c99

C99(1P)                     POSIX Programmer's Manual                    C99(1P)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the corresponding
       Linux manual page for details of Linux behavior), or the interface may
       not be implemented on Linux.

NAME
       c99 — compile standard C programs

SYNOPSIS
       c99 [options...] pathname [[pathname] [-I directory]
           [-L directory] [-l library]]...

DESCRIPTION
       The c99 utility is an interface to the standard C compilation system; it
       shall accept source code conforming to the ISO C standard. The system
       conceptually consists of a compiler and link editor. The input files
       referenced by pathname operands and -l option-arguments shall be compiled
       and linked to produce an executable file. (It is unspecified whether the
       linking occurs entirely within the operation of c99; some implementations
       may produce objects that are not fully resolved until the file is
       executed.)

       If the -c option is specified, for all pathname operands of the form
       file.c, the files:


           $(basename pathname .c).o

       shall be created as the result of successful compilation. If the -c
       option is not specified, it is unspecified whether such .o files are
       created or deleted for the file.c operands.

       If there are no options that prevent link editing (such as -c or -E), and
       all input files compile and link without error, the resulting executable
       file shall be written according to the -o outfile option (if present) or
       to the file a.out.

       The executable file shall be created as specified in Section 1.1.1.4,
       File Read, Write, and Creation, except that the file permission bits
       shall be set to: S_IRWXO | S_IRWXG | S_IRWXU

       and the bits specified by the umask of the process shall be cleared.

OPTIONS
       The c99 utility shall conform to the Base Definitions volume of
       POSIX.1‐2017, Section 12.2, Utility Syntax Guidelines, except that:

        *  Options can be interspersed with operands.

        *  The order of specifying the -L and -l options, and the order of
           specifying -l options with respect to pathname operands is
           significant.

        *  Conforming applications shall specify each option separately; that
           is, grouping option letters (for example, -cO) need not be recognized
           by all implementations.

       The following options shall be supported:

       -c        Suppress the link-edit phase of the compilation, and do not
                 remove any object files that are produced.

       -D name[=value]
                 Define name as if by a C-language #define directive. If no
                 =value is given, a value of 1 shall be used. The -D option has
                 lower precedence than the -U option. That is, if name is used
                 in both a -U and a -D option, name shall be undefined
                 regardless of the order of the options. Additional
                 implementation-defined names may be provided by the compiler.
                 Implementations shall support at least 2048 bytes of -D
                 definitions and 256 names.

       -E        Copy C-language source files to standard output, executing all
                 preprocessor directives; no compilation shall be performed. If
                 any operand is not a text file, the effects are unspecified.

       -g        Produce symbolic information in the object or executable files;
                 the nature of this information is unspecified, and may be
                 modified by implementation-defined interactions with other
                 options.

       -I directory
                 Change the algorithm for searching for headers whose names are
                 not absolute pathnames to look in the directory named by the
                 directory pathname before looking in the usual places. Thus,
                 headers whose names are enclosed in double-quotes ("") shall be
                 searched for first in the directory of the file with the
                 #include line, then in directories named in -I options, and
                 last in the usual places. For headers whose names are enclosed
                 in angle brackets ("<>"), the header shall be searched for only
                 in directories named in -I options and then in the usual
                 places. Directories named in -I options shall be searched in
                 the order specified. If the -I option is used to specify a
                 directory that is one of the usual places searched by default,
                 the results are unspecified. Implementations shall support at
                 least ten instances of this option in a single c99 command
                 invocation.

       -L directory
                 Change the algorithm of searching for the libraries named in
                 the -l objects to look in the directory named by the directory
                 pathname before looking in the usual places. Directories named
                 in -L options shall be searched in the order specified. If the
                 -L option is used to specify a directory that is one of the
                 usual places searched by default, the results are unspecified.
                 Implementations shall support at least ten instances of this
                 option in a single c99 command invocation. If a directory
                 specified by a -L option contains files with names starting
                 with any of the strings "libc.", "libl.", "libpthread.",
                 "libm.", "librt.", "libtrace.", "libxnet.", or "liby.", the
                 results are unspecified.

