can anyone explain me how does this function work? - c

Question

I don't understand what this function do. Can anyone explain me in detail please?
char *my_getline(FILE *stream) {
char *line = NULL;
size_t pos = 0;
int c;
while ((c = getc(stream)) != EOF) {
char *newp = realloc(line, pos + 2);
if (newp == NULL) {
free(line);
return NULL;
}
line = newp;
if (c == '\n')
break;
line[pos++] = (char)c;
}
if (line) {
line[pos] = '\0';
}
return line;
}
If you can add a comment on my code, I think that will help me. I want to search a substring in a string and I found this function code.
This is the main function:
int main(void) {
char *str, *sub;
size_t len1, len2, i, count = 0;
printf("Insert string :\n");
str = my_getline(stdin);
printf("insert substring :\n");
sub = my_getline(stdin);
if (str && sub) {
len1 = strlen(str);
len2 = strlen(sub);
for (i = 0; i + len2 <= len1; i++) {
if (!memcmp(str + i, sub, len2)) {
count++;
printf("Substring found at index : %d\n", i);
}
}
printf("in the number of: %d\n", count);
if (count == 0) {
printf("Substring not found\n");
}
}
free(str);
free(sub);
return 0;
}
I understand the main function but unable to understand the logic in function my_getline.
Please help me in understanding the logic. Thanks!

Answer 1

char *my_getline(FILE *stream) {
// pointer to the line to be read:
char *line = NULL;
// position of the next character:
size_t pos = 0;
// single character:
int c;
while ((c = getc(stream)) != EOF) { // read 1 character at a time until EOF
// allocate a new buffer with room for the char just read + a 0 terminator
// when `line` is NULL, this is the same as `malloc()`, otherwise it
// will change the size of the allocation:
char *newp = realloc(line, pos + 2);
// check for errors:
if (newp == NULL) {
free(line);
return NULL;
}
// no errors, assign new buffer to `line`:
line = newp;
// end of line found: we're done:
if (c == '\n')
break;
// otherwise add new character to the line:
line[pos++] = (char)c;
}
// if there was *anything* to read, add 0 terminator (marks end of string):
if (line) {
line[pos] = '\0';
}
return line;
}
That's about it. Note it's horribly inefficient for two reasons: It reads only one character at a time and it calls realloc() for each and every character.
A better solution would use e.g. fgets() and increase the buffer size in reasonable chunks, for example like this:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define GETLINE_CHUNK 1024
static void xrealloc(void *bufPtr, size_t size)
{
void **buf = bufPtr;
void *tmp = realloc(*buf, size);
if (!tmp)
{
free(*buf);
*buf = 0;
}
*buf = tmp;
}
char *my_getline(FILE *stream)
{
// allocate first chunk:
char *buf = malloc(GETLINE_CHUNK);
if (!buf) return 0;
*buf = 0;
size_t pos = 0;
// read up to GETLINE_CHUNK bytes, until newline:
while (fgets(buf + pos, GETLINE_CHUNK, stream))
{
// look for newline:
char *nlPos = strchr(buf, '\n');
if (nlPos)
{
// found, then our line is complete
*nlPos = 0;
// shrink buffer to needed size
xrealloc(&buf, nlPos-buf+1);
return buf;
}
// set next offset to read
pos = strlen(buf);
// increase buffer size to have room for a whole other GETLINE_CHUNK:
xrealloc(&buf, pos + GETLINE_CHUNK);
if (!buf) return 0;
}
// if nothing was read, free buffer and return NULL:
if (*buf == 0)
{
free(buf);
buf = 0;
}
return buf;
}
int main(void)
{
char *line = my_getline(stdin);
if (line)
{
puts(line);
free(line);
}
else puts("no input!");
return 0;
}

Answer 2

Well this function gives you line, Lets go Step by Step:
char *my_getline(FILE *stream) {
char *line = NULL; //this is just pointer initialization
size_t pos = 0; //position variable definition and init
int c; //a variable to store temporary character
while ((c = getc(stream)) != EOF) //read every character till end of file
{
// To dynamically allocate memory, with reference to the
// number of character and plus '2' is just to compensate null
// character and the character(Since pos is 0)
char *newp = realloc(line, pos + 2);
if (newp == NULL) { // this is to check whether memory was alloacted properly or not.
free(line); //if not free line
return NULL;// break the program and return NULL
}
line = newp;// if memory is allocated properly store allocated memory in line pointer
if (c == '\n') //if new line is detected
break;// break the while loop
line[pos++] = (char)c; // store the character in dynamically allocated memory and new character in new location.
}
if (line) { //if line contains something then add a null character at last, to complete that string
line[pos] = '\0';
}
return line; //returns the content of line.
}
Hope this helps :)