Linux 为什么要动态链接？与静态链接的区别是什么？

// lib.c#include <stdio.h>
void func(int i) { printf("func %d \n", i);}

// Program.cvoid func(int i);
int main() { func(1); return 0;}

$ gcc -fPIC -shared -o lib.so lib.c$ gcc -o test Program.c ./lib.so$ ./test$ func 1

~/test$ readelf -l lib.so
Elf file type is DYN (Shared object file)Entry point 0x530There are 7 program headers, starting at offset 64
Program Headers: Type Offset VirtAddr PhysAddr FileSiz MemSiz Flags Align LOAD 0x0000000000000000 0x0000000000000000 0x0000000000000000 0x00000000000006e4 0x00000000000006e4 R E 0x200000 LOAD 0x0000000000000e10 0x0000000000200e10 0x0000000000200e10 0x0000000000000218 0x0000000000000220 RW 0x200000 DYNAMIC 0x0000000000000e20 0x0000000000200e20 0x0000000000200e20 0x00000000000001c0 0x00000000000001c0 RW 0x8 NOTE 0x00000000000001c8 0x00000000000001c8 0x00000000000001c8 0x0000000000000024 0x0000000000000024 R 0x4 GNU_EH_FRAME 0x0000000000000644 0x0000000000000644 0x0000000000000644 0x0000000000000024 0x0000000000000024 R 0x4 GNU_STACK 0x0000000000000000 0x0000000000000000 0x0000000000000000 0x0000000000000000 0x0000000000000000 RW 0x10 GNU_RELRO 0x0000000000000e10 0x0000000000200e10 0x0000000000200e10 0x00000000000001f0 0x00000000000001f0 R 0x1
Section to Segment mapping: Segment Sections... 00 .note.gnu.build-id .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt .init .plt .plt.got .text .fini .rodata .eh_frame_hdr .eh_frame 01 .init_array .fini_array .dynamic .got .got.plt .data .bss 02 .dynamic 03 .note.gnu.build-id 04 .eh_frame_hdr 05 06 .init_array .fini_array .dynamic .got

// Program.c#include <stdio.h>void func(int i);
int main() { func(1); sleep(-1); return 0;}

~/test$ ./test &[1] 126~/test$ func 1cat /proc/126/maps7ff2c59f0000-7ff2c5bd7000 r-xp 00000000 00:00 516391 /lib/x86_64-linux-gnu/libc-2.27.so7ff2c5bd7000-7ff2c5be0000 ---p 001e7000 00:00 516391 /lib/x86_64-linux-gnu/libc-2.27.so7ff2c5be0000-7ff2c5dd7000 ---p 000001f0 00:00 516391 /lib/x86_64-linux-gnu/libc-2.27.so7ff2c5dd7000-7ff2c5ddb000 r--p 001e7000 00:00 516391 /lib/x86_64-linux-gnu/libc-2.27.so7ff2c5ddb000-7ff2c5ddd000 rw-p 001eb000 00:00 516391 /lib/x86_64-linux-gnu/libc-2.27.so7ff2c5ddd000-7ff2c5de1000 rw-p 00000000 00:00 07ff2c5df0000-7ff2c5df1000 r-xp 00000000 00:00 189022 /mnt/d/wzq/wzq/util/test/lib.so7ff2c5df1000-7ff2c5df2000 ---p 00001000 00:00 189022 /mnt/d/wzq/wzq/util/test/lib.so7ff2c5df2000-7ff2c5ff0000 ---p 00000002 00:00 189022 /mnt/d/wzq/wzq/util/test/lib.so7ff2c5ff0000-7ff2c5ff1000 r--p 00000000 00:00 189022 /mnt/d/wzq/wzq/util/test/lib.so7ff2c5ff1000-7ff2c5ff2000 rw-p 00001000 00:00 189022 /mnt/d/wzq/wzq/util/test/lib.so7ff2c6000000-7ff2c6026000 r-xp 00000000 00:00 516353 /lib/x86_64-linux-gnu/ld-2.27.so7ff2c6026000-7ff2c6027000 r-xp 00026000 00:00 516353 /lib/x86_64-linux-gnu/ld-2.27.so7ff2c6227000-7ff2c6228000 r--p 00027000 00:00 516353 /lib/x86_64-linux-gnu/ld-2.27.so7ff2c6228000-7ff2c6229000 rw-p 00028000 00:00 516353 /lib/x86_64-linux-gnu/ld-2.27.so7ff2c6229000-7ff2c622a000 rw-p 00000000 00:00 07ff2c62e0000-7ff2c62e3000 rw-p 00000000 00:00 07ff2c62f0000-7ff2c62f2000 rw-p 00000000 00:00 07ff2c6400000-7ff2c6401000 r-xp 00000000 00:00 189023 /mnt/d/wzq/wzq/util/test/test7ff2c6600000-7ff2c6601000 r--p 00000000 00:00 189023 /mnt/d/wzq/wzq/util/test/test7ff2c6601000-7ff2c6602000 rw-p 00001000 00:00 189023 /mnt/d/wzq/wzq/util/test/test7fffee96f000-7fffee990000 rw-p 00000000 00:00 0 [heap]7ffff6417000-7ffff6c17000 rw-p 00000000 00:00 0 [stack]7ffff729d000-7ffff729e000 r-xp 00000000 00:00 0 [vdso]

dlopen()：打开动态链接库dlsym()：查找符号dlerror()：错误处理dlclose()：关闭动态链接库

#include <stdio.h>#include <dlfcn.h>
int main() {  void *handle; void (*f)(int); char *error;
 handle = dlopen("./lib.so", RTLD_NOW); if (handle == NULL) { printf("handle null \n"); return -1; } f = dlsym(handle, "func"); do { if ((error = dlerror()) != NULL) { printf("error\n"); break; } f(100); } while (0); dlclose(handle);
 return 0;}

$ gcc -o test program.c -ldl$ ./testfunc 100