       -l library
                 Search the library named liblibrary.a.  A library shall be
                 searched when its name is encountered, so the placement of a -l
                 option is significant. Several standard libraries can be
                 specified in this manner, as described in the EXTENDED
                 DESCRIPTION section. Implementations may recognize
                 implementation-defined suffixes other than .a as denoting
                 libraries.

       -O optlevel
                 Specify the level of code optimization. If the optlevel option-
                 argument is the digit '0', all special code optimizations shall
                 be disabled. If it is the digit '1', the nature of the
                 optimization is unspecified. If the -O option is omitted, the
                 nature of the system's default optimization is unspecified. It
                 is unspecified whether code generated in the presence of the -O
                 0 option is the same as that generated when -O is omitted.
                 Other optlevel values may be supported.

       -o outfile
                 Use the pathname outfile, instead of the default a.out, for the
                 executable file produced. If the -o option is present with -c
                 or -E, the result is unspecified.

       -s        Produce object or executable files, or both, from which
                 symbolic and other information not required for proper
                 execution using the exec family defined in the System
                 Interfaces volume of POSIX.1‐2017 has been removed (stripped).
                 If both -g and -s options are present, the action taken is
                 unspecified.

       -U name   Remove any initial definition of name.

       Multiple instances of the -D, -I, -L, -l, and -U options can be
       specified.

OPERANDS
       The application shall ensure that at least one pathname operand is
       specified. The following forms for pathname operands shall be supported:

       file.c    A C-language source file to be compiled and optionally linked.
                 The application shall ensure that the operand is of this form
                 if the -c option is used.

       file.a    A library of object files typically produced by the ar utility,
                 and passed directly to the link editor. Implementations may
                 recognize implementation-defined suffixes other than .a as
                 denoting object file libraries.

       file.o    An object file produced by c99 -c and passed directly to the
                 link editor. Implementations may recognize implementation-
                 defined suffixes other than .o as denoting object files.

       The processing of other files is implementation-defined.

STDIN
       Not used.

INPUT FILES
       Each input file shall be one of the following: a text file containing a
       C-language source program, an object file in the format produced by c99
       -c, or a library of object files, in the format produced by archiving
       zero or more object files, using ar.  Implementations may supply
       additional utilities that produce files in these formats. Additional
       input file formats are implementation-defined.

ENVIRONMENT VARIABLES
       The following environment variables shall affect the execution of c99:

       LANG      Provide a default value for the internationalization variables
                 that are unset or null. (See the Base Definitions volume of
                 POSIX.1‐2017, Section 8.2, Internationalization Variables for
                 the precedence of internationalization variables used to
                 determine the values of locale categories.)

       LC_ALL    If set to a non-empty string value, override the values of all
                 the other internationalization variables.

       LC_CTYPE  Determine the locale for the interpretation of sequences of
                 bytes of text data as characters (for example, single-byte as
                 opposed to multi-byte characters in arguments and input files).

       LC_MESSAGES
                 Determine the locale that should be used to affect the format
                 and contents of diagnostic messages written to standard error.

       NLSPATH   Determine the location of message catalogs for the processing
                 of LC_MESSAGES.

       TMPDIR    Provide a pathname that should override the default directory
                 for temporary files, if any.  On XSI-conforming systems,
                 provide a pathname that shall override the default directory
                 for temporary files, if any.

ASYNCHRONOUS EVENTS
       Default.

STDOUT
       If more than one pathname operand ending in .c (or possibly other
       unspecified suffixes) is given, for each such file:


           "%s:\n", <pathname>

       may be written. These messages, if written, shall precede the processing
       of each input file; they shall not be written to the standard output if
       they are written to the standard error, as described in the STDERR
       section.

       If the -E option is specified, the standard output shall be a text file
       that represents the results of the preprocessing stage of the language;
       it may contain extra information appropriate for subsequent compilation
       passes.

STDERR
       The standard error shall be used only for diagnostic messages.  If more
       than one pathname operand ending in .c (or possibly other unspecified
       suffixes) is given, for each such file:


           "%s:\n", <pathname>

       may be written to allow identification of the diagnostic and warning
       messages with the appropriate input file. These messages, if written,
       shall precede the processing of each input file; they shall not be
       written to the standard error if they are written to the standard output,
       as described in the STDOUT section.

