Passing Malloc char pointer to function

Question

When I try to pass a malloc'd char pointer from main to use in user_input to get a string in it. After the while loop is done I'm trying to print filename. filename ends up to be empty when it should contain a valid string.

void user_input(char *filename){
    char ch;

    int i = 0;

    printf("Fil att leta i: ");
    while((ch = getchar()) == '\n'){
        filename = realloc(filename, (i+1) * sizeof(char));
        filename[i] = ch;

        i++;
    }
    filename[i] = '\0';
    printf("filnamn = %s", filename);

}

int main(void){
    char *filename;

    filename = (char *)malloc(sizeof(char));
    user_input(filename);

    return 0;
}

You know, sizeof(char) is defined as 1. There's absolutely no reason to ask whether a char is more than one char big. — Deduplicator
– Deduplicator, Commented Oct 19, 2014 at 19:14

Sergey Kalinichenko · Accepted Answer · 2014-10-19 18:52:29Z

3

The filename is a pointer which is passed by value. When you do this in the user_input function

filename = realloc(filename, (i+1) * sizeof(char));

the filename in the main does not change. When you print filename in the main, the pointer has been realloced, so you trigger undefined behavior.

You need to pass filename by pointer. Since it's already a pointer, you end up with a double pointer:

void user_input(char **filenamePtr)

Now you need to take a pointer in the main

user_input(&filename);

and dereference in the user_input:

*filenamePtr = realloc(*filenamePtr, i+2);

Note that sizeof(char) is always 1, so you do not need to multiply by it. Also you add 2, not 1, to i so that you have enough space for the null terminator '\0' character.

edited Oct 19, 2014 at 18:52

answered Oct 19, 2014 at 18:45

Sergey Kalinichenko

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Comments

Bart Friederichs · Accepted Answer · 2014-10-19 18:45:12Z

1

Change this:

while((ch = getchar()) == '\n')

into this:

while((ch = getchar()) != '\n')

you only read the \n. You should read anything but the \n.

answered Oct 19, 2014 at 18:45

Bart Friederichs

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