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popen(3s) [hpux man page]

popen(3S)																 popen(3S)

NAME
popen(), pclose() - initiate pipe I/O to/from a process SYNOPSIS
DESCRIPTION
creates a pipe between the calling program and a command to be executed by the POSIX shell, (see sh-posix(1)). The arguments to are pointers to null-terminated strings containing, respectively, a shell command line and an I/O mode, either for reading or for writing. returns a stream pointer such that one can write to the standard input of the command if the I/O mode is by writing to the file stream; and one can read from the standard output of the command if the I/O mode is by reading from the file stream. Because open files are shared, a type command can be used as an input filter and a type command as an output filter. If mode is other than or the result is undefined. A stream opened by should be closed by which waits for the associated process to terminate and returns the exit status of the command. However, if a call caused the termination status to be unavailable to then returns -1 with errno set to This can happen in one of the fol- lowing conditions: o If the signal handler for is set to o If the signal handler or another thread in the same process issues or call. APPLICATION USAGE
After a stream is associated with a pipe by the stream is byte-oriented (see orientation(5)). RETURN VALUE
Upon successful completion, returns a pointer to an open stream that can be used to read or write to the pipe. Otherwise, it returns a NULL pointer if files or processes cannot be created and set errno to indicate the error. The success of the command execution can be checked by examining the return value of Upon successful return, returns the termination status of the command language interpreter. Otherwise, returns -1 if stream is not associ- ated with a command and set errno to indicate the error. ERRORS
The function fails if: The maximum number of file descriptors allowed are currently open. The function fails if: The status of the child process is not available. WARNINGS
If the original and processes concurrently read or write a common file, neither should use buffered I/O because the buffering will not work properly. Problems with an output filter can be forestalled by careful buffer flushing, e.g., with see fclose(3S). SEE ALSO
pipe(2), wait(2), fclose(3S), fopen(3S), system(3S), orientation(5), thread_safety(5). STANDARDS CONFORMANCE
popen(3S)

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popen(3C)						   Standard C Library Functions 						 popen(3C)

NAME
popen, pclose - initiate a pipe to or from a process SYNOPSIS
#include <stdio.h> FILE *popen(const char *command, const char *mode); int pclose(FILE *stream); DESCRIPTION
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. A trailing F character can also be included in the mode argument as described in fopen(3C) to enable extended FILE facility. The environment of the executed command will be as if a child process were created within the popen() call using fork(2). If the applica- tion is standard-conforming (see standards(5)), the child is created as if invoked with the call: execl("/usr/xpg4/bin/sh", "sh", "-c",command, (char *)0); otherwise, the child is created as if invoked with the call: execl("/usr/bin/sh", "sh", "-c",command, (char *)0); The pclose() function closes a stream opened by popen() by closing the pipe. It 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(3C). See wait.h(3HEAD) for more information on termination status. If, however, a call to waitpid() with a pid argument equal to the process ID of the command line interpreter causes the termination status to be unavailable to pclose(), then pclose() returns -1 with errno set to ECHILD to report this condition. RETURN VALUES
Upon successful completion, popen() returns a pointer to an open stream that can be used to read or write to the pipe. Otherwise, it returns a null pointer and may set errno to indicate the error. Upon successful completion, pclose() returns the termination status of the command language interpreter as returned by waitpid(). Other- wise, it returns -1 and sets errno to indicate the error. ERRORS
The pclose() function will fail if: ECHILD The status of the child process could not be obtained, as described in the DESCRIPTION. The popen() function may fail if: EMFILE There are currently FOPEN_MAX or STREAM_MAX streams open in the calling process. EINVAL The mode argument is invalid. The popen() function may also set errno values as described by fork(2) or pipe(2). USAGE
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 (" "). Even if the process has established a signal handler for SIGCHLD, it will be called when the command terminates. Even if another thread in the same process issues a wait(3C) call, it will interfere with the return value of pclose(). Even if the process's signal handler for SIGCHLD has been set to ignore the signal, there will be no effect on pclose(). EXAMPLES
Example 1 popen() example The following program will print on the standard output (see stdio(3C)) the names of files in the current directory with a .c suffix. #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; } Example 2 system() replacement The following function can be used in a multithreaded process in place of the most common usage of the Unsafe system(3C) function: int my_system(const char *cmd) { FILE *p; if ((p = popen(cmd, "w")) == NULL) return (-1); return (pclose(p)); } ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |See below. | +-----------------------------+-----------------------------+ |MT-Level |Safe | +-----------------------------+-----------------------------+ The F character in the mode argument of popen() is Evolving. In all other respects this function is Standard. The pclose() function is Standard. SEE ALSO
ksh(1), pipe(2), fclose(3C), fopen(3C), posix_spawn(3C), stdio(3C), system(3C), wait(3C), waitpid(3C), wait.h(3HEAD), attributes(5), stan- dards(5) SunOS 5.11 14 Dec 2006 popen(3C)

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