// Copyright 2009 The Go Authors. All rights reserved. // Use of this source code is governed by a BSD-style // license that can be found in the LICENSE file. /* Cgo enables the creation of Go packages that call C code. Using cgo with the go command To use cgo write normal Go code that imports a pseudo-package "C". The Go code can then refer to types such as C.size_t, variables such as C.stdout, or functions such as C.putchar. If the import of "C" is immediately preceded by a comment, that comment, called the preamble, is used as a header when compiling the C parts of the package. For example: // #include <stdio.h> // #include <errno.h> import "C" The preamble may contain any C code, including function and variable declarations and definitions. These may then be referred to from Go code as though they were defined in the package "C". All names declared in the preamble may be used, even if they start with a lower-case letter. Exception: static variables in the preamble may not be referenced from Go code; static functions are permitted. See $GOROOT/misc/cgo/stdio and $GOROOT/misc/cgo/gmp for examples. See "C? Go? Cgo!" for an introduction to using cgo: https://golang.org/doc/articles/c_go_cgo.html. CFLAGS, CPPFLAGS, CXXFLAGS, FFLAGS and LDFLAGS may be defined with pseudo #cgo directives within these comments to tweak the behavior of the C, C++ or Fortran compiler. Values defined in multiple directives are concatenated together. The directive can include a list of build constraints limiting its effect to systems satisfying one of the constraints (see https://golang.org/pkg/go/build/#hdr-Build_Constraints for details about the constraint syntax). For example: // #cgo CFLAGS: -DPNG_DEBUG=1 // #cgo amd64 386 CFLAGS: -DX86=1 // #cgo LDFLAGS: -lpng // #include <png.h> import "C" Alternatively, CPPFLAGS and LDFLAGS may be obtained via the pkg-config tool using a '#cgo pkg-config:' directive followed by the package names. For example: // #cgo pkg-config: png cairo // #include <png.h> import "C" The default pkg-config tool may be changed by setting the PKG_CONFIG environment variable. For security reasons, only a limited set of flags are allowed, notably -D, -I, and -l. To allow additional flags, set CGO_CFLAGS_ALLOW to a regular expression matching the new flags. To disallow flags that would otherwise be allowed, set CGO_CFLAGS_DISALLOW to a regular expression matching arguments that must be disallowed. In both cases the regular expression must match a full argument: to allow -mfoo=bar, use CGO_CFLAGS_ALLOW='-mfoo.*', not just CGO_CFLAGS_ALLOW='-mfoo'. Similarly named variables control the allowed CPPFLAGS, CXXFLAGS, FFLAGS, and LDFLAGS. When building, the CGO_CFLAGS, CGO_CPPFLAGS, CGO_CXXFLAGS, CGO_FFLAGS and CGO_LDFLAGS environment variables are added to the flags derived from these directives. Package-specific flags should be set using the directives, not the environment variables, so that builds work in unmodified environments. Flags obtained from environment variables are not subject to the security limitations described above. All the cgo CPPFLAGS and CFLAGS directives in a package are concatenated and used to compile C files in that package. All the CPPFLAGS and CXXFLAGS directives in a package are concatenated and used to compile C++ files in that package. All the CPPFLAGS and FFLAGS directives in a package are concatenated and used to compile Fortran files in that package. All the LDFLAGS directives in any package in the program are concatenated and used at link time. All the pkg-config directives are concatenated and sent to pkg-config simultaneously to add to each appropriate set of command-line flags. When the cgo directives are parsed, any occurrence of the string ${SRCDIR} will be replaced by the absolute path to the directory containing the source file. This allows pre-compiled static libraries to be included in the package directory and linked properly. For example if package foo is in the directory /go/src/foo: // #cgo LDFLAGS: -L${SRCDIR}/libs -lfoo Will be expanded to: // #cgo LDFLAGS: -L/go/src/foo/libs -lfoo When the Go tool sees that one or more Go files use the special import "C", it will look for other non-Go files in the directory and compile them as part of the Go package. Any .c, .s, or .S files will be compiled with the C compiler. Any .cc, .cpp, or .cxx files will be compiled with the C++ compiler. Any .f, .F, .for or .f90 files will be compiled with the fortran compiler. Any .h, .hh, .hpp, or .hxx files will not be compiled separately, but, if these header files are changed, the C and C++ files will be recompiled. The default C and C++ compilers may be changed by the CC and CXX environment