C - Read by basic I/O reads only "\n" - c

Question

I have a problem at my C-lecture skill practice. My exercise is to read a text document (which is in the same directory like the program) char by char and write it reversed (so from the end to the beginning, char by char) at the Terminal (i have to work at Ubuntu).
Unfortunately it doesn't work - "read" only reads newline-chars (\n).
Can you find my mistake?
#include <sys/stat.h> //mode_t: accessing rights for the file
#include <fcntl.h> //for I/O
#include <unistd.h> //for file descriptors
#include <string.h> //for strlen
short const EXIT_FAILURE = 1;
short const EXIT_SUCCESS = 0;
char const* USAGE_CMD = "usage: write_file filename string_to_write\n";
char const* ERR_OPEN = "error in open\n";
char const* ERR_READ = "error in reading\n";
char const* ERR_CLOSE = "error in close\n";
char const* ERR_WRITE = "error in write\n";
int main(int argc, char** argv){
int fd = open(argv[1], O_RDONLY);
if(fd == -1){
write(STDERR_FILENO, ERR_OPEN, strlen(ERR_OPEN));
return EXIT_FAILURE;
}
int two_char_back = (-1)*sizeof(char); //shift-value for char
int one_back = -1; //shift-value for "no shift"
int length = lseek(fd, one_back, SEEK_END);//setting to one before oef
int i = 0; //for the loop
char buffer;
char* pbuffer = &buffer; //buffer for writing
while (i < length){
if (read(fd, pbuffer, sizeof(buffer)) == -1){ //READING
write(STDERR_FILENO, ERR_READ, strlen(ERR_READ));
return EXIT_FAILURE;
}
if(write(STDOUT_FILENO, pbuffer, sizeof(buffer)) == -1){ //WRITING
write(STDERR_FILENO, ERR_WRITE, strlen(ERR_WRITE));
return EXIT_FAILURE;
}
lseek(fd, two_char_back, SEEK_CUR); //STEPPING
i++;
}
if(close(fd) == -1){ //CLOSING
write(STDERR_FILENO, ERR_CLOSE, strlen(ERR_CLOSE));
return EXIT_FAILURE;
}
return EXIT_SUCCESS;
}

Answer 1

This is wrong:
int two_char_back = (-1)*sizeof(char);
sizeof(char) is 1, you need -2

Answer 2

Haven't tried running it, but looks like two_char_back should be -2. The read advances the cursor, so -1 keeps reading the same one.
Also, just an option, you could make it more efficient by reading the whole file in then reversing it, then writing.

Answer 3

You have a typo in following line:
int two_char_back = (-1)*sizeof(char);
It must be:
int two_char_back = (-2)*sizeof(char);
As read() increments a cursor, you are actually reading the same character all the time e.g:
example text
^
|
After reading:
example text
^
|
After seeking:
example text
^
|

Answer 4

Thanks for your advices a lot!
& Thanks to my colleagues!
Now it works but I created kind of a new version, here it is:
#include <sys/stat.h> //mode_t: accessing rights for the file
#include <fcntl.h> //for I/O
#include <unistd.h> //for file descriptors
#include <string.h> //for strlen
short const EXIT_FAILURE = 1;
short const EXIT_SUCCESS = 0;
char const* USAGE_CMD = "usage: write_file filename string_to_write\n";
char const* ERR_OPEN = "error in open\n";
char const* ERR_READ = "error in reading\n";
char const* ERR_CLOSE = "error in close\n";
char const* ERR_WRITE = "error in write\n";
int main(int argc, char** argv){
int fd = open(argv[1], O_RDONLY); //OPENING
if(fd == -1){
write(STDERR_FILENO, ERR_OPEN, strlen(ERR_OPEN));
return EXIT_FAILURE;
}
int file_size = lseek(fd, 0, SEEK_END); //setting to eof
int i = file_size-1; //for the loop, runs from the end to the start
char buffer;
//the files runs from the end to the back
do{
i--;
lseek(fd, i, SEEK_SET); //STEPPING from the start
if (read(fd, &buffer, sizeof(buffer)) != sizeof(buffer)){ //READING
write(STDERR_FILENO, ERR_READ, strlen(ERR_READ));
return EXIT_FAILURE;
}
if(write(STDOUT_FILENO, &buffer, sizeof(buffer)) != sizeof(buffer)){ //WRITING
write(STDERR_FILENO, ERR_WRITE, strlen(ERR_WRITE));
return EXIT_FAILURE;
}
}while (i != 0);
buffer = '\n';
write(STDOUT_FILENO, &buffer, sizeof(buffer));//no error-det. due to fixed value
if(close(fd) == -1){ //CLOSING
write(STDERR_FILENO, ERR_CLOSE, strlen(ERR_CLOSE));
return EXIT_FAILURE;
}
return EXIT_SUCCESS;
}