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Passes the name of an image to be activated in a child process. This function is nonreentrant.
#include <unistd.h>int execl (const char *file_spec, const char *arg0, ..., (char *)0); (ISO POSIX-1)
int execl (char *file_spec, ...); (COMPATIBILITY)
file_spec
The full file specification of a new image to be activated in the child process.arg0, ...
A sequence of pointers to null-terminated character strings.If the POSIX-1 format is used, at least one argument must be present and must point to a string that is the same as the new process filename (or its last component). (This pointer can also be the NULL pointer, but then execle would accomplish nothing.) The last pointer must be the NULL pointer. This is also the convention if the compatibility format is used.
To understand how the exec functions operate, consider how the OpenVMS system calls any HP C program, as shown in the following syntax:
int main (int argc, char *argv[], char *envp[]);
The identifier argc is the argument count; argv is an array of argument strings. The first member of the array (argv[0]) contains the name of the image. The arguments are placed in subsequent elements of the array. The last element of the array is always the NULL pointer.
An exec function calls a child process in the same way that the run-time system calls any other HP C program. The exec functions pass the name of the image to be activated in the child; this value is placed in argv[0]. However, the functions differ in the way they pass arguments and environment information to the child:
- Arguments can be passed in separate character strings ( execl , execle , and execlp ) or in an array of character strings ( execv , execve , and execvp ).
- The environment can be explicitly passed in an array ( execle and execve ) or taken from the parent's environment ( execl , execv , execlp , and execvp ).
If vfork was called before invoking an exec function, then when the exec function completes, control is returned to the parent process at the point of the vfork call. If vfork was not called, the exec function waits until the child has completed execution and then exits the parent process. See vfork and Chapter 5 for more information.
- 1 Indicates failure.
Passes the name of an image to be activated in a child process. This function is nonreentrant.
#include <unistd.h>int execle (char *file_spec, char *arg0, ..., (char *)0, char *envp[]); (ISO POSIX-1)
int execle (char *file_spec, ...); (COMPATIBILITY)
file_spec
The full file specification of a new image to be activated in the child process.arg0, ...
A sequence of pointers to null-terminated character strings.If the POSIX-1 format is used, at least one argument must be present and must point to a string that is the same as the new process filename (or its last component). (This pointer can also be the NULL pointer, but then execle would accomplish nothing.) The last pointer must be the NULL pointer. This is also the convention if the compatibility format is used.
envp
An array of strings that specifies the program's environment. Each string in envp has the following form:
name = value
The name can be one of the following names and the value is a null-terminated string to be associated with the name:
- HOME---Your login directory
- TERM---The type of terminal being used
- PATH---The default device and directory
- USER---The name of the user who initiated the process
The last element in envp must be the NULL pointer.
When the operating system executes the program, it places a copy of the current environment vector (envp) in the external variable environ.
See execl for a description of how the exec functions operate.
- 1 Indicates failure.
Passes the name of an image to be activated in a child process. This function is nonreentrant.
#include <unistd.h>int execlp (const char *file_name, const char *arg0, ..., (char *)0); (ISO POSIX-1)
int execlp (char *file_name, ...); (COMPATIBILITY)
file_name
The filename of a new image to be activated in the child process. The device and directory specification for the file is obtained by searching the VAXC$PATH environment name.argn
A sequence of pointers to null-terminated character strings. By convention, at least one argument must be present and must point to a string that is the same as the new process filename (or its last component)....
A sequence of pointers to strings. At least one pointer must exist to terminate the list. This pointer must be the NULL pointer.
See execl for a description of how the exec functions operate.
- 1 Indicates failure.
Passes the name of an image to be activated in a child process. This function is nonreentrant.
#include <unistd.h>int execv (char *file_spec, char *argv[]);
file_spec
The full file specification of a new image to be activated in the child process.argv
An array of pointers to null-terminated character strings. These strings constitute the argument list available to the new process. By convention, argv[0] must point to a string that is the same as the new process filename (or its last component). argv is terminated by a NULL pointer.
