data type conversion - converting uint8_t[4] to uint32_t and back again in C -


i wrote test convert uint8[4] uint32_t, how convert output of u32 original uint8[4]? assume can done same way via bit shifting, i'm not sure how...

uint32_t u32(uint8_t b[4]){   uint32_t u;   u = b[0];   u = (u  << 8) + b[1];   u = (u  << 8) + b[2];   u = (u  << 8) + b[3];   return u; }  void p(uint32_t value){         printf("\nuint32: %u\n",value); }  int main(){  uint8_t b[4];  char a[4] = "test";  char tmp[32];   int i;  for(i=0;i<4;i++){    b[i] = a[i];    sprintf(tmp,"%s%u",tmp,b[i]);  }   printf("uint8: ");   for(i=0;i<4;i++)    printf("%u",b[i]);  uint32_t t2 = u32(b);  p(t2);  return 0; }

i tried little bit different , using known functions , not self implemented. main point teach bytes lay in memory interpretation give them depends on context. note there endianness issues 1 should take care. best lesson use bit-wise operations.

#include <stdint.h> #include <stdio.h> #include <arpa/inet.h> int main() {         uint8_t a[4]={0x1,0x2,0x3,0x4};         uint32_t b = *((uint32_t*) a);         /* turning array unit32 */         printf("0x%x\n",htonl(b));         /* turn uint32 array */         uint32_t c = htonl(b);         uint8_t d[4] = {0};         printf("0x%x\n",c);         (int i=0; i<4 ;++i)                 d[i] = ((uint8_t*)&c)[3-i];         (int i=0; i<4 ;++i)                 printf("0x%x\n",d[i]);         return 0; }

the output is:

0x1020304 0x1020304 0x1 0x2 0x3 0x4

disassembling [you can load program using lldb or gdb]:

root# otool -tv a.out a.out: (__text,__text) section _main: 0000000100000e50    pushq   %rbp 0000000100000e51    movq    %rsp, %rbp 0000000100000e54    subq    $0x30, %rsp 0000000100000e58    movl    $0x0, 0xfffffffffffffffc(%rbp) 0000000100000e5f    movl    0x12b(%rip), %eax 0000000100000e65    movl    %eax, 0xfffffffffffffff8(%rbp) 0000000100000e68    movl    0xfffffffffffffff8(%rbp), %eax 0000000100000e6b    movl    %eax, 0xfffffffffffffff4(%rbp) 0000000100000e6e    movl    0xfffffffffffffff4(%rbp), %edi 0000000100000e71    callq   0x100000f50 0000000100000e76    leaq    0x117(%rip), %rdi 0000000100000e7d    movl    %eax, %esi 0000000100000e7f    movb    $0x0, %al 0000000100000e81    callq   0x100000f66 0000000100000e86    movl    0xfffffffffffffff4(%rbp), %edi 0000000100000e89    movl    %eax, 0xffffffffffffffe0(%rbp) 0000000100000e8c    callq   0x100000f50 0000000100000e91    movl    $0x0, %esi 0000000100000e96    movabsq $0x4, %rdx 0000000100000ea0    leaq    0xffffffffffffffec(%rbp), %rcx 0000000100000ea4    movl    %eax, 0xfffffffffffffff0(%rbp) 0000000100000ea7    movq    %rcx, %rdi 0000000100000eaa    callq   0x100000f60 0000000100000eaf    leaq    0xde(%rip), %rdi 0000000100000eb6    movl    0xfffffffffffffff0(%rbp), %esi 0000000100000eb9    movb    $0x0, %al 0000000100000ebb    callq   0x100000f66 0000000100000ec0    movl    $0x0, 0xffffffffffffffe8(%rbp) 0000000100000ec7    movl    %eax, 0xffffffffffffffdc(%rbp) 0000000100000eca    cmpl    $0x4, 0xffffffffffffffe8(%rbp) 0000000100000ed1    jge 0x100000efe 0000000100000ed7    movl    $0x3, %eax 0000000100000edc    subl    0xffffffffffffffe8(%rbp), %eax 0000000100000edf    movslq  %eax, %rcx 0000000100000ee2    movb    0xfffffffffffffff0(%rbp,%rcx), %dl 0000000100000ee6    movslq  0xffffffffffffffe8(%rbp), %rcx 0000000100000eea    movb    %dl, 0xffffffffffffffec(%rbp,%rcx) 0000000100000eee    movl    0xffffffffffffffe8(%rbp), %eax 0000000100000ef1    addl    $0x1, %eax 0000000100000ef6    movl    %eax, 0xffffffffffffffe8(%rbp) 0000000100000ef9    jmpq    0x100000eca 0000000100000efe    movl    $0x0, 0xffffffffffffffe4(%rbp) 0000000100000f05    cmpl    $0x4, 0xffffffffffffffe4(%rbp) 0000000100000f0c    jge 0x100000f3c 0000000100000f12    leaq    0x7b(%rip), %rdi 0000000100000f19    movslq  0xffffffffffffffe4(%rbp), %rax 0000000100000f1d    movzbl  0xffffffffffffffec(%rbp,%rax), %esi 0000000100000f22    movb    $0x0, %al 0000000100000f24    callq   0x100000f66 0000000100000f29    movl    %eax, 0xffffffffffffffd8(%rbp) 0000000100000f2c    movl    0xffffffffffffffe4(%rbp), %eax 0000000100000f2f    addl    $0x1, %eax 0000000100000f34    movl    %eax, 0xffffffffffffffe4(%rbp) 0000000100000f37    jmpq    0x100000f05 0000000100000f3c    movl    $0x0, %eax 0000000100000f41    addq    $0x30, %rsp 0000000100000f45    popq    %rbp 0000000100000f46    ret 0000000100000f47    nopw    (%rax,%rax) __osswapint32: 0000000100000f50    pushq   %rbp 0000000100000f51    movq    %rsp, %rbp 0000000100000f54    movl    %edi, 0xfffffffffffffffc(%rbp) 0000000100000f57    movl    0xfffffffffffffffc(%rbp), %edi 0000000100000f5a    bswapl  %edi 0000000100000f5c    movl    %edi, %eax 0000000100000f5e    popq    %rbp 0000000100000f5f    ret

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