       This utility may produce warning messages about certain conditions that
       do not warrant returning an error (non-zero) exit value.

OUTPUT FILES
       Object files or executable files or both are produced in unspecified
       formats. If the pathname of an object file or executable file to be
       created by c99 resolves to an existing directory entry for a file that is
       not a regular file, it is unspecified whether c99 shall attempt to create
       the file or shall issue a diagnostic and exit with a non-zero exit
       status.

EXTENDED DESCRIPTION
   Standard Libraries
       The c99 utility shall recognize the following -l options for standard
       libraries:

       -l c      This option shall make available all interfaces referenced in
                 the System Interfaces volume of POSIX.1‐2017, with the possible
                 exception of those interfaces listed as residing in <aio.h>,
                 <arpa/inet.h>, <complex.h>, <fenv.h>, <math.h>, <mqueue.h>,
                 <netdb.h>, <net/if.h>, <netinet/in.h>, <pthread.h>, <sched.h>,
                 <semaphore.h>, <spawn.h>, <sys/socket.h>, pthread_kill(), and
                 pthread_sigmask() in <signal.h>, <trace.h>, interfaces marked
                 as optional in <sys/mman.h>, interfaces marked as ADV (Advisory
                 Information) in <fcntl.h>, and interfaces beginning with the
                 prefix clock_ or timer_ in <time.h>.  This option shall not be
                 required to be present to cause a search of this library.

       -l l      This option shall make available all interfaces required by the
                 C-language output of lex that are not made available through
                 the -l c option.

       -l pthread
                 This option shall make available all interfaces referenced in
                 <pthread.h> and pthread_kill() and pthread_sigmask() referenced
                 in <signal.h>.  An implementation may search this library in
                 the absence of this option.

       -l m      This option shall make available all interfaces referenced in
                 <math.h>, <complex.h>, and <fenv.h>.  An implementation may
                 search this library in the absence of this option.

       -l rt     This option shall make available all interfaces referenced in
                 <aio.h>, <mqueue.h>, <sched.h>, <semaphore.h>, and <spawn.h>,
                 interfaces marked as optional in <sys/mman.h>, interfaces
                 marked as ADV (Advisory Information) in <fcntl.h>, and
                 interfaces beginning with the prefix clock_ and timer_ in
                 <time.h>.  An implementation may search this library in the
                 absence of this option.

       -l trace  This option shall make available all interfaces referenced in
                 <trace.h>.  An implementation may search this library in the
                 absence of this option.

       -l xnet   This option shall make available all interfaces referenced in
                 <arpa/inet.h>, <netdb.h>, <net/if.h>, <netinet/in.h>, and
                 <sys/socket.h>.  An implementation may search this library in
                 the absence of this option.

       -l y      This option shall make available all interfaces required by the
                 C-language output of yacc that are not made available through
                 the -l c option.

       In the absence of options that inhibit invocation of the link editor,
       such as -c or -E, the c99 utility shall cause the equivalent of a -l c
       option to be passed to the link editor after the last pathname operand or
       -l option, causing it to be searched after all other object files and
       libraries are loaded.

       It is unspecified whether the libraries libc.a, libl.a, libm.a,
       libpthread.a, librt.a, libtrace.a, libxnet.a, or liby.a exist as regular
       files. The implementation may accept as -l option-arguments names of
       objects that do not exist as regular files.

   External Symbols
       The C compiler and link editor shall support the significance of external
       symbols up to a length of at least 31 bytes; the action taken upon
       encountering symbols exceeding the implementation-defined maximum symbol
       length is unspecified.

       The compiler and link editor shall support a minimum of 511 external
       symbols per source or object file, and a minimum of 4095 external symbols
       in total. A diagnostic message shall be written to the standard output if
       the implementation-defined limit is exceeded; other actions are
       unspecified.

