POPEN(3) Linux Programmer's Manual POPEN(3)
popen, pclose - pipe stream to or from a process
FILE *popen(const char *command, const char *type);
int pclose(FILE *stream);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
popen(), pclose(): _POSIX_C_SOURCE >= 2 || _XOPEN_SOURCE || _BSD_SOURCE || _SVID_SOURCE
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 which
must contain either the letter 'r' for reading or the letter 'w' for writing. Since glibc
2.9, this argument can additionally include the letter 'e', which 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(2) for reasons why this may be useful.
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(3). 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 called popen(), unless this is altered by the command itself. Con-
versely, reading from a "popened" stream reads the command's standard output, and the com-
mand's standard input is the same as that of the process that 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 command as returned by wait4(2).
The popen() function returns NULL if the fork(2) or pipe(2) calls fail, or if it cannot
The pclose() function returns -1 if wait4(2) returns an error, or some other error is
The popen() function does not set errno if memory allocation fails. If the underlying
fork(2) or pipe(2) fails, errno is set appropriately. If the type argument is invalid,
and this condition is detected, errno is set to EINVAL.
If pclose() cannot obtain the child status, errno is set to ECHILD.
The 'e' value for type is a Linux extension.
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 com-
mand'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().
Failure to execute the shell is indistinguishable from the shell's failure to execute com-
mand, or an immediate exit of the command. The only hint is an exit status of 127.
sh(1), fork(2), pipe(2), wait4(2), fclose(3), fflush(3), fopen(3), stdio(3), system(3)
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GNU 2010-02-03 POPEN(3)