I/O redirection in a C program - redirect

Question

I want to implement a simple "cat file1 > file1" command in a C program. I have tried the following, but it does not work...
main () {
pid_t pid;
FILE *ip, *op;
char *args[3];
printf("Name of the executable program\n\t");
scanf("%s", &name[0]); // I entered cat here
printf("Name of the input file\n\t");
scanf("%s", &name[1]); //file1.txt
printf("Name of the output file\n\t");
scanf("%s", &name[0]); //file2.txt
pid = fork();
if(pid == -1)
perror("fork() error");
else if(pid > 0)
waitpid(-1, NULL, 0);
else if (pid == 0) {
op = fopen(name[2], "w");
close(1);
dup(op);
execlp(name[0], name[1], NULL);
}
return 0;
}// end of main()
I thought the execlp() will run cat file1.txt and its output will be redirected to file2.txt, but it's not and I don't know why. How do I do it?

Answer 1

scanf("%s", &name[0]); // I entered cat here
printf("Name of the input file\n\t");
scanf("%s", &name[1]); //file1.txt
printf("Name of the output file\n\t");
scanf("%s", &name[0]); //file2.txt
Clearly not a C&P of actual code - name should be args, and the last one should be "2" instead of 0.
Also, dup works on file descriptors, not FILE*, so need to look at open rather than fopen, or whatever method gets the fd from a FILE*

Answer 2

The first argument to execlp() is the name to be looked up; the second and following arguments are the argv list, starting with argv[0].
int execlp(const char *file, const char *arg0, ... /*, (char *)0 */);
For shell I/O redirection, it is easier to open files with open() than to use standard I/O (<stdio.h> and FILE *); you should also close the file you opened after the dup(), though it is easier to use dup2(). You need to allocate space to read the strings into; on many systems, the original code would crash because the pointers in str don't point anywhere. You should normally aim to exit with status 0 only if everything worked; otherwise, exit with a non-zero exit status.
This leads to:
#include <fcntl.h> /* open() */
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h> /* waitpid() */
#include <unistd.h> /* execlp(), fork(), dup2() */
int main(void)
{
pid_t pid;
pid_t corpse;
int status;
char name[3][50];
printf("Name of the executable program\n\t");
if (scanf("%49s", name[0]) != 1)
return(EXIT_FAILURE);
printf("Name of the input file\n\t");
if (scanf("%49s", name[1]) != 1)
return(EXIT_FAILURE);
printf("Name of the output file\n\t");
if (scanf("%49s", name[2]) != 1)
return(EXIT_FAILURE);
pid = fork();
if (pid == -1)
{
perror("fork() error");
return(EXIT_FAILURE);
}
else if (pid > 0)
corpse = waitpid(-1, &status, 0);
else
{
int fd = open(name[2], O_WRONLY|O_CREAT|O_EXCL, 0644);
if (fd < 0)
{
fprintf(stderr, "Failed to open %s for writing\n", name[2]);
return(EXIT_FAILURE);
}
dup2(fd, 1);
close(fd);
execlp(name[0], name[0], name[1], NULL);
fprintf(stderr, "Failed to exec %s\n", name[0]);
return(EXIT_FAILURE);
}
return(corpse == pid && status == 0 ? EXIT_SUCCESS : EXIT_FAILURE);
}

Answer 3

You have to either use fork() a process and reassign it's file descriptors to previously(manually) open()'ed file, or use system() call to make shell handle it for you.