See execl for a description of how the exec functions operate.
- 1 Indicates failure.
Passes the name of an image to be activated in a child process. This function is nonreentrant.
#include <unistd.h>int execve (const char *file_spec, char *argv[], char *envp[]);
file_spec
The full file specification of a new image to be activated in the child process.argv
An array of pointers to null-terminated character strings. These strings constitute the argument list available to the new process. By convention, argv[0] must point to a string that is the same as the new process filename (or its last component). argv is terminated by a NULL pointer.envp
An array of strings that specifies the program's environment. Each string in envp has the following form:
name = value
The name can be one of the following names and the value is a null-terminated string to be associated with the name:
- HOME---Your login directory
- TERM---The type of terminal being used
- PATH---The default device and directory
- USER---The name of the user who initiated the process
The last element in envp must be the NULL pointer.
When the operating system executes the program, it places a copy of the current environment vector (envp) in the external variable environ .
See execl for a description of how the exec functions operate.
- 1 Indicates failure.
Passes the name of an image to be activated in a child process. This function is nonreentrant.
#include <unistd.h>int execvp (const char *file_name, char *argv[]);
file_name
The filename of a new image to be activated in the child process. The device and directory specification for the file is obtained by searching the environment name VAXC$PATH.argv
An array of pointers to null-terminated character strings. These strings constitute the argument list available to the new process. By convention, argv[0] must point to a string that is the same as the new process filename (or its last component). argv is terminated by a NULL pointer.
See execl for a description of how the exec functions operate.
- 1 Indicates failure.
Terminate execution of the program from which they are called. These functions are nonreentrant.
#include <stdlib.h>void exit (int status);
#include <unistd.h>
void _exit (int status);
status
For non-POSIX behavior, a status value of EXIT_SUCCESS (1), EXIT_FAILURE (2), or a number from 3 to 255, as follows:
- A status value of 0, 1 or EXIT_SUCCESS is translated to the OpenVMS SS$_NORMAL status code to return the OpenVMS success value.
- A status value of 2 or EXIT_FAILURE is translated to an error-level exit status. The status value is passed to the parent process.
- Any other status value is left the same.
For POSIX behavior:
- A status value of 0 is translated to the OpenVMS SS$_NORMAL status code to return the OpenVMS success value.
- Any other status is returned to the parent process as an OpenVMS message symbol with facility set to C, severity set to success, and with the status in the message number field. For more information on the format of message symbols, see "message code" in the HP OpenVMS Command Definition, Librarian, and Message Utilities Manual.
To get POSIX behavior, include <unistd.h> and compile with the _POSIX_EXIT feature-test macro set (either with /DEFINE=_POSIX_EXIT, or with #define _POSIX_EXIT at the top of your file, before any file inclusions). This behavior is available only on OpenVMS Version 7.0 and higher systems.
If the process was invoked by DCL, the status is interpreted by DCL, and a message is displayed.If the process was a child process created using vfork or an exec function, then the child process exits and control returns to the parent. The two functions are identical; the _exit function is retained for reasons of compatibility with VAX C.
The exit and _exit functions make use of the $EXIT system service. If your process is being invoked by the RUN command using any of the hibernation and scheduled wakeup qualifiers, the process might not correctly return to hibernation state when an exit or _exit call is made.
The C compiler command-line qualifier /[NO]MAIN=POSIX_EXIT can be used to direct the compiler to call __posix_exit instead of exit when returning from main . The default is /NOMAIN.
Beginning with OpenVMS Version 8.3, C RTL contains a fix for the problem in which a call to _exit after a failed execl really exits but must not.
In the OpenVMS implementation of vfork , a child process is not actually started as it is started on most UNIX systems. However, the C RTL creates some internal data structures intended to mimic child-process functionality (called the "child context").
A bug occurred whereby after a vfork while in the child context, a call to an exec function justifiably fails, then calls _exit . On UNIX systems, after the failed exec call, the child process continues to execute. A subsequent call to _exit terminates the child. In the OpenVMS implementation, after the failed exec call, the child context terminates. A subsequent call to _exit terminates the parent. The C RTL fix is enabled by a feature logical switch, DECC$EXIT_AFTER_ FAILED_EXEC. Enabling this feature logical allows the child context to continue execution.
