spawn()

Create and execute a new child process

Synopsis:

#include <spawn.h>

pid_t spawn( const char * path,
             int fd_count, 
             const int fd_map[ ], 
             const struct inheritance * inherit, 
             char * const argv[ ], 
             char * const envp[ ] );

Arguments:

path

The full path name of the executable.

fd_count

The number of entries in the fd_map array.

fd_map

An array of file descriptors that you want the child process to inherit. If fd_count isn't 0, fd_map must contain at least fd_count file descriptors, up to OPEN_MAX FDs. The first three entries (if specified) become the child process's standard input, standard output, and standard error (stdin, stdout, and stderr). If fd_count is 0, fd_map is ignored.

If you set fdmap[X] to SPAWN_FDCLOSED instead of to a valid file descriptor, the file descriptor X is closed in the child process.

If fd_count is 0, the child process inherits all file descriptors (except for the ones modified with fcntl()'s FD_CLOEXEC flag).

inherit

A structure, of type struct inheritance, that indicates what you want the child process to inherit from the parent. For more information, see “inheritance structure,” below.

argv

A pointer to an argument vector. The value in argv[0] should represent the filename of the program being loaded, but can be NULL if no arguments are being passed. The last member of argv must be a NULL pointer. The value of argv can't be NULL.

envp

A pointer to an array of character pointers, each pointing to a string defining an environment variable. The array is terminated with a NULL pointer. Each pointer points to a character string of the form:

variable=value
  

that's used to define an environment variable. If the value of envp is NULL, then the child process inherits the environment of the parent process.

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

The spawn() function creates and executes a new child process, named in path.

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If the new child process is a shell script, the first line must start with #!, followed by the path of the program to run to interpret the script, optionally followed by one argument. The script must also be marked as executable. For more information, see “The first line” in the Writing Shell Scripts chapter of the Neutrino User's Guide.

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The spawn() function is a QNX Neutrino function (based on the POSIX 1003.1d draft standard). The C library also includes several specialized spawn*() functions. Their names consist of spawn followed by several letters:

As shown below, these functions eventually call spawn(), which in turn sends a message to the process manager.

To view the documentation for a function, click its name in this diagram:

Most of the spawn*() functions do a lot of work before a message is sent to procnto.

The child process inherits the following attributes of the parent process:

• process group ID (unless SPAWN_SETGROUP is set in inherit.flags)
• session membership
• real user ID and real group ID
• effective user ID and effective group ID
• supplementary group IDs
• priority and scheduling policy
• current working directory and root directory
• file-creation mask
• signal mask (unless SPAWN_SETSIGMASK is set in inherit.flags)
• signal actions specified as SIG_DFL
• signal actions specified as SIG_IGN (except the ones modified by inherit.sigdefault when SPAWN_SETSIGDEF is set in inherit.flags).

The child process has several differences from the parent process:

• Signals set to be caught by the parent process are set to the default action (SIG_DFL).
• The child process's tms_utime, tms_stime, tms_cutime, and tms_cstime are tracked separately from the parent's.
• File locks set by the parent aren't inherited.
• Per-process timers created by the parent aren't inherited.
• Memory locks and mappings set by the parent aren't inherited.

The child process also has these differences from the parent process if you haven't set the SPAWN_EXEC flag:

• The number of seconds left until a SIGALRM signal would be generated is set to zero for the child process.
• The set of pending signals for the child process is empty.

If the child process is spawned on a remote node, the process group ID and the session membership aren't set; the child process is put into a new session and a new process group.

The child process can access its environment by using the environ global variable (found in <unistd.h>).

If the path is on a filesystem mounted with the ST_NOSUID flag set, the effective user ID, effective group ID, saved set-user ID and saved set-group ID are unchanged for the child process. Otherwise, if the set-user ID mode bit is set, the effective user ID of the child process is set to the owner ID of path. Similarly, if the set-group ID mode bit is set, the effective group ID of the child process is set to the group ID of path.

The real user ID, real group ID and supplementary group IDs of the child process remain the same as those of the parent process. The effective user ID and effective group ID of the child process are saved as the saved set-user ID and the saved set-group ID used by the setuid().

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A parent/child relationship doesn't imply that the child process dies when the parent process dies.

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inheritance structure

The inheritance structure contains at least these members:

unsigned long flags

Zero or more of the following bits:

• SPAWN_ALIGN_DEFAULT — use the system's default settings for alignment.
• SPAWN_ALIGN_FAULT — try to always fault data misalignment references.
• SPAWN_ALIGN_NOFAULT — don't fault on misalignment; attempt to fix it (this may be slow).
• SPAWN_CHECK_SCRIPT — start a shell, passing path as a script.
• SPAWN_DEBUG — debug process (this is used only for debugging the kernel itself).
• SPAWN_EXEC — cause the spawn to act like exec*(): replace the calling program in memory with the newly loaded program. If successful, no return is made to the calling program.
• SPAWN_EXPLICIT_CPU — set the runmask and inherit mask equal to the runmask member of the inheritance structure. If this flag isn't set, the child inherits the inherit mask of the calling thread.

  ---

  This flag was added in the QNX Neutrino Core OS 6.3.2.

