popen, pclose - pipe stream to or from a process

FILE *popen( const char *command, const char *type);

int pclose(FILE *stream);

描述

The popen() function opens a process by creating a pipe, forking, and invoking the shell.  Since a pipe is by definition unidirectional, the type argument may specify only reading or writing, not both; the resulting stream is correspondingly read-only or write-only.

The command argument is a pointer to a null-terminated string containing a shell command line. This command is passed to /bin/sh using the -c flag; interpretation, if any, is performed by the shell. The type argument is a pointer to a null-terminated string whichi must contain either the letter ‘r‘ for reading or the lette ‘w‘ for writing. Since glibc 2.9, this argument can additionally include the letter ‘e‘, whichi causes the close-on-exec flag(FD_CLOEXEC) to be set on the underlying file descriptor; see the description of the O_CLOEXEC flag in open for resons why this may be usefull.

The return value from popen() is a normal standard I/O stream in all respects save that it must be closed with pclose() rather than fclose(). Writing to such a stream writes to the standard input of the command; the command‘s standard output is the same as that of the process that call popen(), unless this is altered by the command itself. Conversely, reading from a "popened" stream reads the command‘s standard output, and the command‘s standard input is the same as that of the process the called popen().

Note that output popen() streams are fully buffered by default.

The pclose() function waits for the associated process to terminate and returns the exit status of the comman as returned by wait4.

返回值

The popen() function returns NULL if the fork or pipe calls fail, or if it cannot allocate memory.

The pclose() function returns -1 if wait4 returns an error, or some other error is detected. In the event of an error, these functions set errno to indicate the cause of the error.

BUGS
       Since the standard input of a command opened for reading shares its seek  offset  with  the  process that  called popen(), if the original process has done a buffered read, the command‘s input position may not be as expected.  Similarly, the output from a command opened for writing may  become  intermingled  with  that  of the original process.  The latter can be avoided by calling fflush(3) before popen().

popen&pclose管道方式操作shell命令