Redirecting exec output to a buffer or file

Question

I m writing a C program where I fork    exec    and wait     I d like to take the output of the program I exec ed to write it to file or buffer   For example  if I exec ls  I want to write file1 file2 etc to buffer file   I don t think there is a way to read stdout  so does that mean I have to use a pipe   Is there a general procedure here that I haven t been able to find

User · Answer

You need to decide exactly what you want to do - and preferably explain it a bit more clearly.

Option 1: File

If you know which file you want the output of the executed command to go to, then:

Ensure that the parent and child agree on the name (parent decides name before forking).
Parent forks - you have two processes.
Child reorganizes things so that file descriptor 1 (standard output) goes to the file.
Usually, you can leave standard error alone; you might redirect standard input from /dev/null.
Child then execs relevant command; said command runs and any standard output goes to the file (this is the basic shell I/O redirection).
Executed process then terminates.
Meanwhile, the parent process can adopt one of two main strategies:
- Open the file for reading, and keep reading until it reaches an EOF. It then needs to double check whether the child died (so there won't be any more data to read), or hang around waiting for more input from the child.
- Wait for the child to die and then open the file for reading.
- The advantage of the first is that the parent can do some of its work while the child is also running; the advantage of the second is that you don't have to diddle with the I/O system (repeatedly reading past EOF).

Option 2: Pipe

If you want the parent to read the output from the child, arrange for the child to pipe its output back to the parent.

Use popen() to do this the easy way. It will run the process and send the output to your parent process. Note that the parent must be active while the child is generating the output since pipes have a small buffer size (often 4-5 KB) and if the child generates more data than that while the parent is not reading, the child will block until the parent reads. If the parent is waiting for the child to die, you have a deadlock.
Use pipe() etc to do this the hard way. Parent calls pipe(), then forks. The child sorts out the plumbing so that the write end of the pipe is its standard output, and ensures that all other file descriptors relating to the pipe are closed. This might well use the dup2() system call. It then executes the required process, which sends its standard output down the pipe.
Meanwhile, the parent also closes the unwanted ends of the pipe, and then starts reading. When it gets EOF on the pipe, it knows the child has finished and closed the pipe; it can close its end of the pipe too.

User · Answer

After forking  use dup2 2  to duplicate the file s FD into stdout s FD  then exec

User · Answer

For sending the output to another file  I m leaving out error checking to focus on the important details    if  fork      0           child     int fd   open file  O RDWR   O CREAT  S IRUSR   S IWUSR        dup2 fd  1        make stdout go to file     dup2 fd  2        make stderr go to file - you may choose to not do this                       or perhaps send stderr to another file      close fd          fd no longer needed - the dup ed handles are sufficient      exec           For sending the output to a pipe so you can then read the output into a buffer   int pipefd 2   pipe pipefd    if  fork      0        close pipefd 0          close reading end in the child      dup2 pipefd 1   1       send stdout to the pipe     dup2 pipefd 1   2       send stderr to the pipe      close pipefd 1          this descriptor is no longer needed      exec         else          parent      char buffer 1024        close pipefd 1        close the write end of the pipe in the parent      while  read pipefd 0   buffer  sizeof buffer      0

User · Answer

You could also use the linux sh command and pass it a command that includes the redirection   string cmd     bin ls  gt      filepath   execl   bin sh    sh    -c   cmd c str    0

User · Answer

Since you look like you re going to be using this in a linux cygwin environment  you want to use popen  It s like opening a file  only you ll get the executing programs stdout  so you can use your normal fscanf  fread etc

[c] Redirecting exec output to a buffer or file

Option 1: File

Option 2: Pipe

Examples related to c

Examples related to exec

Examples related to fork