Binary file Reading, add extra characters in c? [duplicate]

Question

I try to read binary(executeable) files, butsome problem as it prints extra character(0xffffff) after every few bytes. But this code work perfectly for text file(Non-binary). The code is:

int main(int argc, char **argv) { uint64_t i=0; FILE * fp=fopen(argv[1],"rb"); uint8_t a; long int size=0; char *msg; size=findSize(argv[1]); msg=(char *)malloc(size+1); while((a=fgetc(fp))!=EOF) { printf("%02x",a); msg[i]=(char)a; i++; if(i==size) break; } i=0; for(i=0;i<size;i++) { printf("%x",msg[i]); } return 0; }

When i try to print the value(s) of a, It works perfectly, while printing msg[i], it prints extra bytes i,e:

Orginal bytes: 0xa31c4b1ab66b900

Output with extra bytes: 0xa31c4ffffffb1ffffffafffffffb66ffffffb900

fgetc() returns an int value. So, a should be defined as an int. — wildplasser
– wildplasser, Commented Aug 7, 2019 at 10:27
... which is especially important when reading binary data, to distinguish EOF from 0xFF. — Weather Vane
– Weather Vane, Commented Aug 7, 2019 at 10:28
Default parameter promotion converts a char to an int when passing to printf. This include sign extensions for negative values. — Gerhardh
– Gerhardh, Commented Aug 7, 2019 at 10:30
and the fgetc thing is relevant when you're working with binary files — Antti Haapala
– Antti Haapala, Commented Aug 7, 2019 at 10:33
also long might not be sufficiently large to contain every file size, but it perhaps wouldn't be a problem. — Antti Haapala
– Antti Haapala, Commented Aug 7, 2019 at 10:34

jpa · Accepted Answer · 2019-08-07 10:29:57Z

char *msg;

Your msg variable has datatype char, which is probably signed on your platform. That means, it represents values from -128 to +127.

printf("%x",msg[i]);

Any arguments to printf() that have an integer datatype smaller than int will be expanded to int. That conversion will keep the arithmetic value, so -128 which is 0x80 in char, will expand to 0xFFFFFF80 for int.

To avoid this, you need to store or cast the values to unsigned char first. The uint8_t type is same as unsigned char and easier to read, so:

printf("%x", (uint8_t)msg[i]);

That way the numeric value will range from 0 to 255, like you would expect, and print as 0x00 to 0xFF even after the implicit conversion to int.

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