Parent -and Child IPC Using a Single Pipe

diff -r 48780e181b38 -r 578be2adaf3e Symbian3/PDK/Source/GUID-AA49FB68-22A1-417F-AB57-9C22CE016B21.dita --- a/Symbian3/PDK/Source/GUID-AA49FB68-22A1-417F-AB57-9C22CE016B21.dita Tue Jul 20 12:00:49 2010 +0100 +++ b/Symbian3/PDK/Source/GUID-AA49FB68-22A1-417F-AB57-9C22CE016B21.dita Fri Aug 13 16:47:46 2010 +0100 @@ -1,179 +1,179 @@ - - - - - -Parent -and Child IPC Using a Single Pipe -

In this scenario, the parent and child processes communicate using a single -pipe. The following pseudo code details the mechanism used by code in the -parent process in a Unix-like system. //Call pipe() to create the pipe. -//Call Fork(). -if (child process) -{ - //Child Process. - //duplicate (via a dup2() call) the read/write end of the pipe using - //prior agreed file descriptor numbers which will subsequently be used by the //child process following the exec() call. - //Close the original file descriptors. - //Call exec() to replace the code image in the child process. -} -else -{ - //Parent process - //Do Whatever -} -

Instead of using the fork()/exec() as -described above, POSIX libraries including P.I.P.S. provide the popen() function -as defined in stdio.h. Rather than using a pipe for the -IPC mechanism the parent process communicates with the child using the stream -returned by the popen() call, while the child process input -or output is using the stdin()/stdout() streams.

However if the parent process requires a file descriptor it can be obtained -by using the fileno() API call. For more information about -the popen() function, see http:\\www.opengroup.org.

Parent process fork() and exec() functions

The following code shows the use of a pipe and subsequent fork()/exec() calls -to create a Pipe for IPC. The example shown is the child process writing data -to the pipe, and the parent process receiving the data.

-//Child Process File Descriptors for the pipe. These should be defined in a common header -//file used to compile both the parent and child process's executable. -//However #define is here for clarity. -#define WRITE_FD 104 -int main(int argc, char *argv[]) -{ - int pipeEnds[2]; //Pipe file descriptors [0] is for read [1] is for write - - //create the pipe - if(pipe(pipeEnds) != 0) - { - //Pipe creation error - return EXIT_FAILURE; - } - else - { - pid_t Childpid = fork(); - - if(Childpid == 0) - { - //close the redundant read FD obtained from the parent process - (void)close(pipeEnds[0]); - - //duplicate the file descriptor for use following exec(). - if(dup2(pipeEnds[1], WRITE_FD) == -1) - { - printf("dup2 error\n"); - return EXIT_FAILURE; - } - //close the redundant write FD obtained from the parent process - (void)close(pipeEnds[1]); - - //exec() to replace the child process executable - char execFileName[] = "/root/PortDoc/Example1_c/Posix/Child/ChildProg"; - execl(execFileName,NULL); - } - else - { - //Parent process. This reads from the pipe. Therefore close the write end of the - //pipe. - close(pipeEnds[1]); - - //declare receive buffer, and clear its contents - char RxBuffer[100]; - memset(RxBuffer,0,sizeof(RxBuffer)); - - //Wait for data from the child process. Child sends a string. - read(pipeEnds[0],RxBuffer,sizeof(RxBuffer)); - - printf(RxBuffer); - - //close the Read end of the pipe - close(pipeEnds[0]); - - //Wait for the child process to terminate - waitpid(Childpid,NULL,0); - } - } - return EXIT_SUCCESS; -} - -

Child process created using fork() or exec() functions

The following code shows an example of the child process source which will -be executed following the exec().

-//Child Process File Descriptors for the pipe. These should be defined in a common header -//file used to compile both the parent and child process's executable. Shown here for -//clarity. -#define WRITE_FD 104 - -int main(void) -{ - char TxMsg[] = "Hello Parent\n"; - - //Send the message to the parent - write(WRITE_FD,TxMsg,sizeof(TxMsg)); - - //close the File Descriptor - close(WRITE_FD); - return EXIT_SUCCESS; -} - -

Parent process P.I.P.S. example for popen() function

The following code shows how the above code can be modified to use popen(), -rather than the fork()/exec() combination.

-int main(int argc, char *argv[]) -{ - //Create child process using popen(). Child process writes to the Parent therefore "r" - //parameter. - FILE* ChildProcessStream = popen("/root/PortDoc/Example1_c/Symbian/Child/ChildProg","r"); - - if(ChildProcessStream == NULL) - { - printf("\n Failure to create child process with popen()\n"); - return EXIT_FAILURE; - } - else - { - //Use a file descriptor rather than a stream - int ChildProcessFD = fileno(ChildProcessStream); - - //Create a receive buffer, and zero contents before receiving. - char RxBuffer[100]; - memset(RxBuffer,0,sizeof(RxBuffer)); - - //Wait for data from the child process. Child sends a string. - int nbytes = read(ChildProcessFD,RxBuffer,sizeof(RxBuffer)); - - //printf is slightly different from the POSIX example to verify that the data output to the - //screen is done by the parent process. - printf("\n Message Received by Parent=%s",RxBuffer); - - //Wait for Child Process to complete - pclose(ChildProcessStream); - } - return EXIT_SUCCESS; -} - -

Child process P.I.P.S. example for popen() function

The following code shows how the child process source can be modified when -it is executed using popen(). Note that rather -than using a file descriptor for the communication via the pipe the stdin() or -the stdout streams are directed to the parent process, as -defined in the usage of the popen() call.

