通过取消引用将字符串与 uint32_t 进行比较

Question

Say for instance I wanted to parse input from the user/stdin, this input was a string ( char* / char [] ). I know I can typecast a string's memory address pointer to a uint32_t pointer and dereference said uint32_t pointer to extract a unsigned int value which is the same across most architectures (that I've tested). Is this safe? Can it lead to bugs/undefined behavior? ie just in general a bad idea?

Example:

#include <stdio.h>
#include <stdint.h>

int main(void) {
    char data[] = "status";
    printf("%u\n", *(uint32_t*)data);

    return 0;
}

Will result in 1952543859 on 64bit and 32bit (windows & linux, not tested on MAC/unix system)

Is it safe to do the below operation?:

char str_from_input[] = "status";
if (*(uint32_t*)str_from_input == 1952543859)
   //... execute some code oriented around "status".

My obvious aim is to reduce computations by not repeatedly calling string parsing functions like strcmp and strncmp by a direct uint32_t check/if statement.

Answer 1

… typecast a string's memory address pointer to a uint32_t pointer… Is this safe?

No. If the string is not properly aligned for a uint32_t , the behavior of the cast is not defined, per C 2018 6.3.2.3 7: “… If the resulting pointer is not correctly aligned for the referenced type, the behavior is undefined…”

Dereferencing the resulting pointer accesses the string using a type that is inappropriate for it, violating the aliasing rule in 6.5 7, and the behavior is not defined by the C standard.

Can it lead to bugs/undefined behavior?

Yes.

ie just in general a bad idea?

Yes.

Is it safe to do the below operation?:

No.

To reinterpret the bytes in a string as a uint32_t , use:

uint32_t t;
memcpy(&t, string, sizeof t);
printf("%" PRIu32 "\n", t);

( memcpy is declared in <string.h> , and PRIu32 is declared in <inttypes.h> .)

To copy the bytes of a uint32_t into a string, use memcpy(string, &t, sizeof t); .

A good C compiler is likely to replace the memcpy calls with direct data move instructions, if feasible.

My obvious aim is to reduce computations by not repeatedly calling string parsing functions like strcmp and strncmp by a direct uint32_t check/if statement.

Avoid using kludges. Write clear code and let the compiler optimize.