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• SPAWN_EXPLICIT_SCHED — set the scheduling policy to the value of the policy member, and the scheduling parameters to the value of the param member.
• SPAWN_HOLD — hold a process for debugging (i.e. send the SIGSTOP signal to the process before it executes its first instruction).
• SPAWN_NOZOMBIE — prevent the child process from becoming a zombie on its death. No child return or exit information will be available.
• SPAWN_PADDR64_SAFE — permit the selective to indicate that the process is known to operate safely with 64-bit addressing or doesn't care about the physical memory location.
• SPAWN_SEARCH_PATH — search the PATH environment variable for the executable.
• SPAWN_SETGROUP — set the child's process group to the value in the pgroup member. If this flag isn't set, the child process is part of the current process group.
• SPAWN_SETND — spawn the child process on the node specified by the nd member.
• SPAWN_SETSID — make the new process a session leader.
• SPAWN_SETSIGDEF — use the sigdefault member to specify the child process's set of defaulted signals. If you don't specify this flag, the child process inherits the parent process's signal actions.
• SPAWN_SETSIGIGN — set the handling for signals defined in the sigignore member to SIG_IGN.
• SPAWN_SETSIGMASK — use the sigmask member to specify the child process's signal mask.
• SPAWN_SETSTACKMAX — set the maximum stack size to the value of the stack_max member.
• SPAWN_TCSETPGROUP — start a new terminal group.

The <spawn.h> file also defines SPAWN_ALIGN_MASK. It's a mask for the alignment flags listed above.

pid_t pgroup

The child process's group if you specify SPAWN_SETGROUP in the flags member.

If SPAWN_SETGROUP is set in inherit.flags and inherit.pgroup is set to SPAWN_NEWPGROUP, the child process starts a new process group with the process group ID set to its process ID.

int runmask

If you specify SPAWN_EXPLICIT_CPU in the flags member, the child process's runmask and inherit mask are set to the value of this member. For more information, see the Multicore Processing User's Guide.

---

This member was added in the QNX Neutrino Core OS 6.3.2.

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sigset_t sigmask

The child process's signal mask if you specify SPAWN_SETSIGMASK in the flags member.

sigset_t sigdefault

The child process's set of defaulted signals if you specify SPAWN_SETSIGDEF in the flags member.

sigset_t sigignore

The child process's set of ignored signals if you specify SPAWN_SETSIGIGN in the flags member.

unsigned long stack_max

The maximum stack size for the child process, if you set SPAWN_SETSTACKMAX in the flags member.

int policy

The scheduling policy for the child process, if you set SPAWN_EXPLICIT_SCHED in the flags member. The policy must be one of the following:

• SCHED_FIFO — a fixed-priority scheduler in which the highest priority ready thread runs until it blocks or is preempted by a higher priority thread.
• SCHED_RR — similar to SCHED_FIFO, except that threads at the same priority level timeslice (round robin) every 4 × the clock period (see ClockPeriod()).
• SCHED_OTHER — currently the same as SCHED_RR.
• SCHED_SPORADIC — sporadic scheduling.

uint32_t nd

The node descriptor of the remote node on which to spawn the child process. This member is used only if you set SPAWN_SETND in the flags member.

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If you want to spawn() remotely, set the nd member to the node descriptor. See the netmgr_strtond() function.

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struct sched_param param

Scheduling parameters for the child process, if you set SPAWN_EXPLICIT_SCHED in the flags member. For more information, see the documentation for sched_param.

Returns:

The process ID of the child process, or -1 if an error occurs (errno is set).

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If you set SPAWN_EXEC in the flags member of the inheritence structure, spawn() doesn't return, unless an error occurred.

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Errors:

E2BIG

The number of bytes used by the argument list and environment list of the new child process is greater than ARG_MAX bytes.

EACCESS

Search permission is denied for a directory listed in the path prefix of the new child process or the child process's file doesn't have the execute bit set or path's filesystem was mounted with the ST_NOEXEC flag.

EAGAIN

Insufficient resources available to create the child process.

EBADF

An entry in fd_map refers to an invalid file descriptor, or an error occurred duplicating open file descriptors to the new process.

EFAULT

One of the buffers specified in the function call is invalid.

ELOOP

Too many levels of symbolic links or prefixes.

EMFILE

Insufficient resources available to load the new executable image or to remap file descriptors in the child process.

ENAMETOOLONG

The length of path exceeds PATH_MAX or a pathname component is longer than NAME_MAX.

ENOENT

The file identified by the path argument is empty, or one or more components of the pathname of the child process don't exist.

ENOEXEC

The child process's file has the correct permissions, but isn't in the correct format for an executable. (This error doesn't occur if SPAWN_CHECK_SCRIPT is set in the flags member of the inheritance structure.)

ENOMEM

Insufficient memory available to create the child process.

ENOSYS

The spawn() function isn't implemented for the filesystem specified in path.

ENOTDIR

A component of the path prefix of the child process isn't a directory.

Classification:

QNX Neutrino

See also:

execl(), execle(), execlp(), execlpe(), execv(), execve(), execvp(), execvpe(), getenv(), netmgr_strtond(), posix_spawn(), posix_spawnp(), putenv(), sched_param, setenv(), sigaddset(), sigdelset(), sigemptyset(), sigfillset(), spawnl(), spawnle(), spawnlp(), spawnlpe(), spawnp(), spawnv(), spawnve(), spawnvp(), spawnvpe(), wait(), waitpid()

Processes and Threads chapter of Getting Started with QNX Neutrino