_spawn, _wspawn Functions

Artigo
06/05/2017

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The latest version of this topic can be found at _spawn, _wspawn Functions.

Each of the _spawn functions creates and executes a new process:

_spawnl, _wspawnl	_spawnv, _wspawnv
_spawnle, _wspawnle	_spawnve, _wspawnve
_spawnlp, _wspawnlp	_spawnvp, _wspawnvp
_spawnlpe, _wspawnlpe	_spawnvpe, _wspawnvpe

The letters at the end of the function name determine the variation.

e
envp, array of pointers to environment settings, is passed to new process.

l
Command-line arguments are passed individually to _spawn function. This suffix is typically used when a number of parameters to a new process is known in advance.

p
PATH environment variable is used to find the file to execute.

v
argv, array of pointers to command-line arguments, is passed to _spawn function. This suffix is typically used when a number of parameters to a new process is variable.

Remarks

The _spawn functions each create and execute a new process. They automatically handle multibyte-character string arguments as appropriate, recognizing multibyte-character sequences according to the multibyte code page currently in use. The _wspawn functions are wide-character versions of the _spawn functions; they do not handle multibyte-character strings. Otherwise, the _wspawn functions behave identically to their _spawn counterparts.

Generic-Text Routine Mappings

Tchar.h routine	_UNICODE and _MBCS not defined	_MBCS defined	_UNICODE defined
`_tspawnl`	`_spawnl`	`_spawnl`	`_wspawnl`
`_tspawnle`	`_spawnle`	`_spawnle`	`_wspawnle`
`_tspawnlp`	`_spawnlp`	`_spawnlp`	`_wspawnlp`
`_tspawnlpe`	`_spawnlpe`	`_spawnlpe`	`_wspawnlpe`
`_tspawnv`	`_spawnv`	`_spawnv`	`_wspawnv`
`_tspawnve`	`_spawnve`	`_spawnve`	`_wspawnve`
`_tspawnvp`	`_spawnvp`	`_spawnvp`	`_wspawnvp`
`_tspawnvpe`	`_spawnvpe`	`_spawnvpe`	`_wspawnvpe`

Enough memory must be available for loading and executing the new process. The mode argument determines the action taken by the calling process before and during _spawn. The following values for mode are defined in Process.h:

_P_OVERLAY
Overlays a calling process with a new process, destroying the calling process (same effect as _exec calls).

_P_WAIT
Suspends a calling thread until execution of the new process is complete (synchronous _spawn).

_P_NOWAIT or _P_NOWAITO
Continues to execute a calling process concurrently with the new process (asynchronous _spawn).

_P_DETACH
Continues to execute the calling process; the new process is run in the background with no access to the console or keyboard. Calls to _cwait against the new process fail (asynchronous _spawn).

The cmdname argument specifies the file that is executed as the new process and can specify a full path (from the root), a partial path (from the current working directory), or just a file name. If cmdname does not have a file name extension or does not end with a period (.), the _spawn function first tries the .com file name extension and then the .exe file name extension, the .bat file name extension, and finally the .cmd file name extension.

If cmdname has a file name extension, only that extension is used. If cmdname ends with a period, the _spawn call searches for cmdname with no file name extension. The _spawnlp, _spawnlpe, _spawnvp, and _spawnvpe functions search for cmdname (using the same procedures) in the directories specified by the PATH environment variable.

If cmdname contains a drive specifier or any slashes (that is, if it is a relative path), the _spawn call searches only for the specified file; no path searching is done.

In the past, some of these functions set errno to zero on success; the current behavior is to leave errno untouched on success, as specified by the C standard. If you need to emulate the old behavior, set errno to zero just before calling these functions.

Note

To ensure proper overlay initialization and termination, do not use the setjmp or longjmp function to enter or leave an overlay routine.

Arguments for the Spawned Process

To pass arguments to the new process, give one or more pointers to character strings as arguments in the _spawn call. These character strings form the argument list for the spawned process. The combined length of the strings forming the argument list for the new process must not exceed 1024 bytes. The terminating null character ('\0') for each string is not included in the count, but space characters (automatically inserted to separate arguments) are included.

Note

Spaces embedded in strings may cause unexpected behavior; for example, passing _spawn the string "hi there" will result in the new process getting two arguments, "hi" and "there". If the intent was to have the new process open a file named "hi there", the process would fail. You can avoid this by quoting the string: "\"hi there\"".

Important

Do not pass user input to _spawn without explicitly checking its content. _spawn will result in a call to CreateProcess so keep in mind that unqualified path names could lead to potential security vulnerabilities.

You can pass argument pointers as separate arguments (in _spawnl, _spawnle, _spawnlp, and _spawnlpe) or as an array of pointers (in _spawnv, _spawnve, _spawnvp, and _spawnvpe). You must pass at least one argument, arg0 or argv[0], to the spawned process. By convention, this argument is the name of the program as you would type it on the command line. A different value does not produce an error.

