popen

Name

popen, pclose
- process I/O

Library

libc.lib

Synopsis

#include <stdio.h>

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

int pclose (FILE *stream);

Return values

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

The pclose function returns -1 if stream is not associated with a "popened" command, if stream already "pclosed," or if
wait4 returns an error.

Detailed description

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 ("r") or write-only "w". If type is anything other than this, then the behavior is undefined.

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 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 called 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 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.

Errors

The popen function does not reliably set errno.

Examples

/*************************************************************************************************************
* Detailed description: Parent Process that demonstrates usage of popen system call in read mode
*
* Preconditions: child process exe should be available in c:\sysnhld.exe in target (assuming its installed)
* EPOCROOT\poc32\release4ld.exe in emulator, before parent process is started.
**************************************************************************************************************/
#include <stdio.h>
#ifdef __WINSCW__
const char* const KChildProcess = "z:\sys\bin\TPopenReadChild.exe";
#else
const char* const KChildProcess = "C:\Sys\Bin\TPopenReadChild.exe";
#endif //__WINSCW__
int main() {
        char string[100];
        int ret;
        FILE* fp = popen(KChildProcess, "r");
        if(fp) {
                ret = fscanf(fp, "%s", string);
                printf("Read %s from child0, string);
                pclose(fp);
        }
        return 0;
}

/*************************************************************************************************************
* Detailed description: Chil Process that will be spawned by Parent as a result of popen system call
* If, this application is spawned independentl, printf flushes "Popened-Child" on to console.
* And if, its spawned as a result of popen (as its done by the parent process), string sent to printf wont be
* displayed onto console, instead, it will be wrote on to the pipe created by the parent between
* parent and child. So, parent process will get this string.
*
**************************************************************************************************************/
#include <stdio.h>
int main() {
        printf("Popened-Child");
        return 0;
}

Output

Read Popened-Child from child

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 before popen.

Failure to execute the shell is indistinguishable from the shell’s failure to execute command, or an immediate exit of the command. The only hint is an exit status of 127.

The popen function always calls
sh, never calls
csh.

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