-int main(void) -{ - //Child process created by popen() so that its stdout is streamed to the parent process - char TxMsg[] = "Hello Parent\n"; - - //Send the message to the parent - printf(TxMsg); - - return EXIT_SUCCESS; -} - + + + + + +Parent +and Child IPC Using a Single Pipe +

In this scenario, the parent and child processes communicate using a single +pipe. The following pseudo code details the mechanism used by code in the +parent process in a Unix-like system. //Call pipe() to create the pipe. +//Call Fork(). +if (child process) +{ + //Child Process. + //duplicate (via a dup2() call) the read/write end of the pipe using + //prior agreed file descriptor numbers which will subsequently be used by the //child process following the exec() call. + //Close the original file descriptors. + //Call exec() to replace the code image in the child process. +} +else +{ + //Parent process + //Do Whatever +} +

Instead of using the fork()/exec() as +described above, POSIX libraries including P.I.P.S. provide the popen() function +as defined in stdio.h. Rather than using a pipe for the +IPC mechanism the parent process communicates with the child using the stream +returned by the popen() call, while the child process input +or output is using the stdin()/stdout() streams.

However if the parent process requires a file descriptor it can be obtained +by using the fileno() API call. For more information about +the popen() function, see http:\\www.opengroup.org.

Parent process fork() and exec() functions

The following code shows the use of a pipe and subsequent fork()/exec() calls +to create a Pipe for IPC. The example shown is the child process writing data +to the pipe, and the parent process receiving the data.

+//Child Process File Descriptors for the pipe. These should be defined in a common header +//file used to compile both the parent and child process's executable. +//However #define is here for clarity. +#define WRITE_FD 104 +int main(int argc, char *argv[]) +{ + int pipeEnds[2]; //Pipe file descriptors [0] is for read [1] is for write + + //create the pipe + if(pipe(pipeEnds) != 0) + { + //Pipe creation error + return EXIT_FAILURE; + } + else + { + pid_t Childpid = fork(); + + if(Childpid == 0) + { + //close the redundant read FD obtained from the parent process + (void)close(pipeEnds[0]); + + //duplicate the file descriptor for use following exec(). + if(dup2(pipeEnds[1], WRITE_FD) == -1) + { + printf("dup2 error\n"); + return EXIT_FAILURE; + } + //close the redundant write FD obtained from the parent process + (void)close(pipeEnds[1]); + + //exec() to replace the child process executable + char execFileName[] = "/root/PortDoc/Example1_c/Posix/Child/ChildProg"; + execl(execFileName,NULL); + } + else + { + //Parent process. This reads from the pipe. Therefore close the write end of the + //pipe. + close(pipeEnds[1]); + + //declare receive buffer, and clear its contents + char RxBuffer[100]; + memset(RxBuffer,0,sizeof(RxBuffer)); + + //Wait for data from the child process. Child sends a string. + read(pipeEnds[0],RxBuffer,sizeof(RxBuffer)); + + printf(RxBuffer); + + //close the Read end of the pipe + close(pipeEnds[0]); + + //Wait for the child process to terminate + waitpid(Childpid,NULL,0); + } + } + return EXIT_SUCCESS; +} + +

Child process created using fork() or exec() functions

The following code shows an example of the child process source which will +be executed following the exec().

+//Child Process File Descriptors for the pipe. These should be defined in a common header +//file used to compile both the parent and child process's executable. Shown here for +//clarity. +#define WRITE_FD 104 + +int main(void) +{ + char TxMsg[] = "Hello Parent\n"; + + //Send the message to the parent + write(WRITE_FD,TxMsg,sizeof(TxMsg)); + + //close the File Descriptor + close(WRITE_FD); + return EXIT_SUCCESS; +} + +

Parent process P.I.P.S. example for popen() function

The following code shows how the above code can be modified to use popen(), +rather than the fork()/exec() combination.

+int main(int argc, char *argv[]) +{ + //Create child process using popen(). Child process writes to the Parent therefore "r" + //parameter. + FILE* ChildProcessStream = popen("/root/PortDoc/Example1_c/Symbian/Child/ChildProg","r"); + + if(ChildProcessStream == NULL) + { + printf("\n Failure to create child process with popen()\n"); + return EXIT_FAILURE; + } + else + { + //Use a file descriptor rather than a stream + int ChildProcessFD = fileno(ChildProcessStream); + + //Create a receive buffer, and zero contents before receiving. + char RxBuffer[100]; + memset(RxBuffer,0,sizeof(RxBuffer)); + + //Wait for data from the child process. Child sends a string. + int nbytes = read(ChildProcessFD,RxBuffer,sizeof(RxBuffer)); + + //printf is slightly different from the POSIX example to verify that the data output to the + //screen is done by the parent process. + printf("\n Message Received by Parent=%s",RxBuffer); + + //Wait for Child Process to complete + pclose(ChildProcessStream); + } + return EXIT_SUCCESS; +} + +

Child process P.I.P.S. example for popen() function

The following code shows how the child process source can be modified when +it is executed using popen(). Note that rather +than using a file descriptor for the communication via the pipe the stdin() or +the stdout streams are directed to the parent process, as +defined in the usage of the popen() call.

+int main(void) +{ + //Child process created by popen() so that its stdout is streamed to the parent process + char TxMsg[] = "Hello Parent\n"; + + //Send the message to the parent + printf(TxMsg); + + return EXIT_SUCCESS; +} + \ No newline at end of file