C fopen段故障

Question

I have a program that takes two parameters, an integer and a string. The first represents the number of lines to be read from the file, whose name is the second arg. Files have an integer value per line.

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

// the size of the data set 
long dataSize = atol(argv[1]);

// an array to store the integers from the file

long dataSet[dataSize];
// open the file
fp = fopen(argv[2], "r");
// exit the program if unable to open file
if(fp == NULL)
{
printf("Couldn't open file, program will now exit.\n");
exit(0);
} // if

I have a file called data10M with 10 million integers. It works fine until I change the first argument to something more than about 1050000, at which point the program throws a segmentation fault at the fopen line.

Answer 1

You are getting a Stack Overflow!

Local variables are placed on the stack. Your C compiler/linker seems to allocate an 8 Mb stack (assuming long is 8 bytes). 1050000 * 8 is more than 8 Mb.

You get the seg fault when you try to allocate an array that doesn't fit.

Try allocationg the array on the heap instead:

// an array to store the integers from the file
long *dataSet = malloc(dataSize * sizeof(long));