[英]C Systems Program - Read/Write Issues During Copy
我正在編寫一個C程序,它從標准的UNIX存檔中提取並創建它存儲的文件。
這是一個例子,如果我在vim中打開它會是什么樣的:
!<arch>
yo 1382105439 501 20 100644 10 `
test1 lol
yo2 1382105444 501 20 100644 10 `
test2 lol
...其中“test1 lol”和“test2 lol”是每個文件的內容,“yo”和“yo2”是兩個不同的文件名,其余的是以對應於標准ar.h的格式存儲的元數據(在這里閱讀更多內容: http : //www.lehman.cuny.edu/cgi-bin/man-cgi?ar.h + 3 )
無論如何,我仍在編寫函數的過程中,但這是我到目前為止所擁有的:
static void extract_files (int argc, char *argv[])
{
int fd;
int new_file_fd;
int num_read = 0;
int new_file_size;
struct ar_hdr current_header;
char name_buffer[16];
char date_buffer[12];
char uid_buffer[6];
char gid_buffer[6];
char mode_buffer[8];
char size_buffer[10];
char fmag_buffer[2];
// grab the fd #
fd = open(argv[2], O_RDWR | O_CREAT, 0666);
// go to the first header
lseek(fd, SARMAG, SEEK_CUR);
// store the number of bits read in a struct current_header
// until its size equal to the size of the entire
// header, or in other words, until the entire
// header is read
while ((num_read = read(fd, (char*) ¤t_header,
sizeof(struct ar_hdr))) == sizeof(struct ar_hdr))
{
// scans the current string in header and stores
// in nameStr array
sscanf(current_header.ar_name, "%s", name_buffer);
sscanf(current_header.ar_date, "%s", date_buffer);
sscanf(current_header.ar_uid, "%s", uid_buffer);
sscanf(current_header.ar_gid, "%s", gid_buffer);
int mode;
sscanf(current_header.ar_mode, "%o", &mode);
sscanf(current_header.ar_size, "%s", size_buffer);
int size = atoi(size_buffer);
sscanf(current_header.ar_fmag, "%s", fmag_buffer);
// Create a new file
new_file_fd = creat(name_buffer, mode);
// Grab new file size
new_file_size = atoi(size_buffer);
int io_size; // buffer size
char buff[size];
int read_cntr = 0;
// from copy.c
while ((io_size = read (fd, buff, new_file_size)) > 0)
{
read_cntr++;
if (read_cntr > new_file_size)
break;
write (new_file_fd, buff, new_file_size);
}
close(new_file_fd);
printf("%s\n", name_buffer);
printf("%s\n", date_buffer);
printf("%s\n", uid_buffer);
printf("%s\n", gid_buffer);
printf("%s\n", mode_buffer);
printf("%s\n", size_buffer);
printf("%s\n", fmag_buffer);
/* Seek to next header. */
lseek(fd, atoi(current_header.ar_size) + (atoi(current_header.ar_size)%2), SEEK_CUR);
}
}
我遇到的問題在於上面代碼中的第二個while循環:
// from copy.c
while ((io_size = read (fd, buff, new_file_size)) > 0)
{
read_cntr++;
if (read_cntr > new_file_size)
break;
write (new_file_fd, buff, new_file_size);
}
由於某種原因,在此while循環中寫入的文件不會運行到write指定的長度。 標准read()/ write()的第三個參數應該是要寫入的字節數。 但是出於某種原因,我的代碼導致整個存檔被讀入並寫入第一個文件。
如果我打開生成的“yo”文件,我發現整個存檔文件已寫入其中
test1 lol
yo2 1382105444 501 20 100644 10 `
test2 lol
在讀取10個字節並給出預期結果“test1 lol”之后,而不是終止。
我還可以確認“new_file_size”值確實為10.所以我的問題是:我在讀取循環時讀錯了什么?
