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| Standard C Library Functions | popen(3C) |
| | popen, pclose - initiate a pipe to or from a process |
SYNOPSIS
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#include <stdio.h> FILE *popen(const char *command, const char *mode); |
| | int pclose(FILE *stream); |
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The popen() function creates a pipe between the calling program and the command to be executed. The arguments to popen() are pointers to null-terminated strings. The command argument consists of a shell command line. The mode argument is an I/O mode, either r for reading or w for writing. The value returned is a stream
pointer such that one can write to the standard input of the command, if the I/O mode is w, by writing to the file stream (see intro(3));
and one can read from the standard output of the command, if the I/O mode is r, by reading from the file stream. Because open files are shared, a type r command may be used as an input filter and a type w as an output
filter.
The environment of the executed command will be as if a child process were created within the popen() call using fork(2). If the application
is standard-conforming (see standards(5)), the child is invoked with the call:
execl("/usr/xpg4/bin/ksh", "sh", "-c", command, (char *)0);
otherwise, the child is invoked with the call:
execl("/usr/bin/sh", "sh", "-c", command, (char *)0);
A stream opened by popen() should be closed by pclose(), which closes the pipe, and waits for the associated process to terminate and returns the termination status of the process running the command language interpreter. This is the value returned
by waitpid(2). See wstat(3xfn) for more information on termination status.
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The popen() function returns a null pointer if files or processes cannot be created.
The pclose() function returns the termination status of the command. It returns -1 if stream is not associated with a popen() command and sets errno to indicate the error.
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The popen() function may fail if:
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EMFILE
- There are currently FOPEN_MAX or STREAM_MAX streams open in the calling process.
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EINVAL
- The mode argument is invalid.
The pclose() function will fail if:
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ECHILD
- The status of the child process could not be obtained, as described above.
The popen() function may also set errno values as described by fork(2) or pipe(2).
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If the original and popen() processes concurrently read or write a common file, neither should use buffered I/O. Problems with an output filter may be forestalled by careful buffer flushing, for example, with fflush() (see fclose(3C)). A security hole exists through the IFS and PATH environment variables. Full pathnames should be used (or PATH reset) and IFS should be set to space and tab (" \t").
The signal handler for SIGCHLD should be set to default when using popen(). If the process has established a signal handler for SIGCHLD, it will be called when the command terminates. If the signal handler or another thread in
the same process issues a wait(2) call, it will interfere with the return value of pclose(). If the process's signal handler for SIGCHLD has been set to ignore the signal, pclose() will fail and errno will be set to ECHILD.
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| | Example 1. popen example
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The following program will print on the standard output (see stdio(3C)) the names of files in the current directory with a .c suffix.
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#include <stdio.h>
#include <stdlib.h>
main()
{
char *cmd = "/usr/bin/ls *.c";
char buf[BUFSIZ];
FILE *ptr;
if ((ptr = popen(cmd, "r")) != NULL)
while (fgets(buf, BUFSIZ, ptr) != NULL)
(void) printf("%s", buf);
(void) pclose(ptr);
return 0;
}
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Example 2. system replacement
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The following code fragment can be used in a multithreaded process in place of the MT-Unsafe system(3C) function:
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See attributes(5) for descriptions of the following attributes:
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
| MT-Level | Safe |
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ksh(1), pipe(2), wait(2), waitpid(2), fclose(3C), fopen(3C), stdio(3C), system(3C), attributes(5), wstat(3xfn), standards(5)
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