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C Programming: Execl

Posted on 2013-11-20
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Last Modified: 2013-11-20
I have a C program on the server that uses a socket to "accept" connections. However, instead of communicating back with the client via that same socket, the professor asked for slightly different requirements.

He asked to fork & execl a program to do some processing, and use that program to talk back to the client.

After forking, inside the child process, I had to override my STDIN_FILENO & STDOUT_FILENO with the ClientFD (socket), so that the execl'ed program can talk to the client via the socket. This was required (for fork'ed program to use STDOUT to talk to client).

csockfd = accept(sockfd, peerSockAddr, &peer_addr_size);
...
// Create Child Process
pid = fork();
...
if( pid == 0) {
   // duplicate client fd to stdin and stdout
   dup2(csockfd, STDIN_FILENO);
   dup2(csockfd, STDOUT_FILENO);
   close(sockfd);
   execl("process","process");
}

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The problem is that I cannot "PRINTF()" to the console. I need to be able to output some text to the console. What's the best way to do that?
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Question by:pzozulka
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by:jkr
ID: 39663831
You could write to 'stderr' instead of 'stdout', since the latter is no longer available,e.g.

fprintf(stderr, "Hello World!\n"); // instead of 'printf()'

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by:pzozulka
ID: 39664274
Is this the correct way to call execl?

execl("twist","twist","www.twitter.com","1","9000","z");

If I was the user running this, I would type:
./twist www.twitter.com 1 9000 z
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jkr earned 500 total points
ID: 39664303
Not quite. The argument list needs to be NULL-terminated, i.e.

execl("twist","twist","www.twitter.com","1","9000","z",(char*)NULL);

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See http://linux.die.net/man/3/execl

The const char *arg and subsequent ellipses in the execl(), execlp(), and execle() functions can be thought of as arg0, arg1, ..., argn. Together they describe a list of one or more pointers to null-terminated strings that represent the argument list available to the executed program. The first argument, by convention, should point to the filename associated with the file being executed. The list of arguments must be terminated by a NULL pointer, and, since these are variadic functions, this pointer must be cast (char *) NULL.
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by:pzozulka
ID: 39664603
fprintf(stderr, "Hello World!\n"); // instead of 'printf()'

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It gives me an error: Input/output error

Twist.c is the client program which connects to deflect.c (acting as server) using a socket. Then deflect.c forks and execl process.c, and then process.c execl twist.c. Should I have fork'd before execl twist.c?

//Inside of deflect
csockfd = accept(sockfd, peerSockAddr, &peer_addr_size);
...
if( pid == 0) {
...
dup2(csockfd, STDIN_FILENO);
dup2(csockfd, STDOUT_FILENO);
close(sockfd);

execl("process","process");
...
}

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// Inside of process.c ----- (execl'ed by deflect.c)
void process( int csockfd, const char *conf) {
	FILE *fp=NULL;

	// Open a log file in write mode.
	fp = fopen ("/home/users1/pz951772/log.txt", "w+");
	fprintf(fp, "Logging info...\n");
	fflush(fp);
        ...
        execl("twist","twist","www.twitter.com","1","9000","z", (char *)NULL);
	exit(0);
}

int main( int argc, char *argv[] ) {
	process(STDIN_FILENO, "config.txt");	
}

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// Inside twist.c
int main( int argc, char *argv[] ) {
   ...
   if(argc > 4) {
		fprintf(stderr, "Hello World\n"); // getting Input/output error
   }
   ...
}

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Expert Comment

by:jkr
ID: 39664750
That's weird, this should work - 'stderr' is he only standard descriptor that you haven't bound to something else. Which user(s) are your processes working under?
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