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#include <unistd.h>extern char **environ;
The initial argument for these functions is the pathname of a file which is to be executed.
The const char *arg and subsequent ellipses in the execl(), execlp(), and execle() functions can be thought of as arg0, arg1, ..., argn. Together they describe a list of one or more pointers to null-terminated strings that represent the argument list available to the executed program. The first argument, by convention, should point to the file name associated with the file being executed. The list of arguments must be terminated by a NULL pointer.
The exect(), execv(), execvp(), and execvP() functions provide an array of pointers to null-terminated strings that represent the argument list available to the new program. The first argument, by convention, should point to the file name associated with the file being executed. The array of pointers must be terminated by a NULL pointer.
The execle() and exect() functions also specify the environment of the executed process by following the NULL pointer that terminates the list of arguments in the argument list or the pointer to the argv array with an additional argument. This additional argument is an array of pointers to null-terminated strings and must be terminated by a NULL pointer. The other functions take the environment for the new process image from the external variable environ in the current process.
Some of these functions have special semantics.
The functions execlp(), execvp(), and execvP() will duplicate the actions of the shell in searching for an executable file if the specified file name does not contain a slash "/" character. For execlp() and execvp(), search path is the path specified in the environment by " PATH" variable. If this variable is not specified, the default path is set according to the _PATH_DEFPATH definition in <paths.h>, which is set to " /sbin:/bin:/usr/sbin:/usr/bin:/usr/local/sbin:/usr/local/bin". For execvP(), the search path is specified as an argument to the function. In addition, certain errors are treated specially.
If an error is ambiguous (for simplicity, we shall consider all errors except ENOEXEC as being ambiguous here, although only the critical error EACCES is really ambiguous), then these functions will act as if they stat the file to determine whether the file exists and has suitable execute permissions. If it does, they will return immediately with the global variable errno restored to the value set by execve(). Otherwise, the search will be continued. If the search completes without performing a successful execve() or terminating due to an error, these functions will return with the global variable errno set to EACCES or ENOENT according to whether at least one file with suitable execute permissions was found.
If the header of a file is not recognized (the attempted execve() returned ENOEXEC), these functions will execute the shell with the path of the file as its first argument. (If this attempt fails, no further searching is done.)
The function exect() executes a file with the program tracing facilities enabled (see ptrace(2)).
The behavior of execlp() and execvp() when errors occur while attempting to execute the file is not quite historic practice, and has not traditionally been documented and is not specified by the POSIX standard.
Traditionally, the functions execlp() and execvp() ignored all errors except for the ones described above and ETXTBSY, upon which they retried after sleeping for several seconds, and ENOMEM and E2BIG, upon which they returned. They now return for ETXTBSY, and determine existence and executability more carefully. In particular, EACCES for inaccessible directories in the path prefix is no longer confused with EACCES for files with unsuitable execute permissions. In BSD 4.4, they returned upon all errors except EACCES, ENOENT, ENOEXEC and ETXTBSY. This was inferior to the traditional error handling, since it breaks the ignoring of errors for path prefixes and only improves the handling of the unusual ambiguous error EFAULT and the unusual error EIO. The behaviour was changed to match the behaviour of sh(1).
The exect() and execv() functions may fail and set errno for any of the errors specified for the library function execve(2).
Due to a fluke of the C standard, on platforms other than FreeBSD the definition of NULL may be the untyped number zero, rather than a (void *)0 expression. To distinguish the concepts, they are referred to as a "null pointer constant" and a "null pointer", respectively. On exotic computer architectures that FreeBSD does not support, the null pointer constant and null pointer may have a different representation. In general, where this document and others reference a NULL value, they actually imply a null pointer. E.g., for portability to non-FreeBSD operating systems on exotic computer architectures, one may use (char *)NULL in place of NULL when invoking execl(), execle(), and execlp().
|EXEC (3)||March 22, 2020|
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|“||On two occasions I have been asked [by members of Parliament], 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question.||”|
|— Charles Babbage|