how to use execlp() with redirected output - c

Question

i tried to write c program in Unix environment that using execlp function.
the command is:
execlp("tsort","tsort","text.txt",">","1.txt",(char *)NULL);
syserr("execlp");
i am always gets the same error.
the error is:
tsort: extra operand `>'
what did i do wrong?

Answer 1

'>' is not a parameter, it is normally interpreted by a shell. If you want to achieve the same effect in C code, you have to do the same thing the shell normally does:
open a file (1.txt) for writing
fork() a new process
[in child] replace the stdout of the new process with the file's descriptor using dup2()
[in child] exec the command
Simplified example code for POSIX:
#define _POSIX_C_SOURCE 200101L
#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <fcntl.h>
#include <unistd.h>
int main(void)
{
int outfd = open("1.txt", O_CREAT|O_WRONLY|O_TRUNC, 0644);
if (!outfd)
{
perror("open");
return EXIT_FAILURE;
}
pid_t pid = fork();
if (pid < 0)
{
close(outfd);
perror("fork");
return EXIT_FAILURE;
}
if (pid)
{
// child code
dup2(outfd, 1); // replace stdout
close(outfd);
// just a "useless cat" for simplicity:
execlp("cat", "cat", "redir.c", 0);
}
else
{
// parent code
close(outfd);
int status;
waitpid(pid, &status, 0);
if (WIFEXITED(status)) return WEXITSTATUS(status);
else return EXIT_FAILURE;
}
}
As per the comment: If you don't mind replacing your process with the called program, you don't even need to fork and the program becomes very short:
#define _POSIX_C_SOURCE 200101L
#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
int main(void)
{
int outfd = open("1.txt", O_CREAT|O_WRONLY|O_TRUNC, 0644);
if (!outfd)
{
perror("open");
return EXIT_FAILURE;
}
dup2(outfd, 1); // replace stdout
close(outfd);
execlp("cat", "cat", "redir.c", 0);
}
This of course is not what an interactive shell does.