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| String Pattern-Matching Library Functions | p2open(3GEN) |
| | p2open, p2close - open, close pipes to and from a command |
SYNOPSIS
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cc [ flag ... ] file ... -lgen [ library ... ]
#include <libgen.h> int p2open(const char *cmd, FILE *fp[2]); |
| | int p2close(FILE *fp[2]); |
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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.
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A common problem is having too few file descriptors. p2close() returns -1 if the two file pointers are not from the same p2open().
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| | Example 1. Example of file descriptors.
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#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);
}
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See attributes(5) for descriptions of the following
attributes:
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
| MT-Level | Unsafe |
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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.
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