variables, respectively; those environment variables may include command line options. The cgo tool is enabled by default for native builds on systems where it is expected to work. It is disabled by default when cross-compiling. You can control this by setting the CGO_ENABLED environment variable when running the go tool: set it to 1 to enable the use of cgo, and to 0 to disable it. The go tool will set the build constraint "cgo" if cgo is enabled. When cross-compiling, you must specify a C cross-compiler for cgo to use. You can do this by setting the generic CC_FOR_TARGET or the more specific CC_FOR_${GOOS}_${GOARCH} (for example, CC_FOR_linux_arm) environment variable when building the toolchain using make.bash, or you can set the CC environment variable any time you run the go tool. The CXX_FOR_TARGET, CXX_FOR_${GOOS}_${GOARCH}, and CXX environment variables work in a similar way for C++ code. Go references to C Within the Go file, C's struct field names that are keywords in Go can be accessed by prefixing them with an underscore: if x points at a C struct with a field named "type", x._type accesses the field. C struct fields that cannot be expressed in Go, such as bit fields or misaligned data, are omitted in the Go struct, replaced by appropriate padding to reach the next field or the end of the struct. The standard C numeric types are available under the names C.char, C.schar (signed char), C.uchar (unsigned char), C.short, C.ushort (unsigned short), C.int, C.uint (unsigned int), C.long, C.ulong (unsigned long), C.longlong (long long), C.ulonglong (unsigned long long), C.float, C.double, C.complexfloat (complex float), and C.complexdouble (complex double). The C type void* is represented by Go's unsafe.Pointer. The C types __int128_t and __uint128_t are represented by [16]byte. A few special C types which would normally be represented by a pointer type in Go are instead represented by a uintptr. See the Special cases section below. To access a struct, union, or enum type directly, prefix it with struct_, union_, or enum_, as in C.struct_stat. The size of any C type T is available as C.sizeof_T, as in C.sizeof_struct_stat. A C function may be declared in the Go file with a parameter type of the special name _GoString_. This function may be called with an ordinary Go string value. The string length, and a pointer to the string contents, may be accessed by calling the C functions size_t _GoStringLen(_GoString_ s); const char *_GoStringPtr(_GoString_ s); These functions are only available in the preamble, not in other C files. The C code must not modify the contents of the pointer returned by _GoStringPtr. Note that the string contents may not have a trailing NUL byte. As Go doesn't have support for C's union type in the general case, C's union types are represented as a Go byte array with the same length. Go structs cannot embed fields with C types. Go code cannot refer to zero-sized fields that occur at the end of non-empty C structs. To get the address of such a field (which is the only operation you can do with a zero-sized field) you must take the address of the struct and add the size of the struct. Cgo translates C types into equivalent unexported Go types. Because the translations are unexported, a Go package should not expose C types in its exported API: a C type used in one Go package is different from the same C type used in another. Any C function (even void functions) may be called in a multiple assignment context to retrieve both the return value (if any) and the C errno variable as an error (use _ to skip the result value if the function returns void). For example: n, err = C.sqrt(-1) _, err := C.voidFunc() var n, err = C.sqrt(1) Calling C function pointers is currently not supported, however you can declare Go variables which hold C function pointers and pass them back and forth between Go and C. C code may call function pointers received from Go. For example: package main // typedef int (*intFunc) (); // // int // bridge_int_func(intFunc f) // { // return f(); // } // // int fortytwo() // { // return 42; // } import "C" import "fmt" func main() { f := C.intFunc(C.fortytwo) fmt.Println(int(C.bridge_int_func(f))) // Output: 42 } In C, a function argument written as a fixed size array actually requires a pointer to the first element of the array. C compilers are aware of this calling convention and adjust the call accordingly, but Go cannot. In Go, you must pass the pointer to the first element explicitly: C.f(&C.x[0]). A few special functions convert between Go and C types by making copies of the data. In pseudo-Go definitions: // Go string to C string // The C string is allocated in the C heap using