我有一个文件指针,它在调用 fread 之前有效,之后为 NULL,我真的很想知道为什么。
以下是相关代码:
244 // open the file for reading
245 clo->heap_file = open_file(heap_path, "rb");
443 // allocate memory to read a page from the file
444 file_page = safe_malloc(clo->page_size);
446 // read a page in to memory from the file
447 read_page(file_page, clo);
void *
safe_malloc(size_t size)
{
void * mem_block = NULL;
mem_block = calloc(1, size);
if (mem_block == NULL) {
fprintf(stderr, "ERROR: safe_malloc() failed to allocate memory.\n");
exit(EXIT_FAILURE);
}
return (mem_block);
}
FILE *
open_file(char * file_name, char * file_mode)
{
FILE * fp;
char * err_msg = ((strcmp(file_mode, "rb") == 0)
? "File not found"
: "File could not be created");
fp = fopen(file_name, file_mode);
/* Print an appropriate error message and exit if open failed */
if (fp == NULL) {
fprintf(stderr, "%s: %s\n", err_msg, file_name);
exit(EXIT_FAILURE);
}
return fp;
}
void
read_page(clo_t * clo, void * file_page)
{
fread(file_page, sizeof(size_t), clo->page_size, clo->heap_file);
if (ferror(clo->heap_file)) {
fprintf(stderr, "ERROR: could not read heap file!\n\n");
free(file_page);
destroy_clo(clo);
exit(EXIT_FAILURE);
}
}
GDB 跟踪:
(gdb) p clo->heap_file
$1 = (FILE *) 0x603070
(gdb) s
read_page (clo=0x6032d0, file_page=0x603010) at dbquery.c:331
331 fread(file_page, clo->page_size, 1, clo->heap_file);
(gdb) s
333 if (ferror(clo->heap_file)) {
(gdb) p clo->heap_file
$2 = (FILE *) 0x0
Valgrind 也没有表明我做错了什么......
我想我很擅长处理返回值,确保指针有效等,但这让我很难过。