With DECC$EXIT_AFTER_FAILED_EXEC disabled or not defined, the current behavior remains the default.
Note
EXIT_SUCCESS and EXIT_FAILURE are portable across any ANSI C compiler to indicate success or failure. On OpenVMS systems, they are mapped to OpenVMS condition codes with the severity set to success or failure, respectively. Values in the range of 3 to 255 can be used by a child process to communicate a small amount of data to the parent. The parent retrieves this data using the wait , wait3 , wait4 , or waitpid functions.
Returns the base e raised to the power of the argument.
#include <math.h>double exp (double x);
float expf (float x); (INTEGRITY SERVERS, ALPHA)
long double expl (long double x); (INTEGRITY SERVERS, ALPHA)
double expm1 (double x); (INTEGRITY SERVERS, ALPHA)
float expm1f (float x); (INTEGRITY SERVERS, ALPHA)
long double expm1l (long double x); (INTEGRITY SERVERS, ALPHA)
x
A real value.
The exp functions compute the value of the exponential function, defined as e**x, where e is the constant used as a base for natural logarithms.The expm1 functions compute exp(x) - 1 accurately, even for tiny x.
If an overflow occurs, the exp functions return the largest possible floating-point value and set errno to ERANGE. The constant HUGE_VAL is defined in the <math.h> header file to be the largest possible floating-point value.
x The exponential value of the argument. HUGE_VAL Overflow occurred; errno is set to ERANGE. 0 Underflow occurred; errno is set to ERANGE. NaN x is NaN; errno is set to EDOM.
Returns the value of 2 raised to the power of the argument.
#include <math.h>double exp2 (double x);
float exp2f (float x);
long double exp2l (long double x);
x
A real value.
The exp2 functions compute the base-2 exponential of x.If an overflow occurs, the exp functions return the largest possible floating-point value and set errno to ERANGE. The constant HUGE_VAL is defined in the <math.h> header file to be the largest possible floating-point value.
n 2** x. HUGE_VAL Overflow occurred; errno is set to ERANGE. 1 x is +0 or - 0; errno is set to ERANGE. 0 x is - Inf or underflow occurred; errno is set to ERANGE. x x is +Inf; errno is set to ERANGE. NaN x is NaN; errno is set to EDOM.
Returns the absolute value of its argument.
#include <math.h>double fabs (double x);
float fabsf (float x); (INTEGRITY SERVERS, ALPHA)
long double fabsl (long double x); (INTEGRITY SERVERS, ALPHA)
x
A real value.
x The absolute value of the argument.
Changes file access permissions.
#include <stat.h>int fchmod (int fildes, mode_t mode);
fildes
An open file descriptor.mode
The bit pattern that determines the access permissions.
The fchmod function is equivalent to the chmod function, except that the file whose permissions are changed is specified by a file descriptor (fildes) rather than a filename.
0 Indicates that the mode is successfully changed. - 1 Indicates that the change attempt has failed.
Changes the owner and group of a file.
#include <unistd.h>int fchown (int fildes, uid_t owner, gid_t group);
fildes
An open file descriptor.owner
A user ID corresponding to the new owner of the file.group
A group ID corresponding to the group of the file.
The fchown function has the same effect as chown except that the file whose owner and group are to be changed is specified by the file descriptor fildes.
0 Indicates success. - 1 Indicates failure. The function sets errno to one of the following values: The fchown function will fail if:
- EBADF -- The fildes argument is not an open file descriptor.
- EPERM -- The effective user ID does not match the owner of the file, or the process does not have appropriate privilege.
- EROFS -- The file referred to by fildes resides on a read-only file system.
The fchown function may fail if:
- EINVAL -- The owner or group ID is not a value supported by the implementation.
- EIO -- A physical I/O error has occurred.
- EINTR -- The fchown function was interrupted by a signal that was intercepted.
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