如何检查两个格式字符串是否兼容? [英] How to check that two format strings are compatible?
问题描述
示例:
"Something %d" and "Something else %d" // Compatible
"Something %d" and "Something else %f" // Not Compatible
"Something %d" and "Something %d else %d" // Not Compatible
"Something %d and %f" and "Something %2$f and %1$d" // Compatible
我认为应该有一些C函数，但是没有得到任何相关的搜索结果.我的意思是编译器正在检查格式字符串和参数是否匹配，因此已经编写了用于检查此格式的代码.唯一的问题是我怎么称呼它.
I figured there should be some C function for this, but I'm not getting any relevant search results. I mean the compiler is checking that the format string and the arguments match, so the code for checking this is already written. The only question is how I can call it.
我正在使用Objective-C，因此如果有针对Objective-C的解决方案也很好.
I'm using Objective-C, so if there is an Objective-C specific solution that's fine too.
推荐答案
检查2个printf()格式字符串是否兼容是格式解析的一项工作.
Checking if 2 printf() format strings are compatible is an exercise in format parsing.
C至少没有标准的运行时比较功能，例如:
C, at least, has no standard run-time compare function such as:
int format_cmp(const char *f1, const char *f2); // Does not exist
像"%d %f"和"%i %e"这样的格式显然是兼容的，因为它们都希望使用int和float/double.注意:float提升为double，而short和signed char提升为int.
Formats like "%d %f" and "%i %e" are obviously compatible in that both expect an int and float/double. Note: float are promoted to double as short and signed char are promoted to int.
格式"%*.*f"和"%i %d %e"是兼容的，但并不明显:都期望int，int和float/double.
Formats "%*.*f" and "%i %d %e" are compatible, but not obvious: both expect an int,int and float/double.
格式"%hhd"和"%d"都希望使用int，即使第一个在打印前将其值强制转换为signed char.
Formats "%hhd" and "%d" both expect an int, even though the first will have it values cast to signed char before printing.
格式"%d"和"%u"不兼容.即使许多系统的表现都符合预期.注意:通常，char将升级为int.
Formats "%d" and "%u" are not compatible. Even though many systems will behaved as hoped. Note: Typically char will promote to int.
格式"%d"和"%ld"严格不兼容.在32位系统上，它是等效的，但一般而言不是.当然可以更改代码以适应此情况. OTOH "%lf"和"%f" 兼容，因为通常将float提升为double.
Formats "%d" and "%ld" are not strictly compatible. On a 32-bit system there are equivalent, but not in general. Of course code can be altered to accommodate this. OTOH "%lf" and "%f" are compatible due to the usual argument promotions of float to double.
格式"%lu"和"zu" 可能兼容，但这取决于unsigned long和size_t的实现.添加代码可以实现此等价方式.
Formats "%lu" and "zu" may be compatible, but that depends on the implementation of unsigned long and size_t. Additions to code could allow this or related equivalences.
修饰符和说明符的某些组合没有像"zp"那样定义.以下内容并不禁止这种深奥的组合-而是对它们进行了比较.
Some combinations of modifiers and specifiers are not defined like "zp". The following does not dis-allow such esoteric combinations - but does compare them.
像"$"这样的修饰符是对标准C的扩展，在下文中未实现.
Modifiers like "$" are extensions to standard C and are not implemented in the following.
printf()的兼容性测试不同于scanf().
The compatibility test for printf() differs from scanf().
