how can I append a char to a string allocating memory dynamically in C?

I wrote this code, but inserts garbage in the start of string:

void append(char *s, char c) {
    int len = strlen(s);
    s[len] = c;
    s[len + 1] = '\0';
}

int main(void) {
    char c, *s;
    int i = 0;
    s = malloc(sizeof(char));
    while ((c = getchar()) != '\n') {
        i++;
        s = realloc(s, i * sizeof(char));
        append(s, c);
    }   
    printf("\n%s",s);   
}

How can I do it?

Solution

There are multiple problems in your code:

you iterate until you read a newline ('\n') from the standard input stream. This will cause an endless loop if the end of file occurs before you read a newline, which would happen if you redirect standard input from an empty file.
c should be defined as int so you can test for EOF properly.
s should be null terminated at all times, you must set the first byte to '\0' after malloc() as this function does not initialize the memory it allocates.
i should be initialized to 1 so the first realloc() extends the array by 1 etc. As coded, your array is one byte too short to accommodate the extra character.
you should check for memory allocation failure.
for good style, you should free the allocated memory before exiting the program
main() should return an int, preferably 0 for success.

Here is a corrected version:

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

/* append a character to a string, assuming s points to an array with enough space */
void append(char *s, char c) {
    size_t len = strlen(s);
    s[len] = c;
    s[len + 1] = '\0';
}

int main(void) {
    int c;
    char *s;
    size_t i = 1;
    s = malloc(i * sizeof(char));
    if (s == NULL) {
        printf("memory allocation failure\n");
        return 1;
    }
    *s = '\0';
    while ((c = getchar()) != EOF && c != '\n') {
        i++;
        s = realloc(s, i * sizeof(char));
        if (s == NULL) {
            printf("memory allocation failure\n");
            return 1;
        }
        append(s, c);
    }
    printf("%s\n", s);
    free(s);
    return 0;
}