How to Execute a Child Process with Piped Standard I/O in C ?

Passing Standard I/O Parameters to Child Processes via Pipes

The objective is to create a C function that executes a child process ("foo") with the supplied string as standard input ("s") and returns the child's standard output in a string variable.

System Calls and POSIX Functions

The following system calls and POSIX functions are required for this task:

• pipe(): Creates a pipe for communication between parent and child processes.
• fork(): Creates a new child process.
• dup2(): Duplicates a file descriptor to another descriptor.
• execve(): Executes a new program image in the current process.
• write(): Writes data to a file descriptor.
• read(): Reads data from a file descriptor.

Function Implementation

The function below follows these steps to execute a child process with piped standard I/O:

1. Create two pipes, one for standard input and one for standard output.
2. Fork a new child process.

3. In the child process:

   • Redirect its standard input to the read end of the input pipe.
   • Redirect its standard output and standard error to the write end of the output pipe.
   • Close all unused file descriptors inherited from the parent.
   • Execute the child program ("foo") using execve().

4. In the parent process:

   • Close the unused ends of the pipes.
   • Write the input string to the write end of the input pipe.
   • Read and store the child's standard output from the read end of the output pipe.
   • Wait for the child process to finish using fork().

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