printf - Page 45 - Developer IT

Using atoi with char

- by John Doe

Is there a way of converting a char into a string in C? I'm trying to do so like this: char *array; array[0] = '1'; int x = atoi(array); printf("%d",x);

String concatenation in C

- by Mechko

if I want to construct a const char * out of several primitive type arguments, is there a way to build the string using a similar to the printf?

print the float value in integer in C language

- by amit singh

#include<stdio.h> int main() { float a=5; printf("%d",a); return 0; } This gives the output: 0 Why is the output zero?

gcc, strict-aliasing, and casting through a union

About a year ago the following paragraph was added to the GCC Manual, version 4.3.4, regarding -fstrict-aliasing: Similarly, access by taking the address, casting the resulting pointer and dereferencing the result has undefined behavior [emphasis added], even if the cast uses a union type, e.g.: union a_union { int i; double d; }; int f() { double d = 3.0; return ((union a_union *)&d)->i; } Does anyone have an example to illustrate this undefined behavior? Note this question is not about what the C99 standard says, or does not say. It is about the actual functioning of gcc, and other existing compilers, today. My simple, naive, attempt fails. For example: #include <stdio.h> union a_union { int i; double d; }; int f1(void) { union a_union t; t.d = 3333333.0; return t.i; // gcc manual: 'type-punning is allowed, provided ...' } int f2(void) { double d = 3333333.0; return ((union a_union *)&d)->i; // gcc manual: 'undefined behavior' } int main(void) { printf("%d\n", f1()); printf("%d\n", f2()); return 0; } works fine, giving on CYGWIN: -2147483648 -2147483648 Also note that taking addresses is obviously wrong (or right, if you are trying to illustrate undefined behavior). For example, just as we know this is wrong: extern void foo(int *, double *); union a_union t; t.d = 3.0; foo(&t.i, &t.d); // UD behavior so is this wrong: extern void foo(int *, double *); double d = 3.0; foo(&((union a_union *)&d)->i, &d); // UD behavior For background discussion about this, see for example: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1422.pdf http://gcc.gnu.org/ml/gcc/2010-01/msg00013.html http://davmac.wordpress.com/2010/02/26/c99-revisited/ http://cellperformance.beyond3d.com/articles/2006/06/understanding-strict-aliasing.html http://stackoverflow.com/questions/98650/what-is-the-strict-aliasing-rule http://stackoverflow.com/questions/2771023/c99-strict-aliasing-rules-in-c-gcc/2771041#2771041 The first link, draft minutes of an ISO meeting seven months ago, notes in section 4.16: Is there anybody that thinks the rules are clear enough? No one is really able to interpret tham.

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