#include <ctype.h>
#include <limits.h>
#include <stdio.h>
#include <string.h>
typedef enum {
type_none,
type_int,
type_unsigned,
type_float,
type_charpointer,
type_voidpointer,
type_intpointer,
type_unknown,
type_type_N = 0xFFFFFF
} type_type;
typedef struct {
const char *format;
int int_queue;
type_type type;
} format_T;
static void format_init(format_T *state, const char *format);
static type_type format_get(format_T *state);
static void format_next(format_T *state);
void format_init(format_T *state, const char *format) {
state->format = format;
state->int_queue = 0;
state->type = type_none;
format_next(state);
}
type_type format_get(format_T *state) {
if (state->int_queue > 0) {
return type_int;
}
return state->type;
}
const char *seek_flag(const char *format) {
while (strchr("-+ #0", *format) != NULL)
format++;
return format;
}
const char *seek_width(const char *format, int *int_queue) {
*int_queue = 0;
if (*format == '*') {
format++;
(*int_queue)++;
} else {
while (isdigit((unsigned char ) *format))
format++;
}
if (*format == '.') {
if (*format == '*') {
format++;
(*int_queue)++;
} else {
while (isdigit((unsigned char ) *format))
format++;
}
}
return format;
}
const char *seek_mod(const char *format, int *mod) {
*mod = 0;
if (format[0] == 'h' && format[1] == 'h') {
format += 2;
} else if (format[0] == 'l' && format[1] == 'l') {
*mod = ('l' << CHAR_BIT) + 'l';
format += 2;
} else if (strchr("ljztL", *format)) {
*mod = *format;
format++;
} else if (strchr("h", *format)) {
format++;
}
return format;
}
const char *seek_specifier(const char *format, int mod, type_type *type) {
if (strchr("di", *format)) {
*type = type_int;
format++;
} else if (strchr("ouxX", *format)) {
*type = type_unsigned;
format++;
} else if (strchr("fFeEgGaA", *format)) {
if (mod == 'l') mod = 0;
*type = type_float;
format++;
} else if (strchr("c", *format)) {
*type = type_int;
format++;
} else if (strchr("s", *format)) {
*type = type_charpointer;
format++;
} else if (strchr("p", *format)) {
*type = type_voidpointer;
format++;
} else if (strchr("n", *format)) {
*type = type_intpointer;
format++;
} else {
*type = type_unknown;
exit(1);
}
*type |= mod << CHAR_BIT; // Bring in modifier
return format;
}
void format_next(format_T *state) {
if (state->int_queue > 0) {
state->int_queue--;
return;
}
while (*state->format) {
if (state->format[0] == '%') {
state->format++;
if (state->format[0] == '%') {
state->format++;
continue;
}
state->format = seek_flag(state->format);
state->format = seek_width(state->format, &state->int_queue);
int mod;
state->format = seek_mod(state->format, &mod);
state->format = seek_specifier(state->format, mod, &state->type);
return;
} else {
state->format++;
}
}
state->type = type_none;
}
// 0 Compatible
// 1 Not Compatible
// 2 Not Comparable
int format_cmp(const char *f1, const char *f2) {
format_T state1;
format_init(&state1, f1);
format_T state2;
format_init(&state2, f2);
while (format_get(&state1) == format_get(&state2)) {
if (format_get(&state1) == type_none)
return 0;
if (format_get(&state1) == type_unknown)
return 2;
format_next(&state1);
format_next(&state2);
}
if (format_get(&state1) == type_unknown)
return 2;
if (format_get(&state2) == type_unknown)
return 2;
return 1;
}
注意:仅进行了最少的测试.可以添加很多其他注意事项.
Note: only minimal testing done. Lots of additional considerations could be added.
已知的缺点:hh,h,l,ll,j,z,t带有n的修饰符. l和s,c.
Known shortcomings: hh,h,l,ll,j,z,t modifiers with n. l with s,c.
OP有关安全性问题的评论.这就改变了职位和比较的性质，从平等的人变成了安全的人.我猜想其中一个模式(A)将是参考模式，下一个(B)将是测试.测试将是"B至少和A一样安全吗?".示例A = "%.20s"和B1 = "%.19s"，B2 = "%.20s"，B3 = "%.21s". B1和B2都通过了安全性测试，因为它们提取的20 char不多. B3是一个问题，因为它超过了20 char的参考极限.进一步的 any 用%s %[ %c限定的非宽度是一个安全问题-在参考或测试模式中.此答案的代码无法解决此问题.
OP comments about security concerns. This changes the nature of the post and the compare from an equality one to a security one. I'd imagine that one of the patterns (A) would be a reference pattern and the next (B) would be the test. The test would be "is B at least as secure as A?". Example A = "%.20s" and B1 = "%.19s", B2 = "%.20s", B3 = "%.21s". B1 and B2 both pass the security test as they do not extract more the 20 char. B3 is a problem as it goes pass the reference limit of 20 char. Further any non-width qualified with %s %[ %c is a security problem - in the reference or test pattern. This answer's code does not address this issue.
如前所述，代码尚未使用"%n"处理修饰符.
As mentioned, code does not yet handle modifiers with "%n".
[2018编辑]
关于格式"%d"和"%u"不兼容.":这通常用于打印值.对于[0..INT_MAX]范围内的值，每种格式都可以按照C11dr§6.5.2.26进行操作.
Concerning "Formats "%d" and "%u" are not compatible.": This is for values to be printed in general. For values in the [0..INT_MAX] range, either format may work per C11dr §6.5.2.2 6.
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