[英]How to make sscanf read until '\0' character
我希望name保留所有字符,直到'\\0'
為止。
#include <stdio.h>
int main(){
char line[] = "1999-08-01,14.547,0.191,United Kingdom";
unsigned int year, month, day;
float temp, uncertainty;
char name[100];
sscanf(line, "%u - %u - %u, %f , %f , %s", &year, &month,
&day, &temp, &uncertainty, name);
printf("%u-%u-%u,%lf,%lf,%s\n", year, month, day, temp, uncertainty, name);
}
我可以像這樣做這項工作:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int main(){
char line[] = "1999-08-01,14.547,0.191,United Kingdom";
char* newline = malloc(strlen(line) + 2);
strcpy(newline, line);
newline[strlen(newline)] = '\n';
newline[strlen(newline)] = '\0';
unsigned int year, month, day;
float temp, uncertainty;
char name[100];
sscanf(line, "%u - %u - %u, %f , %f , %[^\n]", &year, &month,
&day, &temp, &uncertainty, name);
printf("%u-%u-%u,%lf,%lf,%s\n", year, month, day, temp, uncertainty, name);
}
但是我覺得這很不雅致。
sscanf
並不是最優雅的界面,但是它具有很多功能。 其中之一是能夠找出您在輸入字符串中的位置,該功能使您可以提取(或僅指向)“輸入的其余部分”。
例如,之后;
int nchar = -1;
int nfield = sscanf(line, "%u - %u - %u, %f , %f , %n", &year, &month,
&day, &temp, &uncertainty, &nchar);
nchar
將在名稱字段的line
中包含偏移量(除非它仍為-1,表示sscanf
無法與格式字符串匹配)。 如果那場延伸到結束line
,你可以接着直接(使用line + nchar
)或復制到不同的字符串檢查,這是不是太長了。
如果line
與其名稱相反,包含多行,並且您想要提取字符串直到換行符,則可以使用兩種%n
格式,並且中間使用%*[^\\n]
(星號會抑制復制,因此避免出現超限問題):
char name[NAME_MAX + 1];
int nstart = -1, nend = -1;
int nfield = sscanf(line, "%u - %u - %u, %f , %f , %n%*[^\n]%n", &year, &month,
&day, &temp, &uncertainty, &nstart, &nend);
if (nend > 0) {
if (nend - nstart <= NAME_MAX) {
memcpy(name, line + nstart, nend - nstart);
name[nend - nstart] = 0;
}
else {
/* name is too long */
}
}
else if (nstart > 0) {
/* Name was 0 bytes long. Sscanf requires that %[ match at least
* one character; if not, it fails the scan.
*/
name[0] = 0; /* Perhaps you wanted to signal an error
}
else {
/* Line didn't match format */
}
顯然,我可以避免使用固定長度的緩沖區,而不必在知道大小的情況下通過動態分配緩沖區來檢查溢出:
char* name = NULL;
// ...
if (nend > 0)
name = strndup(line + nstart, nend - nstart);
// or, if you don't like strndup
// name = malloc(nend - nstart + 1);
// memcpy(name, line + nstart);
// name[nend - nstart] = 0;
如果您真正想要的是動態分配的字符串,並且您具有兼容Posix的sscanf
,則可以使用m
length修飾符來避免這種麻煩,這是最簡單的全方位解決方案。
char* name = NULL;
int nfield = sscanf(line, "%u - %u - %u, %f , %f , %m[^\n]", &year, &month,
&day, &temp, &uncertainty, &name);
有關詳細信息,請參見您的sscanf
聯機幫助頁。 在動態分配name
所有情況下,完成name
操作后,請不要忘記釋放它。
這應該工作:
char line[] = "1999-08-01,14.547,0.191,United Kingdom";
unsigned int year, month, day;
float temp, uncertainty;
char name[100];
sscanf(line, "%u - %u - %u, %f , %f , %99[^\n]", &year, &month,
&day, &temp, &uncertainty, name);
printf("%u-%u-%u,%lf,%lf,%s\n", year, month, day, temp, uncertainty, name);
不會找到'\\n'
,但是由於不會達到99的限制,因此sscanf
將繼續讀取直到字符串結束標記。
幾天前,我正在閱讀2003年版“ Unix系統編程:通信,並發,線程”一書的第2章,我研究了一個示例,該示例導致使用自定義分隔符將字符串分成令牌(它可以是或_或空格或其他任何字符)。 它使用了strtok()C庫函數。 這是經過調整以某種方式滿足您需求的示例。 我將提供2個文件:
makeargv.c
#include <errno.h>
#include <stdlib.h>
#include <string.h>
/* frees all the memory that was allocated by makeargv */
void freemakeargv(char **argv)
{
if (argv == NULL)
return;