malloc. // It is the caller's responsibility to arrange for it to be // freed, such as by calling C.free (be sure to include stdlib.h // if C.free is needed). func C.CString(string) *C.char // Go []byte slice to C array // The C array is allocated in the C heap using malloc. // It is the caller's responsibility to arrange for it to be // freed, such as by calling C.free (be sure to include stdlib.h // if C.free is needed). func C.CBytes([]byte) unsafe.Pointer // C string to Go string func C.GoString(*C.char) string // C data with explicit length to Go string func C.GoStringN(*C.char, C.int) string // C data with explicit length to Go []byte func C.GoBytes(unsafe.Pointer, C.int) []byte As a special case, C.malloc does not call the C library malloc directly but instead calls a Go helper function that wraps the C library malloc but guarantees never to return nil. If C's malloc indicates out of memory, the helper function crashes the program, like when Go itself runs out of memory. Because C.malloc cannot fail, it has no two-result form that returns errno. C references to Go Go functions can be exported for use by C code in the following way: //export MyFunction func MyFunction(arg1, arg2 int, arg3 string) int64 {...} //export MyFunction2 func MyFunction2(arg1, arg2 int, arg3 string) (int64, *C.char) {...} They will be available in the C code as: extern int64 MyFunction(int arg1, int arg2, GoString arg3); extern struct MyFunction2_return MyFunction2(int arg1, int arg2, GoString arg3); found in the _cgo_export.h generated header, after any preambles copied from the cgo input files. Functions with multiple return values are mapped to functions returning a struct. Not all Go types can be mapped to C types in a useful way. Go struct types are not supported; use a C struct type. Go array types are not supported; use a C pointer. Go functions that take arguments of type string may be called with the C type _GoString_, described above. The _GoString_ type will be automatically defined in the preamble. Note that there is no way for C code to create a value of this type; this is only useful for passing string values from Go to C and back to Go. Using //export in a file places a restriction on the preamble: since it is copied into two different C output files, it must not contain any definitions, only declarations. If a file contains both definitions and declarations, then the two output files will produce duplicate symbols and the linker will fail. To avoid this, definitions must be placed in preambles in other files, or in C source files. Passing pointers Go is a garbage collected language, and the garbage collector needs to know the location of every pointer to Go memory. Because of this, there are restrictions on passing pointers between Go and C. In this section the term Go pointer means a pointer to memory allocated by Go (such as by using the & operator or calling the predefined new function) and the term C pointer means a pointer to memory allocated by C (such as by a call to C.malloc). Whether a pointer is a Go pointer or a C pointer is a dynamic property determined by how the memory was allocated; it has nothing to do with the type of the pointer. Note that values of some Go types, other than the type's zero value, always include Go pointers. This is true of string, slice, interface, channel, map, and function types. A pointer type may hold a Go pointer or a C pointer. Array and struct types may or may not include Go pointers, depending on the element types. All the discussion below about Go pointers applies not just to pointer types, but also to other types that include Go pointers. Go code may pass a Go pointer to C provided the Go memory to which it points does not contain any Go pointers. The C code must preserve this property: it must not store any Go pointers in Go memory, even temporarily. When passing a pointer to a field in a struct, the Go memory in question is the memory occupied by the field, not the entire struct. When passing a pointer to an element in an array or slice, the Go memory in question is the entire array or the entire backing array of the slice. C code may not keep a copy of a Go pointer after the call returns. This includes the _GoString_ type, which, as noted above, includes a Go pointer; _GoString_ values may not be retained by C code. A Go function called by C code may not return a Go pointer (which implies that it may not return a string, slice, channel, and so forth). A Go function called by C code may take C pointers as arguments, and it may store non-pointer or C pointer data through those pointers, but it may not store a Go pointer in memory pointed to by a C pointer. A Go function called by