CODEWITHSUNDEEP
cfgetc
Adam Muller
Adam Muller

Reputation: 7

fgetc misses bytes while reading a file

I use the function fgetc to read each byte of a file, and then write it with printf.

I just noticed that sometimes, fgetc just miss some bytes, when I compare my result with a hex editor.

For example, the first mistake starts around the 118th byte, and a lot of other mistakes randomly ...

Somebody ever experienced this?

This is the code (Windows)

char main(int argc, char* argv[]) {
    FILE* fdIn;
    FILE* fdOut;
    long size = 0;
    long i = 0;
    char c = 0;
    if (argc == 3) {
        if ((fdIn = fopen(argv[1], "rt")) == NULL) {
            printf("FAIL\n");
            return 0;
        }
        if ((fdOut = fopen(argv[2], "w+")) == NULL) {
            printf("FAIL\n");
            return 0;
        }
        fseek(fdIn, 0L, SEEK_END);
        size = ftell(fdIn);
        fseek(fdIn, 0L, 0);
        fprintf(fdOut, "unsigned char shellcode[%ld] = {", size);
        while (i < size) {
            c = fgetc(fdIn);
            if (!(i % 16))
                fprintf(fdOut, "\n\t");
            fprintf(fdOut, "0x%02X", (unsigned char)c);
            if (i != size - 1)
                fprintf(fdOut, ", ");
            i++;
        }
        fprintf(fdOut, "\n};\n");
        fclose(fdIn);
        fclose(fdOut);
        printf("SUCCESS");
        system("PAUSE");
    }
    return 0;
}

Upvotes: 0

Views: 1622

Answers (1)

chux
chux

Reputation: 153457

Open the file in binary mode.

// if ((fdIn = fopen((char*)argv[1], "rt")) == NULL) { 
//                                 >.< 
if ((fdIn = fopen((char*)argv[1], "rb")) == NULL) {

In text mode, and likely a Windows based machine given the "rt", a '\r', '\n' pair is certainly translated into '\n'. IAC, no translations are needed for OP's goal of a hex dump.

2nd issue: fgetc() returns an int in the range of unsigned char or EOF. Use type int to distinguish EOF from all data input.

// char c = 0;
int c = 0;
...
c = fgetc(fdIn);

// also add
if (c == EOF) break;

Upvotes: 2

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