf(fp, "%f\t%f\t%f\t%f\t%f\t%f\t%f\t\n", ax, ay, x, y, vx, vy, time);

               //Collision occurs; implement collision response
               if (y < 0) {

                  //Find time of collision by solving for t in equation vy*t + .5*ay*t^2 + y = 0
                  timeImpact = (-vy + sqrt((vy*vy) - (2*ay*y)) / (2*.5*ay)); //Collision time = 3.7?

                  //velocity = -epsilon*velocity[Impact] + acc*time
                  vy = (-1*ey)*vyImpact + ay*((deltaTime - time) - timeImpact);
                  vx = (-1*ex)*vxImpact + ay*((deltaTime - time) - timeImpact);                            

                  //Position = position[Impact] - epsilon*velocity[Impact]*time + 1/2*acc*time^2
                  x = xImpact - ex*vxImpact*((deltaTime - time) - timeImpact) + .5*ax*       ((deltaTime - time) - timeImpact) * ((deltaTime - time) - timeImpact);
                  y = yImpact - ey*vyImpact*((deltaTime - time) - timeImpact) + .5*ay*((deltaTime - time) - timeImpact) * ((deltaTime - time) - timeImpact);

                  //velocity = v[o] + ay(time)
                  vyImpact = vy + ay*(timeImpact - time);
                  vxImpact = vx + ax*(timeImpact - time); 

                  //position = position[o] + velocity(time) + 1/2*acc*time^2
                  xImpact = x + vx*(timeImpact - time) + .5*ax*(timeImpact - time)*(timeImpact - time);  
                  //yImpact = y + vy*(timeImpact - time) + .5*ay*(timeImpact - time)*(timeImpact - time); 

                  numBounces++; //Increment number of bounces ball takes

                  //fprintf(fp, "%f\t%f\t%f\t%f\t%f\t%f\t%f\t\n", ax, ay, x, y, vx, vy, time);
                  printf("timeImpact: %f\nxImpact: %f\nyImpact: %f\nvxImpact: %f\nvyImpact: %f\n", timeImpact, xImpact, yImpact, vxImpact, vyImpact);
                  printf("Number of Bounce(s): %d\n\n", numBounces); 
               }
     }

     fclose(fp); //Close output file
     fclose(fr); //Close input file

     system ("PAUSE"); 
     return 0;
  }

Sample Input:

ax: 0 ay: -9.8 x: 0 y: 50 vx: 8.66 vy: 5

Answer 1

The program works almost properly. I got this output:

timeImpact: 28.457277
xImpact: 7798.595703
yImpact: 0.000000
vxImpact: 315.561859
vyImpact: 73.370865
Number of Bounce(s): 1

timeImpact: -315.561096
xImpact: 984718.250000
yImpact: 0.000000
vxImpact: -3213.589600
vyImpact: 36.704834
Number of Bounce(s): 2

timeImpact: 3088.760254
xImpact: 94468824.000000
yImpact: 0.000000
vxImpact: 31913.345703
vyImpact: 55.056641
Number of Bounce(s): 3

timeImpact: -30290.189453
xImpact: 9086046208.000000
yImpact: 0.000000
vxImpact: -312763.843750
vyImpact: 45.875000
Number of Bounce(s): 4

timeImpact: 296834.687500
xImpact: 872571076608.000000
yImpact: 0.000000
vxImpact: 3065398.750000
vyImpact: 50.500000
Number of Bounce(s): 5

timeImpact: -2908993.750000
xImpact: 83802579795968.000000
yImpact: 0.000000
vxImpact: -30040802.000000
vyImpact: 48.000000
Number of Bounce(s): 6

timeImpact: 28507292.000000
xImpact: 8047926899113984.000000
yImpact: 0.000000
vxImpact: 294391936.000000
vyImpact: 64.000000
Number of Bounce(s): 7

timeImpact: -279371488.000000
xImpact: 772922520746590208.000000
yImpact: 0.000000
vxImpact: -2885036544.000000
vyImpact: 0.000000
Number of Bounce(s): 8

timeImpact: 2737840640.000000
xImpact: 74231486855715487744.000000
yImpact: 0.000000
vxImpact: 28273358848.000000
vyImpact: 0.000000
Number of Bounce(s): 9

timeImpact: -26831695872.000000
xImpact: 7129642377640744583168.000000
yImpact: 0.000000
vxImpact: -277087289344.000000
vyImpact: 0.000000
Number of Bounce(s): 10

timeImpact: nan
xImpact: nan
yImpact: 0.000000
vxImpact: nan
vyImpact: nan
Number of Bounce(s): 11

sh: PAUSE: command not found

The last line might be the only problem. What the system("PAUSE") was supposed to do?

EXC_BAD_ACCESS appears in the following line:

fscanf(fr, "ax: %f ay: %f x: %f y: %f vx: %f vy: %f\n", &ax, &ay, &x, &y, &vx, &vy);

Setting a breakpoint in it showed that fr was NULL, and that's the problem. The path to the files were set relatively to the executable location, but notice that the executable goes to a build folder, which does not contain the input and output files (these are located in your project folder).

In Xcode, select targets and then right click on your program. Choose the "Show in Finder" item, which will open a folder where the program is located. Drop your input file there and it should work.

---

To clarify the arguments:

In your main function, change the parameters to match this:

int main (int argc, char *argv[])

In short, there are two parameters. Every executable can receive arguments on its launch, and these parameter capture those arguments. For example, consider when you launch your program from the command line:

$ ./myprogram file1.txt file2.txt

The file1.txt and file2.txt are arguments, they will be passed to main. The main function has two arguments: argc and argv . The first is an int, holds the number of arguments, and the second is an array of strings, the arguments themselves.

In the example I gave, argc is set to 3, as the first argument will be the program name (that's a little detail). So:

argv[1] == "file1.txt"
argv[2] == "file2.txt"

To do it in Xcode, first modify your main function to accept parameters. Then, modify this line:

fr = fopen("input_data.txt", "rt"); //Open file for reading

To this:

fr = fopen(argv[1], "rt"); //Open file for reading

Now, considering you're on Xcode4 choose the "Product > Edit Scheme..." menu item. In the run configuration (side bar), choose the arguments tab and add a path to the desired input file in the "Arguments passed on Launch" section.

That's it. Your program will now run as before, but when you need to use another file, you don't need to recompile the program. Just change the path in the argument list. This may appear to be more work, but once you have larger programs and starting using the command line, it will make more sense.

Now go read a good book on C and another on Unix that explain the details I didn't:-)

Answer 2

In most cases you try to send a message or access an instance, which has been deallocated previously.

Work with breakpoints to identify the exact spot where your application crashes.