C code may take a Go pointer as an argument, but it must preserve the property that the Go memory to which it points does not contain any Go pointers. Go code may not store a Go pointer in C memory. C code may store Go pointers in C memory, subject to the rule above: it must stop storing the Go pointer when the C function returns. These rules are checked dynamically at runtime. The checking is controlled by the cgocheck setting of the GODEBUG environment variable. The default setting is GODEBUG=cgocheck=1, which implements reasonably cheap dynamic checks. These checks may be disabled entirely using GODEBUG=cgocheck=0. Complete checking of pointer handling, at some cost in run time, is available via GODEBUG=cgocheck=2. It is possible to defeat this enforcement by using the unsafe package, and of course there is nothing stopping the C code from doing anything it likes. However, programs that break these rules are likely to fail in unexpected and unpredictable ways. Special cases A few special C types which would normally be represented by a pointer type in Go are instead represented by a uintptr. Those include: 1. The *Ref types on Darwin, rooted at CoreFoundation's CFTypeRef type. 2. The object types from Java's JNI interface: jobject jclass jthrowable jstring jarray jbooleanArray jbyteArray jcharArray jshortArray jintArray jlongArray jfloatArray jdoubleArray jobjectArray jweak These types are uintptr on the Go side because they would otherwise confuse the Go garbage collector; they are sometimes not really pointers but data structures encoded in a pointer type. All operations on these types must happen in C. The proper constant to initialize an empty such reference is 0, not nil. These special cases were introduced in Go 1.10. For auto-updating code from Go 1.9 and earlier, use the cftype or jni rewrites in the Go fix tool: go tool fix -r cftype <pkg> go tool fix -r jni <pkg> It will replace nil with 0 in the appropriate places. Using cgo directly Usage: go tool cgo [cgo options] [-- compiler options] gofiles... Cgo transforms the specified input Go source files into several output Go and C source files. The compiler options are passed through uninterpreted when invoking the C compiler to compile the C parts of the package. The following options are available when running cgo directly: -V Print cgo version and exit. -debug-define Debugging option. Print #defines. -debug-gcc Debugging option. Trace C compiler execution and output. -dynimport file Write list of symbols imported by file. Write to -dynout argument or to standard output. Used by go build when building a cgo package. -dynlinker Write dynamic linker as part of -dynimport output. -dynout file Write -dynimport output to file. -dynpackage package Set Go package for -dynimport output. -exportheader file If there are any exported functions, write the generated export declarations to file. C code can #include this to see the declarations. -importpath string The import path for the Go package. Optional; used for nicer comments in the generated files. -import_runtime_cgo If set (which it is by default) import runtime/cgo in generated output. -import_syscall If set (which it is by default) import syscall in generated output. -gccgo Generate output for the gccgo compiler rather than the gc compiler. -gccgoprefix prefix The -fgo-prefix option to be used with gccgo. -gccgopkgpath path The -fgo-pkgpath option to be used with gccgo. -godefs Write out input file in Go syntax replacing C package names with real values. Used to generate files in the syscall package when bootstrapping a new target. -objdir directory Put all generated files in directory. -srcdir directory */ package main /* Implementation details. Cgo provides a way for Go programs to call C code linked into the same address space. This comment explains the operation of cgo. Cgo reads a set of Go source files and looks for statements saying import "C". If the import has a doc comment, that comment is taken as literal C code to be used as a preamble to any C code generated by cgo. A typical preamble #includes necessary definitions: // #include <stdio.h> import "C" For more details about the usage of cgo, see the documentation comment at the top of this file. Understanding C Cgo scans the Go source files that import "C" for uses of that package, such as C.puts. It collects all such identifiers. The next step is to determine each kind of name. In C.xxx the xxx might refer to a type, a function, a constant, or a global variable. Cgo must decide which. The obvious thing for cgo to do is to process the preamble, expanding #includes and processing the corresponding C code. That would require a full C parser and type