strcat eccentric behavior

Question

I wrote this simple C program and couldn't quite figure out this bizarre behavior of strcat

long sum(long col, char* path, char* path2){

    printf("%s\n",path2);
    strcat(path,".endlines");
    printf("%s\n",path2);
    return 0;
}

int main(int argc, char* argv[]) {
    int n=atoi(argv[1]);
    sum(n,argv[2],argv[3]);
    exit(EXIT_SUCCESS);
}

strcat is applied on path, but path2 is eventually modified as well. I would very much appreciate if someone let me know what was happening :) thanks

Run

./program 3 example/mountdir/location2.csv example/rootdir/location2.csv

output:

example/rootdir/location2.csv

endlines

Answer 1

You are overrunning a buffer. The original argv[2] and argv[3] are very likely consecutive in memory. When you strcat onto the end of argv[2], it is writing onto the memory argv[3] points at. You need to allocate new buffers to hold the larger strings you are trying to make.