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Message-ID: <CAFf+5zjSkGyMXTybY7wioFM9gxruhGfKNKh797vvy29NMWsa4g@mail.gmail.com>
Date: Mon, 16 Jan 2023 17:18:29 +0530
From: Amit <amitchoudhary0523@...il.com>
To: linux-kernel@...r.kernel.org
Subject: Alternate implementation of getline() function of standard C library.
Alternate implementation of getline() function of standard C library.
---- Start of Code ----
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#define NO_ERROR 0 // no error happened.
#define INVALID_FD -1 // fd is less than 0.
#define NO_MEMORY -2 // memory not available.
#define FILE_LSEEK_ERROR -3 // error in seeking file
#define BUF_SIZE_INCREMENT 1024
#define NEW_LINE '\n'
char *get_line_from_file(int fd, int *error_num);
/*
* char *get_line_from_file(int fd, int *error_num):
*
* Function get_line_from_file() returns a line from the file represented by fd.
*
* This function supports regular files only (and not pipes, sockets, etc).
*
* If fd is less than 0 then NULL is returned and *error_num is assigned the
* appropriate error value (INVALID_FD in this case).
*
* In case of any error, *error_num is assigned the appropriate error value.
*
* The high level algorithm of this function is:
*
* The function get_line_from_file() reads some bytes in a buffer from the
* file and tries to find newline in the buffer. If a newline is not found
* then it reads more bytes from the file in the buffer. When a newline is
* found in the buffer, then the newline is replaced with null byte and the
* buffer is reallocated to correct size. Then the file offset for reading
* is set to the start of the next line. And then the buffer is returned to
* the user.
*
* Please note that the returned line/buffer doesn't contain any newlines.
*
* The line/buffer returned by this function is allocated using realloc(),
* so it is user's responsibility to free the line (free memory).
*
*/
char *get_line_from_file(int fd, int *error_num)
{
char *buf = NULL;
char *buf_temp = NULL;
long curr_buf_size = 0;
long curr_data_len = 0;
ssize_t bytes_read = -1;
int end_of_file_reached_or_error = 0;
int new_line_found = 0;
long i = 0;
*error_num = NO_ERROR;
if (fd < 0) {
*error_num = INVALID_FD;
return NULL;
}
while (1) {
buf_temp = realloc(buf, (size_t)(curr_buf_size + BUF_SIZE_INCREMENT));
if(!buf_temp) {
*error_num = NO_MEMORY;
free(buf);
return NULL;
}
buf = buf_temp;
curr_buf_size = curr_buf_size + BUF_SIZE_INCREMENT;
// read data from file
while (curr_data_len != curr_buf_size) {
bytes_read = read(fd, buf + curr_data_len,
(size_t)(curr_buf_size - curr_data_len));
if (bytes_read == 0) { // end of file reached
// Check if no bytes were read earlier also in this call to
// get_line_from_file(). If yes, then this means that end of
// test file was reached (but not actually read) in the last
// call to get_line_from_file(). So, this time end of file was
// read and 0 was returned by read. So, since there are no bytes
// to process, free the buffer and return NULL.
if (curr_data_len == 0) {
free(buf);
return NULL;
}
} // end of if bytes_read == 0
if (bytes_read <= 0) {
end_of_file_reached_or_error = 1;
break;
} // end of if bytes_read <= 0
curr_data_len = curr_data_len + bytes_read;
} // end of inner while (1)
new_line_found = 0;
for (i = 0; i < curr_data_len; i++) {
if (buf[i] == NEW_LINE) {
buf[i] = 0;
new_line_found = 1;
break;
}
} // end of for loop
if (new_line_found) {
// realloc and seek
buf_temp = realloc(buf, (size_t)(i + 1));
if(!buf_temp) {
*error_num = NO_MEMORY;
free(buf);
return NULL;
}
if (lseek(fd, (i + 1) - curr_data_len, SEEK_CUR) < 0) {
*error_num = FILE_LSEEK_ERROR;
free(buf);
return NULL;
}
return buf_temp;
} else { // new line not found
// NEW_LINE not found and end of file has been reached or some
// error happened. So, allocate one extra byte for terminating
// null byte and return.
if (end_of_file_reached_or_error) {
buf_temp = realloc(buf, (size_t)(curr_data_len + 1));
if(!buf_temp) {
*error_num = NO_MEMORY;
free(buf);
return NULL;
}
buf_temp[curr_data_len] = 0;
return buf_temp;
} // end of if end_of_file_reached_or_error
} // end of if - else (new_line_found)
} // end of outer while (1)
} // end of get_line_from_file
static void print_usage_and_exit(void)
{
printf("\n");
printf("Usage: get_line_from_file test_file_name\n");
printf("\n");
exit(1);
} // print_usage_and_exit
int main(int argc, char * argv[])
{
int fd = -1;
char *line = NULL;
int error_num = 0;
if (argc != 2) {
printf("\nOnly two arguments are required."
" Program name and test file name.\n");
print_usage_and_exit();
}
fd = open(argv[1], O_RDONLY);
if (fd < 0) {
printf("\n%s: File \"%s\" could not be opened. Exiting..\n\n",
__FILE__, argv[1]);
exit(1);
}
while ((line = get_line_from_file(fd, &error_num)) != NULL) {
printf("%s\n", line);
free(line);
}
if (error_num != 0) {
printf("\nError number returned = %d\n\n", error_num);
}
close(fd);
} // end of main
---- End of Code ----
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