checker that was also aware of any extensions known to the system compiler (for example, all the GNU C extensions) as well as the system-specific header locations and system-specific pre-#defined macros. This is certainly possible to do, but it is an enormous amount of work. Cgo takes a different approach. It determines the meaning of C identifiers not by parsing C code but by feeding carefully constructed programs into the system C compiler and interpreting the generated error messages, debug information, and object files. In practice, parsing these is significantly less work and more robust than parsing C source. Cgo first invokes gcc -E -dM on the preamble, in order to find out about simple #defines for constants and the like. These are recorded for later use. Next, cgo needs to identify the kinds for each identifier. For the identifiers C.foo, cgo generates this C program: <preamble> #line 1 "not-declared" void __cgo_f_1_1(void) { __typeof__(foo) *__cgo_undefined__1; } #line 1 "not-type" void __cgo_f_1_2(void) { foo *__cgo_undefined__2; } #line 1 "not-int-const" void __cgo_f_1_3(void) { enum { __cgo_undefined__3 = (foo)*1 }; } #line 1 "not-num-const" void __cgo_f_1_4(void) { static const double __cgo_undefined__4 = (foo); } #line 1 "not-str-lit" void __cgo_f_1_5(void) { static const char __cgo_undefined__5[] = (foo); } This program will not compile, but cgo can use the presence or absence of an error message on a given line to deduce the information it needs. The program is syntactically valid regardless of whether each name is a type or an ordinary identifier, so there will be no syntax errors that might stop parsing early. An error on not-declared:1 indicates that foo is undeclared. An error on not-type:1 indicates that foo is not a type (if declared at all, it is an identifier). An error on not-int-const:1 indicates that foo is not an integer constant. An error on not-num-const:1 indicates that foo is not a number constant. An error on not-str-lit:1 indicates that foo is not a string literal. An error on not-signed-int-const:1 indicates that foo is not a signed integer constant. The line number specifies the name involved. In the example, 1 is foo. Next, cgo must learn the details of each type, variable, function, or constant. It can do this by reading object files. If cgo has decided that t1 is a type, v2 and v3 are variables or functions, and i4, i5 are integer constants, u6 is an unsigned integer constant, and f7 and f8 are float constants, and s9 and s10 are string constants, it generates: <preamble> __typeof__(t1) *__cgo__1; __typeof__(v2) *__cgo__2; __typeof__(v3) *__cgo__3; __typeof__(i4) *__cgo__4; enum { __cgo_enum__4 = i4 }; __typeof__(i5) *__cgo__5; enum { __cgo_enum__5 = i5 }; __typeof__(u6) *__cgo__6; enum { __cgo_enum__6 = u6 }; __typeof__(f7) *__cgo__7; __typeof__(f8) *__cgo__8; __typeof__(s9) *__cgo__9; __typeof__(s10) *__cgo__10; long long __cgodebug_ints[] = { 0, // t1 0, // v2 0, // v3 i4, i5, u6, 0, // f7 0, // f8 0, // s9 0, // s10 1 }; double __cgodebug_floats[] = { 0, // t1 0, // v2 0, // v3 0, // i4 0, // i5 0, // u6 f7, f8, 0, // s9 0, // s10 1 }; const char __cgodebug_str__9[] = s9; const unsigned long long __cgodebug_strlen__9 = sizeof(s9)-1; const char __cgodebug_str__10[] = s10; const unsigned long long __cgodebug_strlen__10 = sizeof(s10)-1; and again invokes the system C compiler, to produce an object file containing debug information. Cgo parses the DWARF debug information for __cgo__N to learn the type of each identifier. (The types also distinguish functions from global variables.) Cgo reads the constant values from the __cgodebug_* from the object file's data segment. At this point cgo knows the meaning of each C.xxx well enough to start the translation process. Translating Go Given the input Go files x.go and y.go, cgo generates these source files: x.cgo1.go # for gc (cmd/compile) y.cgo1.go # for gc _cgo_gotypes.go # for gc _cgo_import.go # for gc (if -dynout _cgo_import.go) x.cgo2.c # for gcc y.cgo2.c # for gcc _cgo_defun.c # for gcc (if -gccgo) _cgo_export.c # for gcc _cgo_export.h # for gcc _cgo_main.c # for gcc _cgo_flags # for alternative build tools The file x.cgo1.go is a copy of x.go with the import "C" removed and references to C.xxx replaced with names like _Cfunc_xxx or _Ctype_xxx. The definitions of those identifiers, written as Go functions, types, or variables, are provided in _cgo_gotypes.go. Here is a _cgo_gotypes.go containing definitions for needed C types: type _Ctype_char int8 type _Ctype_int int32 type _Ctype_void [0]byte The _cgo_gotypes.go file also contains the definitions of the functions. They all have similar bodies that invoke runtime·cgocall to