fwrite doesn't write properly

Question

I've been trying to write a simple string to a file in C with fwrite().

When I define a simple string as:

char str[]="Hello World!";

and write it to a file as:

fwrite(str, sizeof(char), sizeof(str), fp);

Everything seems to work fine.

But as I make a custom string with sprintf_s, the output file I get is:

﻾﻾﻾﻾﻾﻾﻾﻾﻾﻾﻾﻾畒湮湩⁧湁祬敺浉条⹥硥⁥湯椠杭〰⸱灪㩧爠

Though printf func will print the string correctly

The Complete Code:

#define MAX_ELEMENT_LENGTH 512


void GenerateResultLine(char *resultLineBuffer, int maxLength,
                            const char *result) {
    const char * TEMPLATE= "Running Result: %s";
    sprintf_s(resultLineBuffer, maxLength, TEMPLATE, result);
}


void PrintResultToFile(char* exitCode,  FILE* resultFile) {
    char    resultLine[ MAX_ELEMENT_LENGTH ];

    GenerateResultLine(resultLine, MAX_ELEMENT_LENGTH, exitCode);
    printf("LINE: %s\n",resultLine);


    fwrite(resultLine, sizeof(char), sizeof(resultLine), resultFile);

}

Does anyone have any idea why?? Thanks

Answer 1

Code is writing uninitialized data to the file

// Code writes entire buffer to the file,
// even though only the leading bytes are defined via a previous sprintf_s()
fwrite(resultLine, sizeof(char), sizeof(resultLine), resultFile);

// Instead
size_t n = strlen(resultLine);
n++; // If the final `\0` is desired.
if (n != fwrite(resultLine, sizeof(char), n, resultFile)) {
  handle_error();
}