#include <wordexp.h>
int wordexp(const char *restrict words, wordexp_t *restrict pwordexp,
int flags);
void wordfree(wordexp_t *pwordexp);
The words argument is a pointer to a string containing one or more words to be expanded. The expansions shall be the same as would be performed by the command line interpreter if words were the part of a command line representing the arguments to a utility. Therefore, the application shall ensure that words does not contain an unquoted <newline> character or any of the unquoted shell special characters '|', '&', ';', '<', '>' except in the context of command substitution as specified in the Shell and Utilities volume of POSIX.1-2017, Section 2.6.3, Command Substitution. It also shall not contain unquoted parentheses or braces, except in the context of command or variable substitution. The application shall ensure that every member of words which it expects to have expanded by wordexp() does not contain an unquoted initial comment character. The application shall also ensure that any words which it intends to be ignored (because they begin or continue a comment) are deleted from words. If the argument words contains an unquoted comment character (<number-sign>) that is the beginning of a token, wordexp() shall either treat the comment character as a regular character, or interpret it as a comment indicator and ignore the remainder of words.
The structure type wordexp_t is defined in the <wordexp.h> header and includes at least the following members:
| |||||||||||||||
The wordexp() function shall store the number of generated words into pwordexp->we_wordc and a pointer to a list of pointers to words in pwordexp->we_wordv. Each individual field created during field splitting (see the Shell and Utilities volume of POSIX.1-2017, Section 2.6.5, Field Splitting) or pathname expansion (see the Shell and Utilities volume of POSIX.1-2017, Section 2.6.6, Pathname Expansion) shall be a separate word in the pwordexp->we_wordv list. The words shall be in order as described in the Shell and Utilities volume of POSIX.1-2017, Section 2.6, Word Expansions. The first pointer after the last word pointer shall be a null pointer. The expansion of special parameters described in the Shell and Utilities volume of POSIX.1-2017, Section 2.5.2, Special Parameters is unspecified.
It is the caller's responsibility to allocate the storage pointed to by pwordexp. The wordexp() function shall allocate other space as needed, including memory pointed to by pwordexp->we_wordv. The wordfree() function frees any memory associated with pwordexp from a previous call to wordexp().
The flags argument is used to control the behavior of wordexp(). The value of flags is the bitwise-inclusive OR of zero or more of the following constants, which are defined in <wordexp.h>:
The WRDE_APPEND flag can be used to append a new set of words to those generated by a previous call to wordexp(). The following rules apply to applications when two or more calls to wordexp() are made with the same value of pwordexp and without intervening calls to wordfree():
If the implementation supports the utilities defined in the Shell and Utilities volume of POSIX.1-2017, and words contains an unquoted character---<newline>, '|', '&', ';', '<', '>', '(', ')', '{', '}'---in an inappropriate context, wordexp() shall fail, and the number of expanded words shall be 0.
Unless WRDE_SHOWERR is set in flags, wordexp() shall redirect stderr to /dev/null for any utilities executed as a result of command substitution while expanding words. If WRDE_SHOWERR is set, wordexp() may write messages to stderr if syntax errors are detected while expanding words, unless the stderr stream has wide orientation in which case the behavior is undefined. It is unspecified whether any write errors encountered while outputting such messages will affect the stderr error indicator or the value of errno.
The application shall ensure that if WRDE_DOOFFS is set, then pwordexp->we_offs has the same value for each wordexp() call and wordfree() call using a given pwordexp.
The results are unspecified if WRDE_APPEND and WRDE_REUSE are
both specified.
The following constants are defined as error return values:
The wordfree() function shall not return a value.
The following sections are informative.
The WRDE_NOCMD flag is provided for applications that, for security or other reasons, want to prevent a user from executing shell commands. Disallowing unquoted shell special characters also prevents unwanted side-effects, such as executing a command or writing a file.
POSIX.1-2008 does not require the wordexp() function to be thread-safe if passed an expression referencing an environment variable while any other thread is concurrently modifying any environment variable; see exec.
Even though the WRDE_SHOWERR flag allows the implementation to write messages to stderr during command substitution or syntax errors, this standard does not provide any way to detect write failures during the output of such messages.
Applications which use wide-character output functions with stderr should ensure that any calls to wordexp() do not write to stderr, by avoiding use of the WRDE_SHOWERR flag.
While wordexp() could be implemented entirely as a library routine, it is expected that most implementations run a shell in a subprocess to do the expansion.
Two different approaches have been proposed for how the required information might be presented to the shell and the results returned. They are presented here as examples.
One proposal is to extend the echo utility by adding a -q option. This option would cause echo to add a <backslash> before each <backslash> and <blank> that occurs within an argument. The wordexp() function could then invoke the shell as follows:
(void) strcpy(buffer, "echo -q");
(void) strcat(buffer, words);
if ((flags & WRDE_SHOWERR) == 0)
(void) strcat(buffer, "2>/dev/null");
f = popen(buffer, "r");
The wordexp() function would read the resulting output, remove unquoted <backslash> characters, and break into words at unquoted <blank> characters. If the WRDE_NOCMD flag was set, wordexp() would have to scan words before starting the subshell to make sure that there would be no command substitution. In any case, it would have to scan words for unquoted special characters.
Another proposal is to add the following options to sh:
If an error is encountered during word expansion, sh exits with a non-zero status after writing the former to report any words successfully expanded
With these options, wordexp() could be implemented fairly simply by creating a subprocess using fork() and executing sh using the line:
execl(<shell path>, "sh", "-P", "-w", words, (char *)0);
after directing standard error to /dev/null.
It seemed objectionable for a library routine to write messages to standard error, unless explicitly requested, so wordexp() is required to redirect standard error to /dev/null to ensure that no messages are generated, even for commands executed for command substitution. The WRDE_SHOWERR flag can be specified to request that error messages be written.
The WRDE_REUSE flag allows the implementation to avoid the expense of freeing and reallocating memory, if that is possible. A minimal implementation can call wordfree() when WRDE_REUSE is set.
The Base Definitions volume of POSIX.1-2017, <wordexp.h>
The Shell and Utilities volume of POSIX.1-2017, Chapter 2, Shell Command Language
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