fread position cursor does not seem to advance as expected - c

Question

I'm trying to dynamically realloc memory for a file being read one character at a time. It is not printing the buffer character by character. It looks like the fread function is not advancing 1 character at a time.
int main() {
FILE *fp;
char *newBuffer;
char *buffer = malloc(sizeof(char));
int count = 0;
/* Open file for both reading and writing */
fp = fopen("test.txt", "r");
if (!fp) {
exit(99);
}
/* Seek to the beginning of the file */
fseek(fp, SEEK_SET, 0);
/* Read into memory and display the buffer as its read */
while (1) {
newBuffer = (char*)realloc(buffer, (sizeof(char) * (++count)));
if (newBuffer) {
buffer = newBuffer;
buffer += (count - 1);
fread(buffer, sizeof(char), 1, fp);
if (feof(fp)) {
buffer = newBuffer;
break;
}
buffer = newBuffer;
printf(" %s\n", buffer);
} else {
// realloc failed
free(buffer);
exit(1);
}
}
fclose(fp);
free(newBuffer);
return(0);
}

Answer 1

You do not null terminate the buffer before using it as a string in printf, this is a problem.
Note that you can simplify or improve the code in various ways:
no need to fseek(fp, SEEK_SET, 0); after fopen, the FILE is already at the starting position. Note that you interverted the arguments to fseek: it should be fseek(fp, 0L, SEEK_SET); but you are lucky SEEK_SET is #defined as 0.
reading one byte from the file is much simpler with getc than fread(buffer, sizeof(char), 1, fp);. It allows for a simpler and better test for end of file. Using feof() only works in your example because you only attempt to read a single byte.
no need for the initial malloc, set buffer toNULL.reallocacceptsNULLand behaves likemallocwith such as argument,freeaccepts aNULL` argument and does nothing.
do not cast the return value of malloc, nor realloc.
sizeof(char) is 1 by definition: either use sizeof(*buffer) or elide the sizeof completely.
do not parenthesize the return expression.
the prototype for main without arguments is int main(void)
Here is a simpler version:
int main(void) {
FILE *fp;
char *newBuffer;
char *buffer = NULL;
int count = 0, c;
/* Open file for both reading */
fp = fopen("test.txt", "r");
if (!fp) {
exit(99);
}
/* Read into memory and display the buffer read */
while ((c = getc(fp)) != EOF) {
newBuffer = realloc(buffer, count + 2);
if (newBuffer) {
buffer = newBuffer;
buffer[count++] = c;
buffer[count] = '\0';
printf(" %s\n", buffer);
} else {
// realloc failed
fclose(fp);
free(buffer);
exit(1);
}
}
fclose(fp);
free(buffer);
return 0;
}

Answer 2

Your printf(" %s\n", buffer); expects buffer to end with a '\0' (null) character. Your code doesn't provide the required null.