Segfault when a large parameter is passed - c

Question

My program is supposed to print out every nth character in a string of text. The first parameter defines the "n" or the increment between characters to report on. The second parameter is optional, but if specified, defines how many characters to work on.
#include <stdio.h>
#include <stdlib.h>
#define MAX_STR_LEN 100
int main(int argc, char **argv) {
char string[MAX_STR_LEN];
char *testval="This is a test, this is only a test. For the next sixty seconds, this will be a test of the emergency broadcasting system.";
if (argc<2) {
printf("Expected at least one argument.\n");
return 1;
}
int inc=atoi(argv[1]);
if (inc <=0) {
printf("Expected first argument to be a positive number.\n");
return 1;
}
int max=MAX_STR_LEN;
if (argc==3) {
max=atoi(argv[2]);
if (max<0) max=MAX_STR_LEN;
}
int i=0;
while(i < max) {
string[i]=testval[i]; // gdb says this is where the seg fault occurs
i++;
}
string[i]=0;
i=0;
while(string[i]!=0) {
printf("The %d char is %c\n",i,string[i]);
i = i + inc;
}
return 0;
}
Running "./prog3 5 40" works fine, but running "./prog3 5 150" causes a seg fault.
./prog3 5 150
8
Segmentation Fault

Answer 1

This will work better. you need to make the string buffer large enough to hold all the data you put into it. The segmentation fault you had comes from writing to a memory address not allowed to be used by the program.
#include <stdio.h>
#include <stdlib.h>
#define MAX_STR_LEN 100
int main(int argc, char **argv) {
char *testval="This is a test, this is only a test. For the next sixty seconds, this will be a test of the emergency broadcasting system.";
if (argc<2) {
printf("Expected at least one argument.\n");
return 1;
}
int inc=atoi(argv[1]);
if (inc <=0) {
printf("Expected first argument to be a positive number.\n");
return 1;
}
int max=MAX_STR_LEN;
if (argc==3) {
max=atoi(argv[2]);
if (max<0) max=MAX_STR_LEN;
}
char string[max];
int i=0;
while(i < max) {
string[i]=testval[i];
i++;
}
string[i]=0;
i=0;
while(string[i]!=0) {
printf("The %d char is %c\n",i,string[i]);
i = i + inc;
}
return 0;
}