make a switch from the Go runtime world to the system C (GCC-based) world. For example, here is the definition of _Cfunc_puts: //go:cgo_import_static _cgo_be59f0f25121_Cfunc_puts //go:linkname __cgofn__cgo_be59f0f25121_Cfunc_puts _cgo_be59f0f25121_Cfunc_puts var __cgofn__cgo_be59f0f25121_Cfunc_puts byte var _cgo_be59f0f25121_Cfunc_puts = unsafe.Pointer(&__cgofn__cgo_be59f0f25121_Cfunc_puts) func _Cfunc_puts(p0 *_Ctype_char) (r1 _Ctype_int) { _cgo_runtime_cgocall(_cgo_be59f0f25121_Cfunc_puts, uintptr(unsafe.Pointer(&p0))) return } The hexadecimal number is a hash of cgo's input, chosen to be deterministic yet unlikely to collide with other uses. The actual function _cgo_be59f0f25121_Cfunc_puts is implemented in a C source file compiled by gcc, the file x.cgo2.c: void _cgo_be59f0f25121_Cfunc_puts(void *v) { struct { char* p0; int r; char __pad12[4]; } __attribute__((__packed__, __gcc_struct__)) *a = v; a->r = puts((void*)a->p0); } It extracts the arguments from the pointer to _Cfunc_puts's argument frame, invokes the system C function (in this case, puts), stores the result in the frame, and returns. Linking Once the _cgo_export.c and *.cgo2.c files have been compiled with gcc, they need to be linked into the final binary, along with the libraries they might depend on (in the case of puts, stdio). cmd/link has been extended to understand basic ELF files, but it does not understand ELF in the full complexity that modern C libraries embrace, so it cannot in general generate direct references to the system libraries. Instead, the build process generates an object file using dynamic linkage to the desired libraries. The main function is provided by _cgo_main.c: int main() { return 0; } void crosscall2(void(*fn)(void*, int, uintptr_t), void *a, int c, uintptr_t ctxt) { } uintptr_t _cgo_wait_runtime_init_done() { return 0; } void _cgo_release_context(uintptr_t ctxt) { } char* _cgo_topofstack(void) { return (char*)0; } void _cgo_allocate(void *a, int c) { } void _cgo_panic(void *a, int c) { } void _cgo_reginit(void) { } The extra functions here are stubs to satisfy the references in the C code generated for gcc. The build process links this stub, along with _cgo_export.c and *.cgo2.c, into a dynamic executable and then lets cgo examine the executable. Cgo records the list of shared library references and resolved names and writes them into a new file _cgo_import.go, which looks like: //go:cgo_dynamic_linker "/lib64/ld-linux-x86-64.so.2" //go:cgo_import_dynamic puts puts#GLIBC_2.2.5 "libc.so.6" //go:cgo_import_dynamic __libc_start_main __libc_start_main#GLIBC_2.2.5 "libc.so.6" //go:cgo_import_dynamic stdout stdout#GLIBC_2.2.5 "libc.so.6" //go:cgo_import_dynamic fflush fflush#GLIBC_2.2.5 "libc.so.6" //go:cgo_import_dynamic _ _ "libpthread.so.0" //go:cgo_import_dynamic _ _ "libc.so.6" In the end, the compiled Go package, which will eventually be presented to cmd/link as part of a larger program, contains: _go_.o # gc-compiled object for _cgo_gotypes.go, _cgo_import.go, *.cgo1.go _all.o # gcc-compiled object for _cgo_export.c, *.cgo2.c The final program will be a dynamic executable, so that cmd/link can avoid needing to process arbitrary .o files. It only needs to process the .o files generated from C files that cgo writes, and those are much more limited in the ELF or other features that they use. In essence, the _cgo_import.o file includes the extra linking directives that cmd/link is not sophisticated enough to derive from _all.o on its own. Similarly, the _all.o uses dynamic references to real system object code because cmd/link is not sophisticated enough to process the real code. The main benefits of this system are that cmd/link remains relatively simple (it does not need to implement a complete ELF and Mach-O linker) and that gcc is not needed after the package is compiled. For example, package net uses cgo for access to name resolution functions provided by libc. Although gcc is needed to compile package net, gcc is not needed to link programs that import package net. Runtime When using cgo, Go must not assume that it owns all details of the process. In particular it needs to coordinate with C in the use of threads and thread-local storage. The runtime package declares a few variables: var ( iscgo bool _cgo_init unsafe.Pointer _cgo_thread_start unsafe.Pointer ) Any package using cgo imports "runtime/cgo", which provides initializations for these variables. It sets iscgo to true, _cgo_init to a gcc-compiled function that can be called early during program startup, and _cgo_thread_start to a gcc-compiled function that can be used to create a new thread, in place of the runtime's usual direct system calls. Internal and