Question

I am writing a fantasy football draft program for fun.

I encountered a strange problem. I assign a value to a struct field and that happens, but it also assigns that value to another field in the struct. Apologies for the messy debugging printf statements.

I clearly don’t understand something about struct field assignment.

code:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#define TRUE 1

int QB_count = 0;
int RB_count = 0;
int WR_count = 0;
int TE_count = 0; 
int DEF_count = 0;

struct Player { 
    char *name;
    char *position;
    int age;
    int bye_week;
};

int get_name (struct Player *drafted) {
    char name[20];
    fputs("Enter Player Name: ", stdout);
    fflush(stdout);
    if (fgets(name, sizeof name, stdin) != NULL){
        char *newline = strchr(name, '\n');
        if (newline != NULL){
            *newline = '\0';
        }
        drafted->name = name;
        printf("You've drafted: %s\n", drafted->name);
    }
    return 0;
}

int get_position(struct Player *drafted){
    char position[20];
    int depth;
    char *nametemp = drafted->name;
    printf("nametemp: %s\n", nametemp);
    fputs("Enter Player Position in 'QB/RB/WR/TE/DEF' format: ", stdout);
        fflush(stdout);
        if (fgets(position, sizeof position, stdin) != NULL){
                char *newline = strchr(position, '\n');
                if (newline != NULL){
                        *newline = '\0';
                }
                drafted->position = position;

        if (strcmp(position, "QB") == 0){
            QB_count++;
            depth = QB_count;

        } else if (strcmp(position, "RB") == 0){
                        RB_count++;
                        depth = RB_count;

                } else if (strcmp(position, "WR") == 0){
                WR_count++;
                        depth = WR_count;

                } else if (strcmp(position, "TE") == 0){
                        TE_count++;
                        depth = TE_count;

                } else if (strcmp(position, "DEF") == 0){
                        DEF_count++;
                        depth = DEF_count;

                } else {
            printf("Please re-enter position information using the format 'QB' or 'qb'\n");
            get_position(drafted);
            return 0;
        }
        drafted->name = nametemp;
        printf("NAME: %s\n", drafted->name);
        printf("You've drafted %s at: %s%d\n", drafted->name, drafted->position, depth);
        }
        return 0;

}

int get_age (struct Player *drafted){
    return 0;
}

int get_bye_week (struct Player *drafted){
    return 0;
}

int main (){ 
    int stop = 0;
    char text[20];
    while (TRUE){
        struct Player drafted;
        printf("Welcome to the 2012 Draft Day Program\n");
        get_name (&drafted);
        printf("NAME_MAIN: %s\n", drafted.name);
        get_position(&drafted);
        printf("You've drafted %s at: %s\n", drafted.name, drafted.position);
        get_age(&drafted);
        get_bye_week(&drafted);
        fputs("Would you like to draft another player?\n" 
            "Enter '1' for no, '0' for yes\n", stdout);
        fflush(stdout);
        if(fgets(text, sizeof text, stdin)){
            int number;
            if (sscanf(text, "%d", &number) == 1){
                if (number == 1){
                    printf("Draft Ended!\n");
                    break;
                }       
            }
        }
    }
    return 0;
}

The resulting output is:

Welcome to the 2012 Draft Day Program
Enter Player Name: Aaron Rodgers
You've drafted: Aaron Rodgers
NAME_MAIN: Aaron Rodgers
nametemp: Aaron Rodgers
Enter Player Position in 'QB/RB/WR/TE/DEF' format: QB
NAME: QB
You've drafted QB at: QB1
You've drafted QB at: QB
Would you like to draft another player?
Enter '1' for no, '0' for yes
1
Draft Ended!

Why does drafted.name become “QB”?

Answer 1

In your get_name function you’re assigning to the name field of your struct Player a stack variable name.

In this line:

drafted->name = name;

name is declared in the function and so it’s scope is limited to that function. Once get_name returns, the variable goes out of scope, and attempts to use that memory invoke undefined behavior.

Instead of using the simple assignment, you need to allocate space for drafted->name using malloc, and use strncpy to make a copy of the name. If strdup is available, you can use that to allocate the space and do the copy in a single step. Alternately, you could allocate space for drafted->name before reading the name, and use it in place of the name variable.

As a final option, if you assume a maximum length for names – your current code allows names up to a string length of 19 – you can simply declare an array of that size for each struct Player:

struct Player
{ 
    char name[NAME_MAXLEN];

You have an identical problem with your position field in the get_position function.