atof creates strange result

Question

I am trying to use atof() to convert a string to a double (obviously), but the results are not what I expect. Here is the code and debug information about the variable values BEFORE atof():

d = atof (arg);
next.db = d;

*debug info*
arg = 0x0034f7b0 "10.0"
d = 0.0000000000000000

Once the program has stepped through atof(). the results are as follows:

arg = 0x0034f7b0 "ôþSÄ÷4"
d = 0.0000000000000000

As you can see, before the command, the arg variable does hold a valid double. Though, the return value is 0. What confuses me, is why does the value of arg change?

Also, I do have stdlib.h included. As well, arg is declared as:

char *arg;

If it helps at all, the "10.0" was read from a file.

More code:

void read_instructions()
{
    char *str;
    char *arg;
    int n;
    char c;
    double d = 0;
    instruction next = {0};

    while (!feof(datafile)) {
        // Fetch the next string
        // if (push or pop), get the next argument
        // create instructiwn and add to instruction array
        str = get_next_string();

        if (strncmp (str, "P", 1) == 0) {
            if (strncmp (str, "PUSH", 4) == 0) {
                next.func = pushFunc;
            }
            else {
                next.func = popFunc;
            }
            arg = get_next_string();
            n = arg[0];
            if (n > 64 && n < 71)
                next.ch = arg[0];
            else {
                d = atof (arg);
                next.db = d;
            }
            instr[instr_count] = next;
            instr_count++;
        }
    else {
        c = str[0];


        switch (c) {
        case 'A' :
            next.func = addFunc;
            break;
        case 'S' :
            next.func = subFunc;
            break;
        case 'M' :
            next.func = multFunc;
            break;
        case 'D' :
            next.func = divFunc;
            break;
        case 'H' :
            next.func = haltFunc;
        default :
            printf ("Invalid instruction");
        }
        instr[instr_count] = next;
        instr_count++;
    }
}
fclose (datafile);
}

This is the given code to open and access the file:

FILE *datafile;


int main(int argc, char *argv[]) 
{
    if (argc != 2) {
        printf("error, incorrect number of arguments");
        haltFunc(instr[0]);
    }

    open_file(argv[1]);
    read_instructions();
    execute_instructions();
    return 0;
}

void open_file(char* argv) 
{
    char buf[1024];
    char cwd[512];

    getcwd(cwd, sizeof cwd);
    sprintf(buf, "%s\\%s", cwd, argv);

    if (!(datafile = fopen(buf, "r"))) {
        printf("Error: Make sure your file is located here:\n%s", buf);
    }
}

char* get_next_string() 
{
    char str[15];

    fscanf(datafile, "%s", &str);

    return str;
}

The header file:

#ifndef MAIN_HEADER_H
#define MAIN_HEADER_H

#define INSTR_SIZE 30

typedef struct {
    void (*func)(instruction);
    union {
        double db;
        char ch;
    };
} instruction;

int main(int, char*);
char* get_next_string();
void open_file(char*);
void read_instructions();
void execute_instructions();
void pushFunc(instruction instr);
void popFunc(instruction instr);
void addFunc(instruction instr);
void subFunc(instruction instr);
void multFunc(instruction instr);
void divFunc(instruction instr);
void haltFunc(instruction instr);

#endif

And this is the test file:

PUSH 10.0
PUSH 4.0
PUSH 7.0
PUSH 5.0
POP D
POP E
POP 
PUSH D
ADD
PUSH 5.0
POP B
PUSH 17.0
POP E
PUSH B
PUSH E
SUB
HALT

Answer 1

Your problem is probably caused by the get_next_string() function returning a pointer to a temporary local character array. As soon as the function returns, the stack memory once used by str[] is being overwritten by some other automatic variable. That would explain why arg is being corrupted.

There are several possible fixes.

• The caller could allocate memory to hold the string and pass this pointer to the function for it to fill with data.
• The callee could allocate memory for the string and return that pointer. That also returns ownership to the caller and responsibility for calling free() when done.
• The str[] array could be declared static inside the function. It would no longer be temporary but be aware that every time you call the function the previous string will be overwritten.