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at December 11, 2006 17:55 by CUViper
Initial Code
#include <errno.h> #include <stdlib.h> #include <sys/types.h> #include <unistd.h> #include <fcntl.h> pid_t spawn(const char *file, char *const argv[]) { pid_t pid; int forkpipe[2]; int fd_flags, err, ret; // set up a pipe for communication between forks ret = pipe(forkpipe); if (ret == -1) return -1; pid = fork(); if (pid == -1) { err = errno; // close the write pipe close(forkpipe[1]); goto parent_exit; } if (pid == 0) { // forked child // close the read pipe ret = close(forkpipe[0]); if (ret == -1) goto child_err; // make the write end close-on-exec ret = fcntl(forkpipe[1], F_GETFD); if (ret == -1) goto child_err; fd_flags = ret | FD_CLOEXEC; ret = fcntl(forkpipe[1], F_SETFD, fd_flags); if (ret == -1) goto child_err; // try to run the app execvp(file, argv); // if we continue executing here, an error occurred child_err: // send the error code through the write pipe err = errno; write(forkpipe[1], &err, sizeof(err)); exit(1); } // parent // close the write pipe close(forkpipe[1]); // get the error code from the read pipe ret = read(forkpipe[0], &err, sizeof(err)); if (ret == -1) { err = errno; pid = -1; } else if (ret == sizeof(err)) pid = -1; else err = 0; parent_exit: // close the read pipe close(forkpipe[0]); if (err) errno = err; return pid; }
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With the typical fork+exec method of spawning an application, you are left in the dark about failures in the exec. Once the fork occurs, the "main" process doesn't have any communication channel except waitpid, so it's hard to tell what happened. With this function, a pipe is opened for the forked child to communicate errors. If the exec succeeds, then the pipe is automatically closed, and the main process reports success. Otherwise, the errno from exec is communicated through the pipe.
Initial Title
Safe application spawning with fork+exec
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unix, c, linux
Initial Language
C++