Trying to return char array via pointer and it's giving incorrect results

Question

My code is:

#include <stdio.h>
#include <string.h>
char *getUserInput() {
    char command[65];

    //Ask the user for valid input
    printf("Please enter a command:\n");
    fgets(command, 65, stdin);

    //Remove newline
    command[strcspn(command, "\n")] = 0;
    return command;
}

int main() {
    char *recCommand = getUserInput();
    printf("%s", recCommand);
    return 0;
}

When this code is executed, this is the console:

Please enter a command:
Test <-- this is the command I entered
*weird unknown characters returned to console*

Why are there weird unknown characters being returned to console instead of "Test"?

Local variables are disabled when exiting the scope of the function. — BLUEPIXY
– BLUEPIXY, Commented Apr 10, 2015 at 3:15
So instead of returning a pointer to "command", I should be returning the char array command itself? — Adam Nygate
– Adam Nygate, Commented Apr 10, 2015 at 3:15
try return strdup(command); this make copy string.( secure the area in heap) then free(command); at main. — BLUEPIXY
– BLUEPIXY, Commented Apr 10, 2015 at 3:20
Why do we need to free(command)? Are you referring to here the command located inside the getUserInput scope or inside the main scope? — Adam Nygate
– Adam Nygate, Commented Apr 10, 2015 at 3:21
area of heap isn't release, it will still be secured. So the user must be freed. — BLUEPIXY
– BLUEPIXY, Commented Apr 10, 2015 at 3:24

Deanie · Accepted Answer · 2015-04-10 03:15:27Z

3

It is because you are returning the value of a local variable. Try putting this:

char *getUserInput() {
    static char command[65];

    //Ask the user for valid input
    printf("Please enter a command:\n");
    fgets(command, 65, stdin);

    //Remove newline
    command[strcspn(command, "\n")] = 0;
    return command;
}

answered Apr 10, 2015 at 3:15

Deanie

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3 Comments

Adam Nygate Over a year ago

I come from a C# background, is it possible to instead just return the value of command from the getUserInput function?

Jonathan Wood Over a year ago

@AdamNygate: Yes, it's possible. But it require a bit of work to allocate memory and then make sure it gets released in a timely manner. In C, this is probably your best bet.

Jonathan Leffler Over a year ago

The trouble with this is that if you call getUserInput() twice, the second value overwrites the first. This can cause problems in multi-threaded programs where multiple threads call the function, and it can also cause problems in single-threaded programs where you call the function twice before using the returned value from the first call.

Adam Nygate · Accepted Answer · 2015-04-10 03:35:56Z

3

After doing some more research, it seems that the best way is to actually pass in a char array from the main function in which to store the input from getUserInput.

Here is my revised code:

void getUserInput(char *command) {
    //Ask the user for valid input
    printf("Please enter a command:\n");
    fgets(command, 65, stdin);

    //Remove newline/return carriage
    command[strcspn(command, "\n")] = 0;
}

int main {
    char recCommand[65];
    getUserInput(recCommand);
    printf("%s", recCommand);
    return 0;
}

answered Apr 10, 2015 at 3:35

Adam Nygate

3253 silver badges14 bronze badges

2 Comments

Crowman Over a year ago

Hardcoding the length of your array into your function is a bad idea. At least make it void getUserInput(char * command, size_t size) or similar.

Adam Nygate Over a year ago

I've made the length a macro and defined it in a commonmacros.h file, the requested input will always be 65 chars long (64 + \0)

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