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In this example, two functions are called one after another. The first function, func1(...)*}}is passed an integer entered by a user. That integer is stored in variable: *{{i for the duration of the function. The second function func2()*}}declares a local integer variable: *{{j.   j is not initialized before being checked against a constant value, CONDITION_CHECK. Since {{j}}was not initialized, it assumes whatever value is at that location in the stack, in this case the value of i from func1(). Thus is the user entered 42, the condition the statement if (j == CONDITION_CHECK) succeeds.

#define CONDITION_CHECK  42
void  func1 (int arg) { int i = arg; }
void  func2 (void) {
    int j;

    if (j == CONDITION_CHECK) printf("Condition passed!!\n");
    else printf("ERROR: Condition failed\n");
}
int main(int argc, char **argv) {
    func1(atoi(argv[1]));
    func2();
}

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The local, automatic variable j should be initialized to a default value.

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In this example, user input is copied to a buffer, buf, which has been filled with '\0' characters. The first function, logit(...) copies the user input to another buffer, buffer, and prints it to standard output. Next, the runit(...) routine is called. This routine declares an array of 50 characters, buf, and a character pointer, ptr.

However, since ptr is not initialized it points to whatever data is on the stackreferences data used by the last function called. In this case, the 51st to 55th characters contents of the user controlled data copied in the logit(...) function. When the contents of ptr are data referred to by ptr is copied to buf using an unbounded strcpy(...), ptr is dereferenced and the data in that location is copied to buf. If that data at that location is longer than 50 bytes without a null byte, a buffer overflow will occur.

#define BUF_SIZE 150

void runit(void) {
    char buf[50];
    char *ptr;

    memset(buf,0,50);
    strcpy(buf,ptr);
}

void logit(char *str) {
    char buffer[BUF_SIZE];

    strcpy(buffer, str);
    printf("The message: %s\nhas been logged\n",buffer);
}

int main(int argc, char *argv[]) {
    char buf[BUF_SIZE];
   
    strcpy(buf, argv[1]);
    logit(buf);
    runit();
}

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#define BUF_SIZE 150

void runit(void) {
    char buf[50];
    char *ptr = NULL; /* initialize ptr to 0 */

    memset(buf,0,50);
    strcpy(buf,ptr);
}

void logit(char *str) {
    char buffer[BUF_SIZE];
    int i;
    for (i=0; i < BUF_SIZE; ++i) buffer[i] = '\0';

    strcpy(buffer, str);
    printf("The message: %s\nhas been logged\n",buffer);
}

int main(int argc, char *argv[]) {
    char buf[BUF_SIZE];
   
    strcpy(buf, argv[1]);
    logit(buf);
    runit();
}

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