Weird behaviour on redirection of exec() output

Question

I am writing a program which creates a txt files input.txt and it uses it as input of exec() . I have problems redirecting its output to another file output.txt and they seems to be linked to input.txt writing.

If I use fwrite(array, 4, 1, file) the redirection works. If I use fwrite(array, sizeof(array), 1, file) or fprint , it doesn't.

FILE *finput = fopen("input.txt", "w");
for (int i=0; i<dimension; i++)
{
    char c; // I get it from another file, it isnt't important.
    char arr[1];
    arr[0] = c;

    // WORKING LINE OF CODE
    fwrite(arr, 4, 1, finput);

    // NOT-WORKING LINE OF CODE
    // fwrite(arr, sizeof(arr), 1, finput);

    // ANOTHER NOT-WORKING LINE OF CODE
    // fprintf(finput, "%c", c);
}

pid_t pid;
int status = -1;
pid = fork();
if (pid == -1)
{
    printf("ERROR.");
}
else if ( pid == 0)
{
    int fdOutput = creat("output.txt", 0777);
    dup2(fdOutput, 1);
    dup2(fdOutput, 2);
    close(fdOutput);
    // awkScript is an awk program string
    int executionResult = execl("/usr/bin/gawk", "gawk", awkScript, "input.txt", (char*)NULL);
    // ...
}

If I write the working line of code, at the end of the program I have some text in the output.txt file. Otherwise, it is completely empty.

The weirdest thing is that input.txt works anyway, it is always correctly written. So I can't understand why it should be connected to its writing. If I execute the command:

gawk $awkScript input.txt

The output is printed in all the three ways used to write input.txt

Answer 1

There are multiple issues with this code (such as the size 4 … where does this come from? Your array is 1 byte long; so this is undefined behaviour) but the fundamental reason why it fails is that you fail to close your file. This means the output buffer never gets flushed, and no output written.

Either fclose the file, or at least fflush it after the write.