从文件 fscanf 格式读取十六进制数据编译时警告

Question

I'm reading some data from a file. The format is stated tobe

ASCII text with UNIX-style line-endings, a series of 32-bit signed integers in hexadecimal.

eg

08000000

I'm using fscanf to read in this data.

long data_size;

FILE *fp;
fp=fopen("test01.bin", "r"); // open for reading
if (fp==0) {cerr << "Error openeing file"<<endl; return 1;}

fscanf(fp, "%x", &data_size);

Everything runs ok with my test file but I get the compile-time warning,

warning: format ‘%x’ expects type ‘unsigned int*’, but argument 3 has type ‘long int*’

however a hex value is unsigned and is being cast to a long dose this matter? As long will take the most significant bit as notifying the sign? Or will I end up with problems? Or am I well off the mark in my understanding?

Thanks

Answer 1

You should support the same pointer type as the warning states, or else you will run into serious troubles if you want to port your code to other architectures (eg: 64 bit architectures) where long has a different size than int . This is especially tricky if you are using pointers. (I once had a bug originating from exactly this problem)

Just use int data_size and you will be fine.

Answer 2

The problem is that %x requires an unsigned int * to read the value in, but you have a long * . <stdint.h> header provides value types with fixed length, and <inttypes.h> defines corresponding macros for use with printf , scanf , and their derivatives. I think it'll be better for you to fscanf the data into an int32_t variable using the macro provided by <inttypes.h> :

#include <inttypes.h>

...

int32_t data_size;
fscanf(fp, "%" SCNx32, &data_size);