fread跳过前8个字符

Question

I am trying to read in a file, hello.ms containing the following:

Hello World!

Using this C code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int store_file(char* file_dir, char** buffer)
{
    FILE* file;
    long lSize;
    size_t result;
    char* tempBuffer;

    file = fopen(file_dir, "r");
    if (file==NULL)
    {
        fputs("File error.", stderr);
        exit(1);
    }

    fseek(file, 0, SEEK_END);
    lSize = ftell(file);
    rewind(file);

    tempBuffer = (char*)malloc(sizeof(char)*lSize);
    if (tempBuffer == NULL)
    {
        fputs("Memory error.", stderr);
        exit(2);
    }

    result = fread(tempBuffer, 1, lSize, file);
    if (result != lSize)
    {
        fputs("Reading error.", stderr);
        exit(3);
    }

    *buffer = tempBuffer;
    free(tempBuffer);
    fclose(file);

    return lSize;
}

void fsa_cycle(char* file_dir)
{
    char* buffer;
    int bufferSize = store_file(file_dir, &buffer);

    fwrite(buffer, sizeof(char), bufferSize, stdout);
}

int main(int argc, char* argv[])
{
    if(argc < 2)
    {
        printf("\nSyntax: %s <file-name>\n\n", argv[0]);
        exit(1);
    }

    fsa_cycle(argv[1]);

    return 0;
}

It compiles perfectly fine. No warnings or errors. But it only outputs rld!

When I add 8 spaces to the hello.ms file, it then reads Hello World!

Can anyone tell me why this is happening? I tried writing fseek(file, 0, SEEK_CUR) instead of rewind but that didn't work either. I couldn't find any help on Google that didn't suggest that the programmer forgot to use fseek to get to the beginning. Any assistance would be awesome!

Answer 1

You are using free to release tempBuffer memory, but that memory block is in fact assigned to buffer .

free might work different with different compilers, but mine works as yours, just remove free(tempBuffer);

Answer 2

You are freeing the buffer too early.
You should free the memory after you use the buffer. The line:

fwrite(buffer, sizeof(char), bufferSize, stdout);

Answer 3

I ran it in gdb and things go wrong after the free(tempBuffer);

(gdb) 31 if (result != lSize)

(gdb) print tmpBuffer

No symbol "tmpBuffer" in current context.

(gdb) print tempBuffer

$3 = 0x602250 "Hello World!\\n"

(gdb) n

37 *buffer = tempBuffer;

(gdb)

38 free(tempBuffer);

(gdb) print buffer

$4 = (char **) 0x7fffffffe180

(gdb) print *buffer

$5 = 0x602250 "Hello World!\\n"

(gdb) n

39 fclose(file);

(gdb)

41 return lSize;

(gdb) print *buffer

$6 = 0x602250 ""

(gdb) print tempBuffer

$7 = 0x602250 ""

Answer 4

try to use in this way

    char *buff;
buff = calloc(sizeOfFileInBytes + 1, sizeof(char));//replace sizeOfFileInBytes with file size (use stat to get file size)
if (!buff) {
    printf("error : %s", strerror(errno));
    fclose(f);
}
fread(buff, sizeOfFileInBytes, 1, f);

fclose(f);
printf("%s\n", (char *) buff);//lets echo what we get from fread;
free(buff);//lets return memory to heap