   Header Search
       If a file with the same name as one of the standard headers defined in
       the Base Definitions volume of POSIX.1‐2017, Chapter 13, Headers, not
       provided as part of the implementation, is placed in any of the usual
       places that are searched by default for headers, the results are
       unspecified.

   Programming Environments
       All implementations shall support one of the following programming
       environments as a default. Implementations may support more than one of
       the following programming environments. Applications can use sysconf() or
       getconf to determine which programming environments are supported.

                    Table 4-4: Programming Environments: Type Sizes

           ┌────────────────────────┬─────────┬─────────┬─────────┬─────────┐
           │Programming Environment Bits in Bits in Bits in Bits in │
           │     getconf Name       int   long   pointer off_t  │
           ├────────────────────────┼─────────┼─────────┼─────────┼─────────┤
           │_POSIX_V7_ILP32_OFF32   │    32   │    32   │    32   │    32   │
           │_POSIX_V7_ILP32_OFFBIG  │    32   │    32   │    32   │   ≥64   │
           │_POSIX_V7_LP64_OFF64    │    32   │    64   │    64   │    64   │
           │_POSIX_V7_LPBIG_OFFBIG  │   ≥32   │   ≥64   │   ≥64   │   ≥64   │
           └────────────────────────┴─────────┴─────────┴─────────┴─────────┘
       All implementations shall support one or more environments where the
       widths of the following types are no greater than the width of type long:

                          blksize_t   ptrdiff_t     tcflag_t
                          cc_t        size_t        wchar_t
                          mode_t      speed_t       wint_t
                          nfds_t      ssize_t
                          pid_t       suseconds_t

       The executable files created when these environments are selected shall
       be in a proper format for execution by the exec family of functions. Each
       environment may be one of the ones in Table 4-4, Programming
       Environments: Type Sizes, or it may be another environment. The names for
       the environments that meet this requirement shall be output by a getconf
       command using the POSIX_V7_WIDTH_RESTRICTED_ENVS argument, as a
       <newline>-separated list of names suitable for use with the getconf -v
       option. If more than one environment meets the requirement, the names of
       all such environments shall be output on separate lines. Any of these
       names can then be used in a subsequent getconf command to obtain the
       flags specific to that environment with the following suffixes added as
       appropriate:

       _CFLAGS   To get the C compiler flags.

       _LDFLAGS  To get the linker/loader flags.

       _LIBS     To get the libraries.

       This requirement may be removed in a future version.

       When this utility processes a file containing a function called main(),
       it shall be defined with a return type equivalent to int.  Using return
       from the initial call to main() shall be equivalent (other than with
       respect to language scope issues) to calling exit() with the returned
       value. Reaching the end of the initial call to main() shall be equivalent
       to calling exit(0).  The implementation shall not declare a prototype for
       this function.

       Implementations provide configuration strings for C compiler flags,
       linker/loader flags, and libraries for each supported environment.  When
       an application needs to use a specific programming environment rather
       than the implementation default programming environment while compiling,
       the application shall first verify that the implementation supports the
       desired environment. If the desired programming environment is supported,
       the application shall then invoke c99 with the appropriate C compiler
       flags as the first options for the compile, the appropriate linker/loader
       flags after any other options except -l but before any operands or -l
       options, and the appropriate libraries at the end of the operands and -l
       options.

       Conforming applications shall not attempt to link together object files
       compiled for different programming models. Applications shall also be
       aware that binary data placed in shared memory or in files might not be
       recognized by applications built for other programming models.

