Thread: passing strings from functions as the "return" part

Hi all, I am a beginner to C. The following is not a completed program, or even a part of a program, but I see no point in writing any more before I can get this to compile...

Code:

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

int get_filename();

int main(void){



//declarations
    FILE    *input_stream;
  
//program statements
    input_stream = fopen( get_filename(), "r" ) ); //line 37
         //there will be error checking here, in the form of an if, eventually
    }

int get_filename(){

//declarations
    char filename[101];

//program statements
    printf("enter filename:   ");
    scanf( "%s", filename);
    return (filename); //line 47
    
    }

The problem is already obvious to you guys, but I am getting the following errors:

37 passing arg 1 of `fopen' from incompatible pointer type
37 syntax error before ')' token
In function `get_filename':
47 [Warning] return makes integer from pointer without a cast
47 [Warning] function returns address of local variable

To me this appears to be basically one error, that manifests in several places. I think it is to do with the returning of the contents of the variable filename. Of course, I could just put it in main, but that would be like giving up, and I will need to know how to do this in the future, anyway. Can you return strings out of functions like that? I have tried the FAQ and everything, to no avail....any ideas?

Thanks in advance...

fraktal

First of all, your function says it returns an int, but you are trying to return a pointer to a char. Secondly, the array you allocated goes out of scode when the function returns. The way this is usually done, is you pass the string buffer into the function like so:

Code:

void get_filename(char *filename){

    printf("enter filename:   ");
    fflush(stdout);  // you should put this line here
    scanf( "%s", filename);
    
}

Note that the above code can cause a buffer overflow error if the user inputs a string that is longer than the space you allocated for the file name. The correct way to avoid this would be to pass the size of the buffer to the get_filename function, and use fgets() to get the user input, or some other input function that checks buffer sizes. To use the above code, you would do:

Code:

char filename[100];
get_filename(filename);
printf("User entered %s\n",filename);

Right, I see. It makes sense now. Unfortunately, I think I am likely to encounter more problems further down the line. Until then, thanks very much, you have been extremely helpful. I am very grateful.

okay, sorry to keep coming back, but I really can't see what's wrong with my code here.
I have progressed somewhat since my last posts. I have written a program that implements linked lists before, but now with very similar code in this program I can't get it to work. The problem i'm having is with sscanf, I will include the relevant part of my code:

Code:

        struct rootnode{
            float gauge_diameter;
            float gauge_length;
            float force;
            float extension;
            struct node *nextnodeaddr;
            } *node1_addr;

        node1_addr = malloc( sizeof(struct rootnode) );

        fgets(  line, sizeof(line), input_stream  );
        sscanf(  line, "%s %lf", null, &node1_addr->gauge_diameter);
        printf(  "\nread: %s %lf\n",       null,
        node1_addr->gauge_diameter);

all variables have been declared. line is a character string containing the ascii string:
Gauge_diameter: 20.0e-3 m.
The output on the screen is read: Gauge_diameter: 89128.960938 which is obviously wrong. The thing is, when I use the workaround

Code:

        sscanf(  line, "%s %lf", null, gauge_diameter);
        node1_addr->gauge_diameter=gauge_diameter;

where gauge_diameter is declared as a float, of course, the answer is right, which suggests that the problem is the way the struct is shown to sscanf. Any ideas?


Thanks again

%lf expects a pointer to a double, you have a pointer to a float.

Are you using gcc for your compiler?

oh yeah! I fixed that, and it all works fine now. I don't know what I was using "float" for...Thanks a lot!

(by the way: yes, I am using gcc.)

gcc -W -Wall -ansi -pedantic -O2 prog.c
Will diagnose an awful lot of problems, including mis-matches between the format string and parameters of printf and scanf type functions.