The _spawnl, _spawnle, _spawnlp, and _spawnlpe calls are typically used in cases where the number of arguments is known in advance. The arg0 argument is usually a pointer to cmdname. The arguments arg1 through argn are pointers to the character strings forming the new argument list. Following argn, there must be a NULL pointer to mark the end of the argument list.

The _spawnv, _spawnve, _spawnvp, and _spawnvpe calls are useful when there is a variable number of arguments to the new process. Pointers to the arguments are passed as an array, argv. The argument argv[0] is usually a pointer to a path in real mode or to the program name in protected mode, and argv[1] through argv[n] are pointers to the character strings forming the new argument list. The argument argv[n +1] must be a NULL pointer to mark the end of the argument list.

Environment of the Spawned Process

Files that are open when a _spawn call is made remain open in the new process. In the _spawnl, _spawnlp, _spawnv, and _spawnvp calls, the new process inherits the environment of the calling process. You can use the _spawnle, _spawnlpe, _spawnve, and _spawnvpe calls to alter the environment for the new process by passing a list of environment settings through the envp argument. The argument envp is an array of character pointers, each element (except the final element) of which points to a null-terminated string defining an environment variable. Such a string usually has the form NAME=value where NAME is the name of an environment variable and value is the string value to which that variable is set. (Note that value is not enclosed in double quotation marks.) The final element of the envp array should be NULL. When envp itself is NULL, the spawned process inherits the environment settings of the parent process.

The _spawn functions can pass all information about open files, including the translation mode, to the new process. This information is passed in real mode through the C_FILE_INFO entry in the environment. The startup code normally processes this entry and then deletes it from the environment. However, if a _spawn function spawns a non-C process, this entry remains in the environment. Printing the environment shows graphics characters in the definition string for this entry because the environment information is passed in binary form in real mode. It should not have any other effect on normal operations. In protected mode, the environment information is passed in text form and therefore contains no graphics characters.

You must explicitly flush (using fflush or _flushall) or close any stream before calling a _spawn function.

New processes created by calls to _spawn routines do not preserve signal settings. Instead, the spawned process resets signal settings to the default.

Redirecting Output

If you are calling _spawn from a DLL or a GUI application and want to redirect the output to a pipe, you have two options:

Use the Win32 API to create a pipe, then call AllocConsole, set the handle values in the startup structure, and call CreateProcess.
Call _popen, _wpopen which will create a pipe and invoke the app using cmd.exe /c (or command.exe /c).

Example

// crt_spawn.c  
// This program accepts a number in the range  
// 1-8 from the command line. Based on the number it receives,  
// it executes one of the eight different procedures that  
// spawn the process named child. For some of these procedures,  
// the CHILD.EXE file must be in the same directory; for  
// others, it only has to be in the same path.  
//  
  
#include <stdio.h>  
#include <process.h>  
  
char *my_env[] =  
{  
   "THIS=environment will be",  
   "PASSED=to child.exe by the",  
   "_SPAWNLE=and",  
   "_SPAWNLPE=and",  
   "_SPAWNVE=and",  
   "_SPAWNVPE=functions",  
   NULL  
};  
  
int main( int argc, char *argv[] )  
{  
   char *args[4];  
  
   // Set up parameters to be sent:   
   args[0] = "child";  
   args[1] = "spawn??";  
   args[2] = "two";  
   args[3] = NULL;  
  
   if (argc <= 2)  
   {  
      printf( "SYNTAX: SPAWN <1-8> <childprogram>\n" );  
      exit( 1 );  
   }  
  
   switch (argv[1][0])   // Based on first letter of argument   
   {  
   case '1':  
      _spawnl( _P_WAIT, argv[2], argv[2], "_spawnl", "two", NULL );  
      break;  
   case '2':  
      _spawnle( _P_WAIT, argv[2], argv[2], "_spawnle", "two",   
               NULL, my_env );  
      break;  
   case '3':  
      _spawnlp( _P_WAIT, argv[2], argv[2], "_spawnlp", "two", NULL );  
      break;  
   case '4':  
      _spawnlpe( _P_WAIT, argv[2], argv[2], "_spawnlpe", "two",   
                NULL, my_env );  
      break;  
   case '5':  
      _spawnv( _P_OVERLAY, argv[2], args );  
      break;  
   case '6':  
      _spawnve( _P_OVERLAY, argv[2], args, my_env );  
      break;  
   case '7':  
      _spawnvp( _P_OVERLAY, argv[2], args );  
      break;  
   case '8':  
      _spawnvpe( _P_OVERLAY, argv[2], args, my_env );  
      break;  
   default:  
      printf( "SYNTAX: SPAWN <1-8> <childprogram>\n" );  
      exit( 1 );  
   }  
   printf( "from SPAWN!\n" );  
}

child process output  
from SPAWN!

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