Debugging with GDB: Output

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23.1.4 Commands for Controlled Output

During the execution of a command file or a user-defined command, normal GDB output is suppressed; the only output that appears is what is explicitly printed by the commands in the definition. This section describes three commands useful for generating exactly the output you want.

echo text

Print text. Nonprinting characters can be included in text using C escape sequences, such as ‘\n’ to print a newline. No newline is printed unless you specify one. In addition to the standard C escape sequences, a backslash followed by a space stands for a space. This is useful for displaying a string with spaces at the beginning or the end, since leading and trailing spaces are otherwise trimmed from all arguments. To print ‘ and foo =  ’, use the command ‘echo \ and foo = \  ’.

A backslash at the end of text can be used, as in C, to continue the command onto subsequent lines. For example, 

echo This is some text\n\
which is continued\n\
onto several lines.\n

produces the same output as 

echo This is some text\n
echo which is continued\n
echo onto several lines.\n
output expression

Print the value of expression and nothing but that value: no newlines, no ‘$nn = ’. The value is not entered in the value history either. See Expressions, for more information on expressions.

output/fmt expression

Print the value of expression in format fmt. You can use the same formats as for print. See Output Formats, for more information.

printf template, expressions…

Print the values of one or more expressions under the control of the string template. To print several values, make expressions be a comma-separated list of individual expressions, which may be either numbers or pointers. Their values are printed as specified by template, exactly as a C program would do by executing the code below: 

printf (template, expressions…);

As in C printf, ordinary characters in template are printed verbatim, while conversion specification introduced by the ‘%’ character cause subsequent expressions to be evaluated, their values converted and formatted according to type and style information encoded in the conversion specifications, and then printed.

For example, you can print two values in hex like this: 

printf "foo, bar-foo = 0x%x, 0x%x\n", foo, bar-foo

printf supports all the standard C conversion specifications, including the flags and modifiers between the ‘%’ character and the conversion letter, with the following exceptions:

  • The argument-ordering modifiers, such as ‘2$’, are not supported.
  • The modifier ‘*’ is not supported for specifying precision or width.
  • The ‘'’ flag (for separation of digits into groups according to LC_NUMERIC') is not supported.
  • The type modifiers ‘hh’, ‘j’, ‘t’, and ‘z’ are not supported.
  • The conversion letter ‘n’ (as in ‘%n’) is not supported.
  • The conversion letters ‘a’ and ‘A’ are not supported.

Note that the ‘ll’ type modifier is supported only if the underlying C implementation used to build GDB supports the long long int type, and the ‘L’ type modifier is supported only if long double type is available.

As in C, printf supports simple backslash-escape sequences, such as \n, ‘\t’, ‘\\’, ‘\"’, ‘\a’, and ‘\f’, that consist of backslash followed by a single character. Octal and hexadecimal escape sequences are not supported.

Additionally, printf supports conversion specifications for DFP (Decimal Floating Point) types using the following length modifiers together with a floating point specifier. letters:

  • ‘H’ for printing Decimal32 types.
  • ‘D’ for printing Decimal64 types.
  • ‘DD’ for printing Decimal128 types.

If the underlying C implementation used to build GDB has support for the three length modifiers for DFP types, other modifiers such as width and precision will also be available for GDB to use.

In case there is no such C support, no additional modifiers will be available and the value will be printed in the standard way.

Here’s an example of printing DFP types using the above conversion letters: 

printf "D32: %Hf - D64: %Df - D128: %DDf\n",1.2345df,1.2E10dd,1.2E1dl
eval template, expressions…

Convert the values of one or more expressions under the control of the string template to a command line, and call it.

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