How to swap STDIO.H library functions with strictly system calls? - c

Question

A simple program that takes an input file specified at the terminal, and alters the text to be reversed. How can the <stdio.h> functions be converted to only linux system calls? (I assume using only libraries like <unistd.h>)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char* concat(const char *str1, const char *str2)
{
char *answer = malloc(strlen(str1) + strlen(str2) + 1);
strcpy(answer, str1);
strcat(answer, str2);
return answer;
}
int main(int argc, char** argv) {
FILE * fp;
char * line = NULL;
size_t len = 0;
fp = fopen(argv[1], "r");
if (fp == NULL){
perror("\nError ");
exit(1);
}
char *rev1 = "rev ";
char *rev2 = {argv[1]};
char *rev3 = concat(rev1,rev2);
system(rev3);
fclose(fp);
return 0;
}
Thank you for any help. Company only wants me to use system calls for some reason, this internship is not going great!

Answer 1

don't understand how to properly implement [read and write]
Assuming you mean call them (not implement them), the catch with read and write is that they may read or write less than requested, so you have to call them in a loop.
size_t to_write = strlen(str);
while (to_write) {
ssize_t written = write(fd, str, to_write);
if (written < 0) {
perror(NULL);
exit(1);
}
str += written;
to_write -= written;
}
Reading works the same way if you know how much you need to read or if you're trying to read an entire file. (To read the entire file, read chunks until read returns 0. Factors of 8*1024 are nice chunk sizes.)
Otherwise, it gets far more complicated. How do you know how much to read before you read it? If you want to read a line, for example, you have no idea how long the line is until you encounter the terminating line feed. You could read a character at a time, but that's very inefficient. You could do like like stdio does and use a buffer that holds the excess. At which point you might as well use stdio.

Answer 2

//#include<stdio.h> not used
#include<stdlib.h>
#include<string.h>
#include<unistd.h>
#include<sys/types.h>
#include<sys/stat.h>
#include<fcntl.h>
char* swap(const char *one, const char *two)
{
char *result = malloc(strlen(one) + strlen(two) + 1);
strcpy(result, one);
strcat(result, two);
return result;
}
int main(int argc, char** argv) {
if (argc != 2){
printf("Error, wrong number of arguments!\n");
exit(1);
}
int fd = open(argv[1], O_RDONLY); //<------------------<
char * line = NULL;
size_t len = 0;
char *string1 = "rev ";
char *string2 = {argv[1]};
char *result = swap(string1,string2);
system(result);
return 0;
}