#include <unistd.h> int getopt(int argc, char * const argv[], const char *optstring); extern char *optarg; extern int opterr, optind, optopt;
The parameters argc and argv are the argument count and argument array as passed to main() (see exec()). The argument optstring is a string of recognized option characters; if a character is followed by a <colon>, the option takes an argument. All option characters allowed by Utility Syntax Guideline 3 are allowed in optstring. The implementation may accept other characters as an extension.
The variable optind is the index of the next element of the argv[] vector to be processed. It shall be initialized to 1 by the system, and getopt() shall update it when it finishes with each element of argv[]. If the application sets optind to zero before calling getopt(), the behavior is unspecified. When an element of argv[] contains multiple option characters, it is unspecified how getopt() determines which options have already been processed.
The getopt() function shall return the next option character (if one is found) from argv that matches a character in optstring, if there is one that matches. If the option takes an argument, getopt() shall set the variable optarg to point to the option-argument as follows:
If, when getopt() is called:
argv[optind] is a null pointer *argv[optind] is not the character - argv[optind] points to the string "-"
getopt() shall return -1 without changing optind. If:
argv[optind] points to the string "--"
getopt() shall return -1 after incrementing optind.
If getopt() encounters an option character that is not contained in optstring, it shall return the <question-mark> ('?') character. If it detects a missing option-argument, it shall return the <colon> character (':') if the first character of optstring was a <colon>, or a <question-mark> character ('?') otherwise. In either case, getopt() shall set the variable optopt to the option character that caused the error. If the application has not set the variable opterr to 0 and the first character of optstring is not a <colon>, getopt() shall also print a diagnostic message to stderr in the format specified for the getopts utility, unless the stderr stream has wide orientation, in which case the behavior is undefined.
The getopt() function need not be thread-safe.
A <colon> (':') shall be returned if getopt() detects a missing argument and the first character of optstring was a <colon> (':').
A <question-mark> ('?') shall be returned if getopt() encounters an option character not in optstring or detects a missing argument and the first character of optstring was not a <colon> (':').
Otherwise, getopt() shall return -1 when all command line options are parsed.
The following sections are informative.
The following code fragment shows how you might process the arguments for a utility that can take the mutually-exclusive options a and b and the options f and o, both of which require arguments:
#include <stdio.h> #include <stdlib.h> #include <unistd.h> int main(int argc, char *argv[ ]) { int c; int bflg = 0, aflg = 0, errflg = 0; char *ifile; char *ofile; . . . while ((c = getopt(argc, argv, ":abf:o:")) != -1) { switch(c) { case 'a': if (bflg) errflg++; else aflg++; break; case 'b': if (aflg) errflg++; else bflg++; break; case 'f': ifile = optarg; break; case 'o': ofile = optarg; break; case ':': /* -f or -o without operand */ fprintf(stderr, "Option -%c requires an operand\n", optopt); errflg++; break; case '?': fprintf(stderr, "Unrecognized option: '-%c'\n", optopt); errflg++; } } if (errflg) { fprintf(stderr, "usage: . . . "); exit(2); } for ( ; optind < argc; optind++) { if (access(argv[optind], R_OK)) { . . . }
This code accepts any of the following as equivalent:
cmd -ao arg path path cmd -a -o arg path path cmd -o arg -a path path cmd -a -o arg -- path path cmd -a -oarg path path cmd -aoarg path path
The following example selects the type of database routines the user wants to use based on the Options argument.
#include <unistd.h> #include <string.h> ... const char *Options = "hdbtl"; ... int dbtype, c; char *st; ... dbtype = 0; while ((c = getopt(argc, argv, Options)) != -1) { if ((st = strchr(Options, c)) != NULL) { dbtype = st - Options; break; } }
Applications which use wide-character output functions with stderr should ensure that any calls to getopt() do not write to stderr, either by setting opterr to 0 or by ensuring the first character of optstring is always a <colon>.
While ferror(stderr) may be used to detect failures to write a diagnostic to stderr when getopt() returns '?', the value of errno is unspecified in such a condition. Applications desiring more control over handling write failures should set opterr to 0 and independently perform output to stderr, rather than relying on getopt() to do the output.
The description has been written to make it clear that getopt(), like the getopts utility, deals with option-arguments whether separated from the option by <blank> characters or not. Note that the requirements on getopt() and getopts are more stringent than the Utility Syntax Guidelines.
The getopt() function shall return -1, rather than EOF, so that <stdio.h> is not required.
The special significance of a <colon> as the first character of optstring makes getopt() consistent with the getopts utility. It allows an application to make a distinction between a missing argument and an incorrect option letter without having to examine the option letter. It is true that a missing argument can only be detected in one case, but that is a case that has to be considered.
The Base Definitions volume of POSIX.1-2017, Section 12.2, Utility Syntax Guidelines, <unistd.h>
The Shell and Utilities volume of POSIX.1-2017, getopts
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