strftime (对字符串进行格式化以表示系统日期和时间)

The strftime() function formats the broken-down time tm according to the format specification format and places the result in the character array s of size max.

The format specification is a null-terminated string and may contain special character sequences called conversion specifications, each introduced by a '%' character and terminated by some other character known as a conversion specifier character. All other character sequences are ordinary character sequences.

The characters of ordinary character sequences (including the null byte) are copied verbatim from format to s.

Conversion characters are replaced as follows.

#include 
size_t strftime(char *s, size_t max, const char *format, const struct tm *tm);

Conversion Characters

Some conversion specifications can be modified by preceding the conversion specifier character by the E or O modifier to indicate that an alternative format should be used. If the alternative format or specification does not exist for the current locale, the behavior will be as if the unmodified conversion specification were used. The Single UNIX Specification (SU) mentions %Ec, %EC, %Ex, %EX, %Ey, %EY, %Od, %Oe, %OH, %OI, %Om, %OM, %OS, %Ou, %OU, %OV, %Ow, %OW, %Oy, where the effect of the O modifier is to use alternative numeric symbols (say, roman numerals), and that of the E modifier is to use a locale-dependent alternative representation.

The broken-down time structure tm is defined in . See also ctime

Return Value

The strftime() function returns the number of bytes placed in the array s, not including the terminating null byte, provided the string, including the terminating null byte, fits. Otherwise, it returns 0, and the contents of the array is undefined. This behavior applies since at least libc 4.4.4; very old versions of libc, such as libc 4.4.1, would return max if the array was too small. Note that the return value 0 does not necessarily indicate an error; for example, in many locales %p yields an empty string, which is equivalent to zero.

Environment

The environment variables TZ and LC_TIME are used.

Notes

ISO 8601 Week Dates %G, %g, and %V yield values calculated from the week-based year defined by the ISO 8601 standard. In this system, weeks start on a Monday, and are numbered from 01, for the first week, up to 52 or 53, for the last week. Week 1 is the first week where four or more days fall within the new year (or, synonymously, week 01 is: the first week of the year that contains a Thursday; or, the week that has 4 January in it).

When three of fewer days of the first calendar week of the new year fall within that year, then the ISO 8601 week-based system counts those days as part of week 53 of the preceding year. For example, 1 January 2010 is a Friday, meaning that just three days of that calendar week fall in 2010. Thus, the ISO 8601 week-based system considers these days to be part of week 53 (%V) of the year 2009 (%G) ; week 01 of ISO 8601 year 2010 starts on Monday, 4 January 2010.

Glibc Notes

Glibc provides some extensions for conversion specifications. These extensions are not specified in POSIX.1-2001, but a few other systems provide similar features. Between the '%' character and the conversion specifier character, an optional flag and field width may be specified. These precede the E or O modifiers, if present.

The following flag characters are permitted:

An optional decimal width specifier may follow the (possibly absent) flag. If the natural size of the field is smaller than this width, then the result string is padded (on the left) to the specified width.

"%a, %b %d %l:%M.%S"

The above string would format the date and time to look like: Sat, Jul 13 1:32.59

Here's an example of a program that uses strftime() to interpret such a string:

#include 
#include 
#include 
int main(int argc, char *argv[])
{
  char outstr[200];
  time_t t;
  struct tm *tmp;
  t = time(NULL);
  tmp = localtime(&t);
  if (tmp == NULL) {
  perror("localtime");
  exit(EXIT_FAILURE);
  }
   if (strftime(outstr, sizeof(outstr), argv[1], tmp) == 0)
   {
   fprintf(stderr, "strftime returned 0");
   exit(EXIT_FAILURE);
   }
   printf("Result string is \"%s\"\n", outstr);
   exit(EXIT_SUCCESS);
}

For example, if the above program were compiled as the executable file a.out, it could be used from the command line like this:

./a.out '%m'

Result string is "11"

./a.out '%5m'

Result string is "00011"

./a.out '%_5m'

Result string is " 11"