printf

问题 I'm trying to redirect stdout so that printf in a windows application will go to a file of my choice. I'm doing this: outFile = fopen("log.txt", "w"); *stdout = *outFile; setvbuf(stdout, NULL, _IONBF, 0); but printf still writes to the console (or nowhere on a GUI based win32 application) I can redirect 'std::cout' by doing this: outFileStr = std::ofstream(outFile); std::cout.rdbuf(outFileStr.rdbuf()); But printf seems to be doing its own thing. Unfortunately, I need to redirect printf as I'm

问题 How do functions such as printf extract digits from a floating point number? I understand how this could be done in principle. Given a number x , of which you want the first n digits, scale x by a power of 10 so that x is between pow(10, n) and pow(10, n-1) . Then convert x into an integer, and take the digits of the integer. I tried this, and it worked. Sort of. My answer was identical to the answer given by printf for the first 16 decimal digits, but tended to differ on the ones after that.

问题 How do functions such as printf extract digits from a floating point number? I understand how this could be done in principle. Given a number x , of which you want the first n digits, scale x by a power of 10 so that x is between pow(10, n) and pow(10, n-1) . Then convert x into an integer, and take the digits of the integer. I tried this, and it worked. Sort of. My answer was identical to the answer given by printf for the first 16 decimal digits, but tended to differ on the ones after that.

问题 I wish to know which of these two options is the more secure one to use: #define MAXLEN 255 char buff[MAXLEN + 1] sprintf(buff, "%.*s", MAXLEN, name) snprintf(buff, MAXLEN, "%s", name) My understanding is that both are same. Please suggest. 回答1: The two expressions you gave are not equivalent: sprintf takes no argument specifying the maximum number of bytes to write; it simply takes a destination buffer, a format string, and a bunch of arguments. Therefore, it may write more bytes than your

问题 I wish to know which of these two options is the more secure one to use: #define MAXLEN 255 char buff[MAXLEN + 1] sprintf(buff, "%.*s", MAXLEN, name) snprintf(buff, MAXLEN, "%s", name) My understanding is that both are same. Please suggest. 回答1: The two expressions you gave are not equivalent: sprintf takes no argument specifying the maximum number of bytes to write; it simply takes a destination buffer, a format string, and a bunch of arguments. Therefore, it may write more bytes than your

问题 On Unix (Clang 3.8.1), this code outputs: 6: 32 8: a8e On Windows (MSVC 19.00.24215.1), this code outputs: 6: 12345 6: a12345e #include <iostream> #include <stdarg.h> static std::string getFormattedString(const char* fmt, va_list ap) { int count = vsnprintf(NULL, 0, fmt, ap) + 1; std::cout << count << ": "; if (count <= 0) { return "unable to format message"; } std::string result = std::string(count, '\0'); if (vsnprintf(&result[0], count, fmt, ap) < 0) { return "error";} return result; }

问题 This question already has answers here : Closed 8 years ago . Possible Duplicate: Why scanf must take the address of operator Why do we pass the variable in the case of printf(), whereas the address of the variable in the case of scanf()? why to pass address in scanf 回答1: why to use '&' in scanf( ) but not in printf( ) 'printf'()' only need the values in order to output them. 'scanf()' stores values, so it needs a place to store them. This is done by providing the addresses (in pointers) of

问题 I am writing a basic C program which takes a word from the user as input and prints that word twice on the same row. The issue that I am facing in printing the word twice is mentioned after the code that I have written below to do this job void print_word() { char ch; char str[15]; int i = 0; /* i will be used as index to access the elements of str */ printf ("\n Enter a word of your choice : ") ; ch = getch() ; /* Dont echo character */ while ( !isspace(ch) && i < 14) /* while loop executes

问题 I am writing a basic C program which takes a word from the user as input and prints that word twice on the same row. The issue that I am facing in printing the word twice is mentioned after the code that I have written below to do this job void print_word() { char ch; char str[15]; int i = 0; /* i will be used as index to access the elements of str */ printf ("\n Enter a word of your choice : ") ; ch = getch() ; /* Dont echo character */ while ( !isspace(ch) && i < 14) /* while loop executes

问题 I have a very simple C program. In main, I have this operation: int main(){ int theNumber = 9009; printf("%s", (char *)theNumber); } And when I run it, it gives me a seg fault. Any idea why? Am I doing this wrong? Expected Output This code should convert theNumber to a string and then print it. So the output would be: 9009 Actual Output A segmentation fault. 回答1: This is trying to print whatever is found at the address 9009 . Seeing as the operating system is not giving your program access to