malloc with user input

Question

I'm trying to make a program where a user inputs a string then if they want to enter a letter they want to replace and what with. I want to use malloc to set the array but how would I do it with scanf?

Please can someone help.

Thanks!

This is what the program looks before going to the replace method:

char *s,x,y;

printf("Please enter String \n");
scanf("%s ", malloc(s));

printf("Please enter the character you want to replace\n");
scanf("%c ", &x); 

printf("Please enter replacment \n");
scanf("%c ", &y);

prinf("%s",s);

I think you want POSIX getline() (not part of the C99 Standard). — pmg
– pmg, Commented Apr 1, 2014 at 11:46
Your use of malloc is horribly wrong: Malloc takes the number of bytes and returns a pointer. You pass it a pointer. — mic_e
– mic_e, Commented Apr 1, 2014 at 11:58

mic_e · Accepted Answer · 2014-04-01 12:03:35Z

You can't know the size of the user input beforehand, so you need to dynamically allocate more memory if the user input hasn't ended yet.

An example would be:

//don't forget to free() the result when done!
char *read_with_alloc(FILE *f) {
    size_t bufsize = 8;
    char *buf = (char *) malloc(bufsize);
    size_t pos = 0;

    while (1) {
        int c = fgetc(f);

        //read until EOF, 0 or newline is read
        if (c < 0 or c == '\0' or c == '\n') {
            buf[pos] = '\0';
            return buf;
        }

        buf[pos++] = (char) c;

        //enlarge buf to hold whole string
        if (pos == bufsize) {
            bufsize *= 2;
            buf = (char *) realloc((void *) buf, bufsize);
        }
    }
}

A pragmatic alternative solution would be to limit the buf size (for example, to 256 characters), and to make sure that only that number of bytes is read:

char buf[256]; //alternative: char *buf = malloc(256), make sure you understand the precise difference between these two!
if (scanf("%255s", buf) != 1) {
   //something went wrong! your error handling here.
}

Pranit Kothari · Accepted Answer · 2014-04-01 11:39:43Z

0

scanf("%s ", malloc(s));

What does this mean? s uninitialized is pointer, it can have any value, say 0x54654, it is Undefined Behavior.

Your code should be,

int size_of_intput = 100; //decide size of string
s = malloc(size_of_intput);
scanf("%s ", s);

answered Apr 1, 2014 at 11:39

Pranit Kothari

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2 Comments

mic_e Over a year ago

You should always make sure that no buffer overflows are caused. A >= 100-character user input might very well cause a heap corruption with your code! Pass scanf("%99s ", s) instead, and check the return value of scanf.

Pranit Kothari Over a year ago

@mic_e Great advice. Not knowing %99s possible.

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