                  Table 4-5: Programming Environments: c99 Arguments

    ┌────────────────────────┬─────────────────────┬───────────────────────────────┐
    │Programming Environment │                     │         c99 Arguments         │
    │     getconf Name       Use         getconf Name          │
    ├────────────────────────┼─────────────────────┼───────────────────────────────┤
    │_POSIX_V7_ILP32_OFF32   │ C Compiler Flags    │ POSIX_V7_ILP32_OFF32_CFLAGS   │
    │                        │ Linker/Loader Flags │ POSIX_V7_ILP32_OFF32_LDFLAGS  │
    │                        │ Libraries           │ POSIX_V7_ILP32_OFF32_LIBS     │
    ├────────────────────────┼─────────────────────┼───────────────────────────────┤
    │_POSIX_V7_ILP32_OFFBIG  │ C Compiler Flags    │ POSIX_V7_ILP32_OFFBIG_CFLAGS  │
    │                        │ Linker/Loader Flags │ POSIX_V7_ILP32_OFFBIG_LDFLAGS │
    │                        │ Libraries           │ POSIX_V7_ILP32_OFFBIG_LIBS    │
    ├────────────────────────┼─────────────────────┼───────────────────────────────┤
    │_POSIX_V7_LP64_OFF64    │ C Compiler Flags    │ POSIX_V7_LP64_OFF64_CFLAGS    │
    │                        │ Linker/Loader Flags │ POSIX_V7_LP64_OFF64_LDFLAGS   │
    │                        │ Libraries           │ POSIX_V7_LP64_OFF64_LIBS      │
    ├────────────────────────┼─────────────────────┼───────────────────────────────┤
    │_POSIX_V7_LPBIG_OFFBIG  │ C Compiler Flags    │ POSIX_V7_LPBIG_OFFBIG_CFLAGS  │
    │                        │ Linker/Loader Flags │ POSIX_V7_LPBIG_OFFBIG_LDFLAGS │
    │                        │ Libraries           │ POSIX_V7_LPBIG_OFFBIG_LIBS    │
    └────────────────────────┴─────────────────────┴───────────────────────────────┘
       In addition to the type size programming environments above, all
       implementations also support a multi-threaded programming environment
       that is orthogonal to all of the programming environments listed above.
       The getconf utility can be used to get flags for the threaded programming
       environment, as indicated in Table 4-6, Threaded Programming Environment:
       c99 Arguments.

              Table 4-6: Threaded Programming Environment: c99 Arguments

       ┌────────────────────────┬─────────────────────┬──────────────────────────┐
       │Programming Environment │                     │      c99 Arguments       │
       │     getconf Name       Use         getconf Name       │
       ├────────────────────────┼─────────────────────┼──────────────────────────┤
       │_POSIX_THREADS          │ C Compiler Flags    │ POSIX_V7_THREADS_CFLAGS  │
       │                        │ Linker/Loader Flags │ POSIX_V7_THREADS_LDFLAGS │
       └────────────────────────┴─────────────────────┴──────────────────────────┘
       These programming environment flags may be used in conjunction with any
       of the type size programming environments supported by the
       implementation.

EXIT STATUS
       The following exit values shall be returned:

        0    Successful compilation or link edit.

       >0    An error occurred.

CONSEQUENCES OF ERRORS
       When c99 encounters a compilation error that causes an object file not to
       be created, it shall write a diagnostic to standard error and continue to
       compile other source code operands, but it shall not perform the link
       phase and it shall return a non-zero exit status. If the link edit is
       unsuccessful, a diagnostic message shall be written to standard error and
       c99 exits with a non-zero status. A conforming application shall rely on
       the exit status of c99, rather than on the existence or mode of the
       executable file.

       The following sections are informative.

APPLICATION USAGE
       Since the c99 utility usually creates files in the current directory
       during the compilation process, it is typically necessary to run the c99
       utility in a directory in which a file can be created.

       On systems providing POSIX Conformance (see the Base Definitions volume
       of POSIX.1‐2017, Chapter 2, Conformance), c99 is required only with the
       C-Language Development option; XSI-conformant systems always provide c99.

       Some historical implementations have created .o files when -c is not
       specified and more than one source file is given. Since this area is left
       unspecified, the application cannot rely on .o files being created, but
       it also must be prepared for any related .o files that already exist
       being deleted at the completion of the link edit.

       There is the possible implication that if a user supplies versions of the
       standard functions (before they would be encountered by an implicit -l c
       or explicit -l m), that those versions would be used in place of the
       standard versions.  There are various reasons this might not be true
       (functions defined as macros, manipulations for clean name space, and so
       on), so the existence of files named in the same manner as the standard
       libraries within the -L directories is explicitly stated to produce
       unspecified behavior.

