p2open(3G)

p2open, p2close -- open, close pipes to and from a command

Synopsis

   cc [flag . . . ] file . . . -lgen [library] . . .
   
   #include <libgen.h>
   

   int p2open (const char cmd, FILE fp[2]);
   

   int p2close (FILE fp[2]);

Description

p2open forks and execs a shell running the command line pointed to by cmd. On return, fp[0] points to a FILE pointer to write the command's standard input and fp[1] points to a FILE pointer to read from the command's standard output. In this way the program has control over the input and output of the command.

The function returns 0 if successful; otherwise it returns -1.

p2close is used to close the file pointers that p2open opened. It waits for the process to terminate and returns the process status. It returns 0 if successful; otherwise it returns -1.

Examples

   #include <stdio.h>
   #include <libgen.h>
   
   main(argc,argv)
   int argc;
   char argv;
   {
   	FILE fp[2];
   	pid_t pid;
   	char buf[16];
   

   	pid=p2open("/usr/bin/cat", fp);
   	if ( pid == 0 ) {
   		fprintf(stderr, "p2open failed\n");
   		exit(1);
   	}
   	write(fileno(fp[0]),"This is a test\n", 16);
   	if(read(fileno(fp[1]), buf, 16) <=0)
   		fprintf(stderr, "p2open failed\n");
   	else
   		write(1, buf, 16);
   	(void)p2close(fp);
   }

References

fclose(3S), popen(3S), setbuf(3S)

Return values

A common problem is having too few file descriptors. p2close returns -1 if the two file pointers are not from the same p2open.

Notices

Buffered writes on fp[0] can make it appear that the command is not listening. Judiciously placed fflush calls or unbuffering fp[0] can be a big help; see fclose(3S).

Many commands use buffered output when connected to a pipe. That, too, can make it appear as if things are not working.

Usage is not the same as for popen, although it is closely related.