if (*argv != NULL)
free(*argv);
free(argv);
}
/* Now the function that breaks string s into tokens */
int makeargv(const char *s, const char *delimiters, char ***argvp)
{
int error; int i;
int numtokens;
const char *snew;
char *t;
if ((s == NULL) || (delimiters == NULL) || (argvp == NULL))
{ errno = EINVAL; return -1; }
*argvp = NULL; /* so that a failed call to malloc,will leave it NULL */
/* now we consume any initial delimiters characters of input s */
snew = s + strspn(s, delimiters); /* snew is real start of string */
if ((t = malloc(strlen(snew) + 1)) == NULL)
return -1;
strcpy(t, snew);
numtokens = 0;
if (strtok(t, delimiters) != NULL) /* count number of tokens in s */
for (numtokens = 1; strtok(NULL, delimiters) != NULL; numtokens++) ;
/* next,create argument array for ptrs to the tokens */
if ((*argvp = malloc((numtokens + 1)*sizeof(char *))) == NULL)
{
error = errno;
free(t);
errno = error;
return -1;
}
/* now insert pointers-to-tokens into the argument array */
if (numtokens == 0) free(t);
else
{
strcpy(t, snew);
**argvp = strtok(t, delimiters);
for (i = 1; i < numtokens; i++)
*((*argvp) + i) = strtok(NULL, delimiters);
}
*((*argvp) + numtokens) = NULL; /* append final NULL pointer */
return numtokens;
}
main.c
#include <stdio.h>
#include <stdlib.h>
int makeargv(const char *s, const char *delimiters, char ***argvp);
void freemakeargv(char **argv);
int main()
{
char delim[] = ",";
int i, numtokens;
char **myargv; /* memory will be allocated dynamicaly, and has to bee freed before exit */
char line[] = "1999-08-01,14.547,0.191,United Kingdom-UK";
if ((numtokens = makeargv(line, delim, &myargv)) == -1) /* nakeargv() allocates memory for myargv, it may fail */
{
fprintf(stderr, "Failed to construct an argument array for %s\n", line);
return 1;
}
printf(" The argument array contains:\n");
for (i = 0; i < numtokens; i++)
printf("%d:%s\n", i, myargv[i]);
freemakeargv(myargv); /* do not forget to free the memory! */
return 0;
}
gcc -Wall -std = c99 -o標記程序main.c makeargv.c
並運行它
./令牌
@rici的好方法的變體:
如何使sscanf讀取到'\\ 0'字符
使用"%n"
來應對“其余部分”。
"%n"
記錄到該點為止的掃描偏移量(如果達到此距離)。
%*[^\\n]
進行掃描,但不會按照OP的“保留在行中”將所有字符保存到一個'\\n'
使用它來分配結束字符串。
// Some untested code
typedef struct {
unsigned int year, month, day;
float temp, uncertainty;
char *name;
} data_T;
// return 0 on success
int foo(data_T *dest, const char *line) {
int start = 0;
int end = 0;
memset(dest, 0, sizeof *dest); // zero `dest`
sscanf(line, "%u - %u - %u, %f , %f , %n%*[^\n]%n",
&dest->year, &dest->month, &dest->day, &dest->temp, &dest->uncertainty,
&start, &end);
if (start == 0) {
// line did nor scan properly, return error
return 1;
}
if (end == 0) {
end = start; // there was no non-white-space text after the `,`
}
size_t len = end - start;
dest->name = malloc(len + 1u);
if (dest->name == NULL) {
// Out of memory
return 1;
}
memcpy(dest->name, line + start, len);
dest->name[len] = '\0';
printf("%u-%u-%u,%f,%f, %s\n",
dest->year, dest->month, dest->day, dest->temp, dest->uncertainty, dest->name);
return 0; // be sure to free dest->name when done with it.
}
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