       All of the functions specified in the System Interfaces volume of
       POSIX.1‐2017 may be made visible by implementations when the Standard C
       Library is searched. Conforming applications must explicitly request
       searching the other standard libraries when functions made visible by
       those libraries are used.

       In the ISO C standard the mapping from physical source characters to the
       C source character set is implementation-defined. Implementations may
       strip white-space characters before the terminating <newline> of a
       (physical) line as part of this mapping and, as a consequence of this,
       one or more white-space characters (and no other characters) between a
       <backslash> character and the <newline> character that terminates the
       line produces implementation-defined results. Portable applications
       should not use such constructs.

       Some c99 compilers not conforming to POSIX.1‐2008 do not support
       trigraphs by default.

EXAMPLES
        1. The following usage example compiles foo.c and creates the executable
           file foo:


               c99 -o foo foo.c

           The following usage example compiles foo.c and creates the object
           file foo.o:


               c99 -c foo.c

           The following usage example compiles foo.c and creates the executable
           file a.out:


               c99 foo.c

           The following usage example compiles foo.c, links it with bar.o, and
           creates the executable file a.out.  It may also create and leave
           foo.o:


               c99 foo.c bar.o

        2. The following example shows how an application using threads
           interfaces can test for support of and use a programming environment
           supporting 32-bit int, long, and pointer types and an off_t type
           using at least 64 bits:


               offbig_env=$(getconf _POSIX_V7_ILP32_OFFBIG)
               if [ $offbig_env != "-1" ] && [ $offbig_env != "undefined" ]
               then
                   c99 $(getconf POSIX_V7_ILP32_OFFBIG_CFLAGS) \
                   $(getconf POSIX_V7_THREADS_CFLAGS) -D_XOPEN_SOURCE=700 \
                   $(getconf POSIX_V7_ILP32_OFFBIG_LDFLAGS) \
                   $(getconf POSIX_V7_THREADS_LDFLAGS) foo.c -o foo \
                   $(getconf POSIX_V7_ILP32_OFFBIG_LIBS) \
                   -l pthread
               else
                   echo ILP32_OFFBIG programming environment not supported
                   exit 1
               fi

        3. The following examples clarify the use and interactions of -L and -l
           options.

           Consider the case in which module a.c calls function f() in library
           libQ.a, and module b.c calls function g() in library libp.a.  Assume
           that both libraries reside in /a/b/c.  The command line to compile
           and link in the desired way is:


               c99 -L /a/b/c main.o a.c -l Q b.c -l p

           In this case the -L option need only precede the first -l option,
           since both libQ.a and libp.a reside in the same directory.

           Multiple -L options can be used when library name collisions occur.
           Building on the previous example, suppose that the user wants to use
           a new libp.a, in /a/a/a, but still wants f() from /a/b/c/libQ.a:


               c99 -L /a/a/a -L /a/b/c main.o a.c -l Q b.c -l p

           In this example, the linker searches the -L options in the order
           specified, and finds /a/a/a/libp.a before /a/b/c/libp.a when
           resolving references for b.c.  The order of the -l options is still
           important, however.

        4. The following example shows how an application can use a programming
           environment where the widths of the following types: blksize_t, cc_t,
           mode_t, nfds_t, pid_t, ptrdiff_t, size_t, speed_t, ssize_t,
           suseconds_t, tcflag_t, wchar_t, wint_t

           are no greater than the width of type long:


               # First choose one of the listed environments ...

               # ... if there are no additional constraints, the first one will do:
               CENV=$(getconf POSIX_V7_WIDTH_RESTRICTED_ENVS | head -n l)

               # ... or, if an environment that supports large files is preferred,
               # look for names that contain "OFF64" or "OFFBIG". (This chooses
               # the last one in the list if none match.)
               for CENV in $(getconf POSIX_V7_WIDTH_RESTRICTED_ENVS)
               do
                   case $CENV in
                   *OFF64*|*OFFBIG*) break ;;
                   esac
               done

               # The chosen environment name can now be used like this:

               c99 $(getconf ${CENV}_CFLAGS) -D _POSIX_C_SOURCE=200809L \
               $(getconf ${CENV}_LDFLAGS) foo.c -o foo \
               $(getconf ${CENV}_LIBS)

RATIONALE
       The c99 utility is based on the c89 utility originally introduced in the
       ISO POSIX‐2:1993 standard.

