Question

Is this safe to do? Does fgets terminate the buffer with null or should I be setting the 20th byte to null after the call to fgets and before I call clean?

// strip new lines
void clean(char *data)
{
    while (*data)
    {
        if (*data == '\n' || *data == '\r') *data = '\0';
        data++;
    }
}

// for this, assume that the file contains 1 line no longer than 19 bytes
// buffer is freed elsewhere
char *load_latest_info(char *file)
{
    FILE *f;
    char *buffer = (char*) malloc(20);
    if (f = fopen(file, "r"))
        if (fgets(buffer, 20, f))
        {
            clean(buffer);
            return buffer;
        }
    free(buffer);
    return NULL;
}

Answer 1

Yes fgets() always properly null-terminates the buffer. From the man page:

The fgets() function reads at most one less than the number of characters specified by n from the given stream and stores them in the string s. Reading stops when
a newline character is found, at end-of-file or error. The newline, if any, is retained. If any characters are read and there is no error, a ‘\0‘ character is
appended to end the string.