What to do with buffer in C - c

Question

I am trying to use a function that returns char * buffer to trap random user input from standard input and return a char* (that is, a string) with the input until '\n' or '\0' is detected as input.
However, the function user_input() crashed at the second occurence of c = getchar(); near the end of the function (at the //Error Here comment).
What am I doing wrong?
char* user_input(){
char *buffer=NULL;
size_t num_read = 0;
size_t buffer_size = 100;
char c = '\0';
char *new_buffer=NULL;
buffer = malloc(buffer_size);
fflush(stdin);
c = getchar();
while ( c != '\n' || c !='\0' ) {
if (num_read >= buffer_size) {
new_buffer=NULL;
buffer_size *= 2; // try a buffer that's twice as big as before
new_buffer = realloc(buffer, buffer_size);
if (new_buffer == NULL) {
free(buffer);
return NULL;
/* Abort - out of memory */
}
buffer = new_buffer;
}
buffer[num_read] = c;
num_read++;
c = getchar(); //Error Here
}//while
buffer[num_read] = '\0';
return buffer;
}

Answer 1

while ( c != '\n' || c !='\0' ) {
should use && instead of || or the loop will never exit
while ( c != '\n' && c !='\0' ) {
I built it and it worked although there are other bugs in the code such as the "buffer[num_read] = '\0';" at the end could overflow the buffer if there are exactly buffer_size items in the array.

Answer 2

Here are the issues that I found:
You cannot call fflush() on stdin. It leads to undefined behavior. fflush() is only for output streams.
The type for c should be an int. The getchar() will return EOF when there is no more input, and EOF is a value not equal to any valid value for unsigned char. That is why getchar() returns an int, so your variable receiving the return value of getchar() should match that type.
The sense of the while check is incorrect. You will only leave the loop if c equals both \n and \0 at the same time, which is not possible. Thus the loop will never exit. If you actually ended the input given to your program, you probably entered an infinite loop, continually allocating until your system ran out of memory. So, you want to leave the loop if c equals \n or \0.
You should add a check to leave the loop if c is EOF.
You should make sure num_read is less than buffer_size before you assign the \0 character. This can be done by moving your realloc() code to be at the bottom of the while loop instead of at the top.
With these changes, the code looks like:
char *user_input() {
char *buffer = NULL;
size_t num_read = 0;
size_t buffer_size = 100;
int c = '\0';
char *new_buffer = NULL;
buffer = malloc(buffer_size);
c = getchar();
while (!(c == EOF || c == '\n' || c == '\0')) {
assert(num_read < buffer_size);
buffer[num_read++] = c;
if (num_read >= buffer_size) {
buffer_size *= 2; // try a buffer that's twice as big as before
new_buffer = realloc(buffer, buffer_size);
if (new_buffer == NULL) {
free(buffer);
return NULL;
}
buffer = new_buffer;
}
c = getchar();
} //while
assert(num_read < buffer_size);
buffer[num_read] = '\0';
return buffer;
}

Answer 3

I have modified your code below. Try to run it as is, using the 'q' character at first, then you can experiment with other exit criteria later. By the way, your code worked without modification and did not crash for me (unless buffer overrun occured, but it would never leave the while loop otherwise (as Brian Walker pointed out). In any case, Try this in any ANSI C compiler: (just don't enter more than 1000 entries :)
#include <ansi_c.h>
char* user_input(void);
int main(void)
{
char *buf;
buf = malloc(1000);
//SearchDirectory("C:\\dev");
sprintf(buf, "%s", user_input());
printf("%s", buf);
free(buf);
return 0;
}
char* user_input(void){
char *buffer=NULL;
size_t num_read = 0;
size_t buffer_size = 100;
char c = '\0';
char *new_buffer=NULL;
buffer = malloc(buffer_size);
fflush(stdin);
c = getchar();
while ( c != 'q' ) {
if (num_read >= buffer_size) {
new_buffer=NULL;
buffer_size *= 2; // try a buffer that's twice as big as before
new_buffer = realloc(buffer, buffer_size);
if (new_buffer == NULL) {
free(buffer);
return NULL;
/* Abort - out of memory */
}
buffer = new_buffer;
}
buffer[num_read] = c;
num_read++;
c = getchar();
}//while
buffer[num_read] = '\0';
return buffer;
}