What is cgo in Go and what are its performance trade-offs?

cgo allows Go programs to call C functions and vice versa. It is used for binding to C libraries (OpenSSL, SQLite, CUDA), OS system calls not exposed in the Go standard library, and legacy C codebases.

// Simple cgo example
package main

// #include 
// #include 
// char* greet(const char* name) {
//     char* result = malloc(100);
//     snprintf(result, 100, "Hello, %s!", name);
//     return result;
// }
import "C"
import (
    "fmt"
    "unsafe"
)

func main() {
    cname := C.CString("World")     // Go string → C string (malloc)
    defer C.free(unsafe.Pointer(cname)) // must free C memory!

    cresult := C.greet(cname)           // call C function
    result := C.GoString(cresult)        // C string → Go string (copy)
    C.free(unsafe.Pointer(cresult))

    fmt.Println(result) // Hello, World!
}

// cgo overhead per call:
// 60-200 ns per C call (vs ~1 ns for a plain Go function call)
// The runtime must save goroutine state, switch stack frames, etc.

// Cross-compilation challenge:
// CGO_ENABLED=0 disables cgo and allows full static binary cross-compilation
// CGO_ENABLED=1 (default) requires a C compiler on the target platform

// Alternatives to cgo:
// - Pure Go implementations (avoid cgo entirely)
// - WASI / WebAssembly for sandboxed native code
// - gRPC subprocess calling a C binary

Key costs of cgo: (1) each C call is ~60–200 ns overhead — avoid calling C in tight loops. (2) goroutines calling C must have the C function run on an OS thread — the Go scheduler complexity increases. (3) C memory is not managed by the Go GC — you must explicitly free it. (4) cross-compilation is much harder with cgo enabled.

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