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man pages section 3: Extended Library Functions, Volume 2

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Updated: Thursday, June 13, 2019
 
 

p2close(3GEN)

Name

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

The p2open()gfunction 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.

The p2close() function 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.

Return Values

A common problem is having too few file descriptors. The p2close() function returns −1 if the two file pointers are not from the same p2open().

Examples

Example 1 Example of file descriptors.
#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 == −1 ) {
		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);
}

Attributes

See attributes(7) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
MT-Level
Unsafe

See Also

fclose(3C), popen(3C), setbuf(3C), attributes(7)

Notes

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(3C).

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.