man popen

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(3) 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.