为什么这个缓冲区指向不可寻址的字节？

Question

编辑：感谢repliers我已经改变了FREAD至（......的sizeof缓冲，1，...），但现在我得到这个错误的valgrind：

==2409== Invalid read of size 4 
==2409== at 0x51AB8D0: fread (iofread.c:41) 
==2409== by 0x4007B6: main (recover2.c:31) 
==2409== Address 0x5502000 is not stack'd, malloc'd or (recently) free'd 
==2409== 
==2409== Use of uninitialised value of size 8 
==2409== at 0x51B8787: _IO_sgetn (genops.c:495) 
==2409== by 0x51AB93E: fread (iofread.c:42) 
==2409== by 0x4007B6: main (recover2.c:31) 
==2409== 
==2409== Invalid read of size 8 
==2409== at 0x51B8787: _IO_sgetn (genops.c:495) 
==2409== by 0x51AB93E: fread (iofread.c:42) 
==2409== by 0x4007B6: main (recover2.c:31) 
==2409== Address 0x40 is not stack'd, malloc'd or (recently) free'd 
==2409== 
==2409== 
==2409== Process terminating with default action of signal 11 (SIGSEGV) 
==2409== Access not within mapped region at address 0x40 
==2409== at 0x51B8787: _IO_sgetn (genops.c:495) 
==2409== by 0x51AB93E: fread (iofread.c:42) 
==2409== by 0x4007B6: main (recover2.c:31) 
==2409== If you believe this happened as a result of a stack 
==2409== overflow in your program's main thread (unlikely but 
==2409== possible), you can try to increase the size of the 
==2409== main thread stack using the --main-stacksize= flag. 
==2409== The main thread stack size used in this run was 8388608. 

---

我是新来的所以我希望这是有道理的。我正在编写此代码以从文件中检索数据并将其复制到jpeg文件。该代码旨在通过其标题找到一个jpg文件，然后将其写入文件。该代码是：

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

int main(int argc, char* argv[]) 
{ 

FILE* file = fopen("card.raw", "r"); 
if (file == NULL) 
{ 
    printf("Could not open file!\n"); 
    return 1; 
} 

char title[7]; 
int currentImage = 0; 
uint8_t buffer[512]; 
FILE* img; 
while (fread(buffer, sizeof(buffer), 512, file) == 1) 
{ 
printf("found data!\n"); 
if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff) 
{ 
    if (buffer[3] == 0xe0 || buffer[3] == 0xe1 || buffer[3] == 0xe2 || buffer[3] == 0xe3 || buffer[3] == 0xe4 || buffer[3] == 0xe5 || buffer[3] == 0xe6 || buffer[3] == 0xe7 || buffer[3] == 0xe8 || buffer[3] == 0xe9 || buffer[3] == 0xea || buffer[3] == 0xeb || buffer[3] == 0xec || buffer[3] == 0xed || buffer[3] == 0xee || buffer[3] == 0xef) 
    { 
     printf("Found new jpg!\n"); 
     sprintf(title, "%03d.jpg", currentImage); 
     img = fopen(title, "a"); 
     currentImage++; 
     printf("size of buffer to print is %lu\n", sizeof(buffer)); 
     fwrite(buffer, sizeof(buffer), 1, img); 
     } 
} 
else if (currentImage > 0) 
{ 
     fwrite(buffer, sizeof(buffer), 1, img); 


} 

} 
} 

我得到一个分段错误，一旦发现有JPEG，并执行FWRITE，然后返回到while循环。

的Valgrind的错误是：

==1866== Syscall param read(buf) points to unaddressable byte(s) 
==1866== at 0x5228810: __read_nocancel (syscall-template.S:81) 
==1866== by 0x51B63B8: _IO_file_xsgetn (fileops.c:1438) 
==1866== by 0x51AB93E: fread (iofread.c:42) 
==1866== by 0x4007C3: main (recover2.c:31) 
==1866== Address 0xfff001000 is not stack'd, malloc'd or (recently) free'd 
==1866== 
==1866== Jump to the invalid address stated on the next line 
==1866== at 0x0: ??? 
==1866== Address 0x0 is not stack'd, malloc'd or (recently) free'd 
==1866== 
==1866== 
==1866== Process terminating with default action of signal 11 (SIGSEGV) 
==1866== Bad permissions for mapped region at address 0x0 
==1866== at 0x0: ??? 

我是新来这使仍在学习和记忆等，因此在理解它为什么出了错任何帮助，将不胜感激。

Answer 1

通过这样做

fread(buffer, sizeof(buffer), 512, file) 

你问fread阅读512块，每块是sizeof(buffer)字节长。即您正尝试将512 * 512 = 262144字节读入声明为uint8_t buffer[512]的数组中。这当然不适合。

如果你只是想将数据读入buffer阵列，这将是要么

fread(buffer, sizeof buffer, 1, file) 

或

fread(buffer, 1, sizeof buffer, file) 

，或者，如果你愿意的话，根据

fread(buffer, sizeof *buffer, sizeof buffer/sizeof *buffer, file) 

读取操作中您认为是“原子”数据块。

---

另外

sprintf(title, "%03d.jpg", currentImage); 

将产生至少7个字符长的字符串（例如001.jpg），这意味着title必须是长，以容纳零终止至少8个字符。但是，您的title被宣布为

char title[7]; 

这太小了。

Answer 2

作为每fread()的man page，签名是

size_t fread(void *ptr, size_t size, size_t nmemb, FILE * stream);

所述描述是

功能fread()读取数据的nmemb元素，每个size字节长，来自指向的流210，将它们存储在ptr给出的位置。

所以，你的代码应该是

while (fread(buffer, sizeof(buffer[0]), 512, file) == 1) 

否则，你就结束了要求读取和存储512个数每个大小的块，512个字节的，这是错误的，会导致缓冲区溢出，如valgrind报告。