c - Segmentation fault when joining two programs

Question

I'm writing this program to evaluate post fix expressions and the program runs fine on its own. But when I join this program with another program of mine, it gives me segmentation fault.

Like this:

typedef struct node2
{
    int info;
    struct node2 *following;

}node2;

typedef struct eval
{
    int size;
    node2 *head1;
}eval;

int init(eval **exp)
{
    *exp = (eval*)malloc(sizeof(eval));
    if(*exp==NULL)
    {
        return EXIT_FAILURE;
    }

    (*exp)->head1 = NULL;
    (*exp)->size = 0;

    return EXIT_SUCCESS;
}

void postpush(eval *exp, int num)
{
    node2 *newn=(node2*)malloc(sizeof(node2));

    newn->following=exp->head1;
    exp->head1=newn;
    newn->info=num;
    (exp->size)++;

}

void postpop(eval *exp)
{
    node2 *postpop=NULL;

    postpop = exp->head1;
    exp->head1 = postpop->following;
    (exp->size)--;
    free(postpop);
}

int operand(char op)
{
    if (op == '1'|| op == '2'||op == '3' || op == '4' || op == '5' || op == '6' || op == '7' || op == '8' || op == '9' || op == '0' )
        return 1;
    return 0;
}

void expre(int head2, int temp, char operand, eval* exp)
{
    int result=0;

    switch(operand)
    {
    case '+':
        result=head2 + temp;
        break;
    case '-':
        result=head2 - temp;
        break;
    case '/':
        result=head2/temp;
        break;
    case '*':
        result= head2*temp;
        break;
    case '^':
        result= pow(head2,temp);
        break;
    default:
        return;
    }
    postpop(exp);
    postpush(exp,result);
}

int operators(char op)
{
    if (op == '+' || op == '-' || op == '*' || op=='/' || op=='^')
        return 1;

    return 0;
}

int evaluationpost()
{
    eval *exp = NULL;
    char input[MAX];

    init(&exp);

    char* op;

    printf("Please enter a postfix expression (with spaces between the operators and operands):");
    gets(input);

    int temp;

    op=input;

    while(*op!='\0')
    {
        if(operand(*op))
        {
            postpush(exp, abs((int)(*op)-48));
        }

        if (operators(*op))
        {
            temp=(exp->head1)->info;
            postpop(exp);
            expre((exp->head1)->info, temp, *op, exp);

        }
        op++;
    }

    printf("Result: %d\n", (exp->head1)->info);

    return 0;
}

int main()
{
    evaluationpost();
}

It will run fine and there won't be any errors. But once I type this code into another program and try to call the function, it gives me a "Segmentation fault".

This is the part of the code I'm trying to call the function evaluationpost in:

case 3: //if the expression is a postfix expression
    printf("This is a postfix expression. Would you like to:\n"); //telling the user its a postfix expression
    printf("A- Convert it to infix\n");                           //and asking them what they want to do
    printf("B- Convert it to prefix\n");
    printf("C- Evaluate the expression\n");
    printf("D- Exit the program.\n");
    printf("Your choice: "); //asking the user for their choice
    scanf("%c", &option);    //getting the user's choice

    if(option=='A' || option =='a') //if the user chooses to convert the expression to infix
    {
        printf("The infix expression is: %s\n", ConvertPostfix(input, 2)); //calling the function that changes postfix to infix
                                                                           //and displaying the converted expression to the user
        //saving the output in the output.txt file
        output=ConvertPostfix(input, 2); //assigning the converted expression to output
        fprintf(PTR, "Postfix expression converted to an infix expression: %s\n", output); //putting the converted expression in the file
        printf("Output was saved to output file.\n"); //telling the user the output was saved in output.txt
        fclose(PTR); //closing the file using its pointer
    }
    else if(option=='B' || option=='b') //if the user chooses to convert the expression to prefix
    {
        printf("The prefix expression is: %s\n", ConvertPostfix(input, 1)); //calling the function that changes postfix to prefix
                                                                            //and displaying the converted expression to the user
        //saving the output in the output.txt file
        output=ConvertPostfix(input,1); //assigning the converted expression to output
        fprintf(PTR, "Postfix expression converted to an prefix expression: %s\n", output); //putting the converted expression in the file
        printf("Output was saved to output file.\n"); //telling the user the output was saved in output.txt
        fclose(PTR); //closing the file using its pointer
    }
    else if(option=='C' || option =='c') //if the user chooses to evaluate the expression
    {
        evaluationpost(); //calling the function that evaluates postfix expressions
    }

When I run my program, type in a postfix expression and press C to evaluate it. It gives me "Segmentation Fault".

I don't understand why it's giving me a segmentation fault here when it works well on its own.

标签： csegmentation-fault

Answer 1

这条线

scanf("%c", &option);

读入一个字符。但是当你输入时c<ENTER>，你发送两个字符，第二个是换行符 \n。

该gets()函数（无论如何你都不应该使用它，听你的编译器！）

gets(input);

现在只读取（终止）换行符并存储一个空字符串（换行符被删除）。

while 条件现在立即为假，因此永远不会执行循环体：

while(*op!='\0')

现在当你取消引用

printf("Result: %d\n", (exp->head1)->info);

thenexp->head1仍然是一个NULL指针，由您的函数初始化init()，导致分段错误。

您必须处理空字符串的条件并使用

scanf("%c ", &option);

吞下输入缓冲区中的换行符