External Linking The text above describes "internal" linking, in which cmd/link parses and links host object files (ELF, Mach-O, PE, and so on) into the final executable itself. Keeping cmd/link simple means we cannot possibly implement the full semantics of the host linker, so the kinds of objects that can be linked directly into the binary is limited (other code can only be used as a dynamic library). On the other hand, when using internal linking, cmd/link can generate Go binaries by itself. In order to allow linking arbitrary object files without requiring dynamic libraries, cgo supports an "external" linking mode too. In external linking mode, cmd/link does not process any host object files. Instead, it collects all the Go code and writes a single go.o object file containing it. Then it invokes the host linker (usually gcc) to combine the go.o object file and any supporting non-Go code into a final executable. External linking avoids the dynamic library requirement but introduces a requirement that the host linker be present to create such a binary. Most builds both compile source code and invoke the linker to create a binary. When cgo is involved, the compile step already requires gcc, so it is not problematic for the link step to require gcc too. An important exception is builds using a pre-compiled copy of the standard library. In particular, package net uses cgo on most systems, and we want to preserve the ability to compile pure Go code that imports net without requiring gcc to be present at link time. (In this case, the dynamic library requirement is less significant, because the only library involved is libc.so, which can usually be assumed present.) This conflict between functionality and the gcc requirement means we must support both internal and external linking, depending on the circumstances: if net is the only cgo-using package, then internal linking is probably fine, but if other packages are involved, so that there are dependencies on libraries beyond libc, external linking is likely to work better. The compilation of a package records the relevant information to support both linking modes, leaving the decision to be made when linking the final binary. Linking Directives In either linking mode, package-specific directives must be passed through to cmd/link. These are communicated by writing //go: directives in a Go source file compiled by gc. The directives are copied into the .o object file and then processed by the linker. The directives are: //go:cgo_import_dynamic <local> [<remote> ["<library>"]] In internal linking mode, allow an unresolved reference to <local>, assuming it will be resolved by a dynamic library symbol. The optional <remote> specifies the symbol's name and possibly version in the dynamic library, and the optional "<library>" names the specific library where the symbol should be found. In the <remote>, # or @ can be used to introduce a symbol version. Examples: //go:cgo_import_dynamic puts //go:cgo_import_dynamic puts puts#GLIBC_2.2.5 //go:cgo_import_dynamic puts puts#GLIBC_2.2.5 "libc.so.6" A side effect of the cgo_import_dynamic directive with a library is to make the final binary depend on that dynamic library. To get the dependency without importing any specific symbols, use _ for local and remote. Example: //go:cgo_import_dynamic _ _ "libc.so.6" For compatibility with current versions of SWIG, #pragma dynimport is an alias for //go:cgo_import_dynamic. //go:cgo_dynamic_linker "<path>" In internal linking mode, use "<path>" as the dynamic linker in the final binary. This directive is only needed from one package when constructing a binary; by convention it is supplied by runtime/cgo. Example: //go:cgo_dynamic_linker "/lib/ld-linux.so.2" //go:cgo_export_dynamic <local> <remote> In internal linking mode, put the Go symbol named <local> into the program's exported symbol table as <remote>, so that C code can refer to it by that name. This mechanism makes it possible for C code to call back into Go or to share Go's data. For compatibility with current versions of SWIG, #pragma dynexport is an alias for //go:cgo_export_dynamic. //go:cgo_import_static <local> In external linking mode, allow unresolved references to <local> in the go.o object file prepared for the host linker, under the assumption that <local> will be supplied by the other object files that will be linked with go.o. Example: //go:cgo_import_static puts_wrapper //go:cgo_export_static <local> <remote> In external linking mode, put the Go symbol named <local> into the program's exported symbol table as <remote>, so that C code can refer to it by that name. This mechanism makes it possible for C code to call back into Go