Is using malloc within scanf compliant to c ansi standard

Question

I want to read the input of a user and save it. What i have now does work but i need to know if its legit (following ansi standard - c90) that scanf is first assigning the variable "length" before it allocates memory for the input, or if its just a quirk of the compiler.

#include <stdio.h>
int main()
{
    char* text;
    int length = 0;
    scanf("%s%n", text = malloc(length+1), &length);

    printf("%s", text);

    return 0;
}  

Answer 1

This will not work as you expect.

At the time you call malloc, length still has the value 0, so you're only allocating one byte. length isn't updated until after scanf returns. So any non-empty string will write past the bounds of the allocated buffer, invoking undefined behavior.

While not exactly the same, what you can do is use getline, assuming you're running on a POSIX system such as Linux. This function reads a line of text (including the newline) and allocates space for that line.

char *text = NULL;
size_t n = 0;
ssite_t rval = getline(&text, &n, stdin);

if (rval == -1) {
    perror("getline failed");
} else {
    printf("%s", text);
}
free(text);