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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]);

MT-Level

Unsafe

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.

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().

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 == -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);
}

See Also

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

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(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.

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