or to share Go's data. //go:cgo_ldflag "<arg>" In external linking mode, invoke the host linker (usually gcc) with "<arg>" as a command-line argument following the .o files. Note that the arguments are for "gcc", not "ld". Example: //go:cgo_ldflag "-lpthread" //go:cgo_ldflag "-L/usr/local/sqlite3/lib" A package compiled with cgo will include directives for both internal and external linking; the linker will select the appropriate subset for the chosen linking mode. Example As a simple example, consider a package that uses cgo to call C.sin. The following code will be generated by cgo: // compiled by gc //go:cgo_ldflag "-lm" type _Ctype_double float64 //go:cgo_import_static _cgo_gcc_Cfunc_sin //go:linkname __cgo_gcc_Cfunc_sin _cgo_gcc_Cfunc_sin var __cgo_gcc_Cfunc_sin byte var _cgo_gcc_Cfunc_sin = unsafe.Pointer(&__cgo_gcc_Cfunc_sin) func _Cfunc_sin(p0 _Ctype_double) (r1 _Ctype_double) { _cgo_runtime_cgocall(_cgo_gcc_Cfunc_sin, uintptr(unsafe.Pointer(&p0))) return } // compiled by gcc, into foo.cgo2.o void _cgo_gcc_Cfunc_sin(void *v) { struct { double p0; double r; } __attribute__((__packed__)) *a = v; a->r = sin(a->p0); } What happens at link time depends on whether the final binary is linked using the internal or external mode. If other packages are compiled in "external only" mode, then the final link will be an external one. Otherwise the link will be an internal one. The linking directives are used according to the kind of final link used. In internal mode, cmd/link itself processes all the host object files, in particular foo.cgo2.o. To do so, it uses the cgo_import_dynamic and cgo_dynamic_linker directives to learn that the otherwise undefined reference to sin in foo.cgo2.o should be rewritten to refer to the symbol sin with version GLIBC_2.2.5 from the dynamic library "libm.so.6", and the binary should request "/lib/ld-linux.so.2" as its runtime dynamic linker. In external mode, cmd/link does not process any host object files, in particular foo.cgo2.o. It links together the gc-generated object files, along with any other Go code, into a go.o file. While doing that, cmd/link will discover that there is no definition for _cgo_gcc_Cfunc_sin, referred to by the gc-compiled source file. This is okay, because cmd/link also processes the cgo_import_static directive and knows that _cgo_gcc_Cfunc_sin is expected to be supplied by a host object file, so cmd/link does not treat the missing symbol as an error when creating go.o. Indeed, the definition for _cgo_gcc_Cfunc_sin will be provided to the host linker by foo2.cgo.o, which in turn will need the symbol 'sin'. cmd/link also processes the cgo_ldflag directives, so that it knows that the eventual host link command must include the -lm argument, so that the host linker will be able to find 'sin' in the math library. cmd/link Command Line Interface The go command and any other Go-aware build systems invoke cmd/link to link a collection of packages into a single binary. By default, cmd/link will present the same interface it does today: cmd/link main.a produces a file named a.out, even if cmd/link does so by invoking the host linker in external linking mode. By default, cmd/link will decide the linking mode as follows: if the only packages using cgo are those on a whitelist of standard library packages (net, os/user, runtime/cgo), cmd/link will use internal linking mode. Otherwise, there are non-standard cgo packages involved, and cmd/link will use external linking mode. The first rule means that a build of the godoc binary, which uses net but no other cgo, can run without needing gcc available. The second rule means that a build of a cgo-wrapped library like sqlite3 can generate a standalone executable instead of needing to refer to a dynamic library. The specific choice can be overridden using a command line flag: cmd/link -linkmode=internal or cmd/link -linkmode=external. In an external link, cmd/link will create a temporary directory, write any host object files found in package archives to that directory (renamed to avoid conflicts), write the go.o file to that directory, and invoke the host linker. The default value for the host linker is $CC, split into fields, or else "gcc". The specific host linker command line can be overridden using command line flags: cmd/link -extld=clang -extldflags='-ggdb -O3'. If any package in a build includes a .cc or other file compiled by the C++ compiler, the go tool will use the -extld option to set the host linker to the C++ compiler. These defaults mean that Go-aware build systems can ignore the linking changes and keep running plain 'cmd/link' and get reasonable results, but they can also control the linking details if desired. */