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Questions tagged [stdio]

This tag is for questions regarding "Standard I/O", i.e. I/O using the facilities in the C header or using the standard streams stdin, stdout, stderr.

The C standard header <stdio.h> defines facilities for using data streams via FILE objects and also declares the pre-defined standard streams, stdin, stdout and stderr.

The standard IO streams can be used from C in two ways:

  1. Using standard IO streams as implemented by the standard header <stdio.h> e.g. fprintf (stdout, "hello, world\n");
  2. Using the underlying file descriptors directly using the facilities in <unistd.h> e.g. write (STDOUT_FILENO, "hello, world\n", 13);. Note that this is not ISO C, but POSIX.
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whitespace in the format string (scanf)

Consider the following code: #include int main() { int i=3, j=4; scanf("%d c %d",&i,&j); printf("%d %d",i,j); return 0; } It works if I give 2c3 or 2 c 3 or 2c 3 as input if I have to change the value of variables. What…
Vaibhav Agarwal
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What does "< /dev/null >& /dev/null" at the end of a command do?

One of the scripts I run over ssh was hanging and I found a solution for it on this site: http://www.snailbook.com/faq/background-jobs.auto.html The site resolves the problem by adding this to the end of the command: xterm < /dev/null >& /dev/null…
user215997
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C fgets versus fgetc for reading line

I need to read a line of text (terminated by a newline) without making assumptions about the length. So I now face to possibilities: Use fgets and check each time if the last character is a newline and continuously append to a buffer Read each…
nc3b
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String Stream in C

print2fp(const void *buffer, size_t size, FILE *stream) { if(fwrite(buffer, 1, size, stream) != size) return -1; return 0; } How to write the data into string stream instead of File stream?
Said
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Where to find struct _IO_FILE

I was looking through /usr/include/stdio.h and happened to come across the following piece of code. /* Standard streams. */ extern struct _IO_FILE *stdin; /* Standard input stream. */ extern struct _IO_FILE *stdout; /* Standard…
ArunMKumar
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Prevent inclusion of stdio.h (or other standard header)

A codebase I work with has historically tried--purposefully--to avoid dependencies on stdio.h creeping in. It has its own print formatting and mechanisms, and those are what's supposed to be used instead of printf etc. But someone adds a dependency…
HostileFork says dont trust SE
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Java + Eclipse: Synchronize stdout and stderr

I use Eclipse. When I have an application like this: write 20 times 'Hello World\n' to stdout write 'ERROR\n' to stderr write 5 times 'Hello World\n' to stdout The output looks many times like this: Hello World Hello World Hello World Hello…
Martijn Courteaux
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Unable to open a file with fopen()

I've been trying to open a file and output text, but I keep getting errors. So I thought I would start at the very beginning and just try opening the file. This is my code: #include #include #define CORRECT_PARAMETERS 3 int…
jet
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printf() prints string without newline to stdout in line buffer mode after scanf()

I am aware that most terminals are by default in line buffer mode. i.e. output is buffered and not directed to stdout until a new-line character is encountered. So I would expect this to print nothing (at least before the buffer is filled up): int…
Chung Lun Yuen
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Why is buffered writing to a fmemopen()'ed FILE faster than unbuffered?

For sure, buffered I/O to a file on disk is faster than unbuffered. But why is there a benefit even when writing to a memory buffer? The following benchmark code example was compiled with gcc 5.40 using optimization option -O3, linked against glibc…
ralfg
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Is creating two FILEs for the same file descriptor well-defined?

POSIX specifies an fdopen function that creates a FILE for a file descriptor. POSIX also specifies a fileno function that returns the file descriptor for a FILE. Together, these two could be used to create a second FILE accessing the same underlying…
fuz
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Redirecting stdio from a command in os.system() in Python

Usually I can change stdout in Python by changing the value of sys.stdout. However, this only seems to affect print statements. So, is there any way I can suppress the output (to the console), of a program that is run via the os.system() command…
Leif Andersen
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Is close/fclose on stdin guaranteed to be correct?

It seems as though the following calls do what you'd expect (close the stream and not allow any further input - anything waiting for input on the stream returns error), but is it guaranteed to be correct across all…
Greg Rogers
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Does sprintf/snprintf allocate additional memory?

I'm writing a library and want to make it absolutely resource-agnostic which also means that library should live with user-supplied memory allocation functions. Library allows user to set their own error handler function too, which are called with…
firegurafiku
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forcing a program to flush its standard output when redirected

i have a closed source program that prints output to standard output. i need to parse the output. so i redirect the output to a fifo (from which i can read in the parent process that forks and execs the binary) using dup2 and then exec the program.…
Rohit Banga
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