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

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

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

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

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 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

[c] Redirecting exec output to a buffer or file

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