       Some of the changes from c89 include the ability to intersperse options
       and operands (which many c89 implementations allowed despite it not being
       specified), the description of -l as an option instead of an operand, and
       the modification to the contents of the Standard Libraries section to
       account for new headers and options; for example, <spawn.h> added to the
       description of -l rt, and -l trace added for the Tracing option.

       POSIX.1‐2008 specifies that the c99 utility must be able to use regular
       files for *.o files and for a.out files. Implementations are free to
       overwrite existing files of other types when attempting to create object
       files and executable files, but are not required to do so. If something
       other than a regular file is specified and using it fails for any reason,
       c99 is required to issue a diagnostic message and exit with a non-zero
       exit status. But for some file types, the problem may not be noticed for
       a long time. For example, if a FIFO named a.out exists in the current
       directory, c99 may attempt to open a.out and will hang in the open() call
       until another process opens the FIFO for reading. Then c99 may write most
       of the a.out to the FIFO and fail when it tries to seek back close to the
       start of the file to insert a timestamp (FIFOs are not seekable files).
       The c99 utility is also allowed to issue a diagnostic immediately if it
       encounters an a.out or *.o file that is not a regular file. For portable
       use, applications should ensure that any a.out, -o option-argument, or
       *.o files corresponding to any *.c files do not conflict with names
       already in use that are not regular files or symbolic links that point to
       regular files.

       On many systems, multi-threaded applications run in a programming
       environment that is distinct from that used by single-threaded
       applications. This multi-threaded programming environment (in addition to
       needing to specify -l pthread at link time) may require additional flags
       to be set when headers are processed at compile time (-D_REENTRANT being
       common). This programming environment is orthogonal to the type size
       programming environments discussed above and listed in Table 4-4,
       Programming Environments: Type Sizes.  This version of the standard adds
       getconf utility calls to provide the C compiler flags and linker/loader
       flags needed to support multi-threaded applications. Note that on a
       system where single-threaded applications are a special case of a multi-
       threaded application, both of these getconf calls may return NULL
       strings; on other implementations both of these strings may be non-NULL
       strings.

       The C standardization committee invented trigraphs (e.g., "??!" to
       represent '|') to address character portability problems in development
       environments based on national variants of the 7-bit ISO/IEC 646:1991
       standard character set. However, these environments were already obsolete
       by the time the first ISO C standard was published, and in practice
       trigraphs have not been used for their intended purpose, and usually are
       intended to have their original meaning in K&R C.  For example, in
       practice a C-language source string like "What??!" is usually intended to
       end in two <question-mark> characters and an <exclamation-mark>, not in
       '|'.

       When the -E option is used, execution of some #pragma preprocessor
       directives may simply result in a copy of the directive being included in
       the output as part of the allowed extra information used by subsequent
       compilation passes (see STDOUT).

FUTURE DIRECTIONS
       Unlike all of the other non-OB-shaded utilities in this standard, a
       utility by this name probably will not appear in the next version of this
       standard.  This utility's name is tied to the current revision of the
       ISO C standard at the time this standard is approved. Since the ISO C
       standard and this standard are maintained by different organizations on
       different schedules, we cannot predict what the compiler will be named in
       the next version of the standard.

SEE ALSO
       Section 1.1.1.4, File Read, Write, and Creation, ar, getconf, make, nm,
       strip, umask

       The Base Definitions volume of POSIX.1‐2017, Chapter 8, Environment
       Variables, Section 12.2, Utility Syntax Guidelines, Chapter 13, Headers

       The System Interfaces volume of POSIX.1‐2017, exec, sysconf()

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute
       of Electrical and Electronics Engineers, Inc and The Open Group.  In the
       event of any discrepancy between this version and the original IEEE and
       The Open Group Standard, the original IEEE and The Open Group Standard is
       the referee document. The original Standard can be obtained online at
       http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are most
       likely to have been introduced during the conversion of the source files
       to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                   2017                               C99(1P)