注意:預期輸入將是命令行參數,類似於:./ extractor.c -x name_of_archive_file
我認為我需要在此函數中處理的唯一相關信息是存檔文件的名稱,我在extract_files的開頭獲取了fd。
補充:雜項 - 運行時的輸出:
yo
1382105439
501
20
X
10
`
正如你所看到的,它永遠不會看到yo2文件或打印出它的標題,因為它會被寫入“yo”之前可能發生...因為這個流浪而循環:(
你的while()
循環應該在它之后有大括號( { ... }
),否則你只是在不做任何其他事情的情況下遞增read_cntr
。
你讀取一個值size_buffer,並將其分配給size和new_file_size,你還創建一個相同大小的buffer[size]
,
int size = atoi(size_buffer);
sscanf(current_header.ar_fmag, "%s", fmag_buffer);
//...
new_file_size = atoi(size_buffer);
//...
char buff[size];
Read返回范圍[0..new_file_size]
的ssize_t字節數,你設置為io_size,意識到read(2)可能return < new_file_size
字節,這就是你需要while循環的原因。 因此,您需要編寫已閱讀的所有內容,直到達到寫入限制為止。 我已經做了一些評論來指導你。
// from copy.c
while ((io_size = read (fd, buff, new_file_size)) > 0)
{
read_cntr++;
//perhaps you mean read_cntr += io_size;
//you probably mean to write io_size bytes here, regardless
//write(new_file_fd, buff, io_size);
if (read_cntr > new_file_size) //probably you want >= here
break;
//you may have broke before you write...
write (new_file_fd, buff, new_file_size);
}
這個副本的一個更典型的習慣用法是你選擇一個讀/寫緩沖區大小,比如4*1024 (4K)
, 16*1024 (16K)
等,並讀取塊大小,直到你剩下的塊大小少於; 例如,
//decide how big to make buffer for read()
#define BUFSIZE (16*1024) //16K
//you need min(
#define min(x,y) ( ((x)<(y)) ? (x) : (y) )
ssize_t fdreader(int fd, int ofd, ssize_t new_file_size )
{
ssize_t remaining = new_file_size;
ssize_t readtotal = 0;
ssize_t readcount;
unsigned char buffer[BUFSIZE];
for( ; readcount=read(fd,buffer,min(sizeof(buffer),remaining)); )
{
readtotal += readcount;
if( readcount > remaining ) //only keep remaining
readcount = remaining;
write( ofd, buffer, readcount);
remaining -= readcount;
if( remaining <= 0 ) break; //done
}
return readtotal;
}
嘗試這個,
#include<stdio.h>
#include<stdlib.h>
void usage(char*progname)
{
printf("need 2 files\n");
printf("%s <infile> <outfile>\n",progname);
}
//decide how big to make buffer for read()
#define BUFSIZE (16*1024) //16K
//you need min(
#define min(x,y) ( ((x)<(y)) ? (x) : (y) )
ssize_t fdreader(int fd, int ofd, ssize_t new_file_size )
{
ssize_t remaining = new_file_size;
ssize_t readtotal = 0;
ssize_t readcount;
unsigned char buffer[BUFSIZE];
for( ; readcount=read(fd,buffer,min(sizeof(buffer),remaining)); )
{
readtotal += readcount;
if( readcount > remaining ) //only keep remaining
readcount = remaining;
write( ofd, buffer, readcount);
remaining -= readcount;
if( remaining <= 0 ) break; //done
}
return readtotal;
}
int main(int argc,char**argv)
{
int i=0; /* the infamous 'i' */
FILE*infh;
FILE*outfh;
if( argc < 3 )
{
usage(argv[0]);
return 0;
}
printf("%s %s\n",argv[1],argv[2]); fflush(stdout);
if( !(infh=fopen(argv[1],"r")) )
{
printf("cannot open %s\n",argv[2]); fflush(stdout);
return(2);
}
if( !(outfh=fopen(argv[2],"w+")) )
{
printf("cannot open %s\n",argv[3]); fflush(stdout);
return(3);
}
int x = fdreader(fileno(infh), fileno(outfh), 512 );
return 0;
}
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